Upon reading this post’s title, you may be inclined to stop right there. (That’s why I have an eye-catching photo to lure you in.) While logic may seem irrelevant to your enjoyment of gardening, I can guarantee that reading this blog post will challenge many seemingly logical assumptions you’ve heard or read about. Recognizing unsubstantiated assumptions and avoiding their pitfalls means you can make wise choices about how you care for your gardens and landscapes.
A few definitions are needed before we get started:
Correlation refers to variables whose changes mirror one another. For instance, the addition of nitrogen fertilizer to container plants is correlated to plant growth: as nitrogen levels increase so does plant growth. You can also have inverse correlation, where the variables move in opposite directions. An example is water availability in soil and planting density: the more plants you have in a specified area, the less water is in the soil.
Causation takes correlation one step further: it establishes that one of those variables is causing the change in the other. Using the same examples, we know through published evidence that the increase in nitrogen is causing the increase in plant growth, and the increase in planting density is causing the decrease in soil water because of competing roots. These relationships are obvious to us, but what’s important is that these causative effects have been established through scientific experiments.
Sometimes scientific evidence doesn’t exist to demonstrate causation. That may be because it’s impractical or impossible to run an experiment that tests for a causative effect, or it may be because the experiments just haven’t been conducted yet. The latter is the unfortunate reality for those of us interested in managing gardens and landscapes: there is no major funding agency that supports field research for us. There is research being done, but it’s on a small scale with a shoestring budget…so the body of literature develops very slowly. In such situations, we must rely on established applied plant physiology and soil science to ask whether a suggested correlation might be elevated to causation.
Which brings me to my current source of online irritation: the constant blaming of tree failure on mulch volcanoes. Yes, tree failure is definitely correlated with mulch volcanoes – because lots and lots of newly planted trees fail. But is the mulch to blame? No one seems to care much that there is NO published work to show that mounds of appropriate mulch materials will somehow kill otherwise healthy trees. Instead, observers jump to the conclusion that thick layers of wood chip mulch kill trees. They are elevating correlation to causation in the absence of either experimental research OR known plant physiology. In fact, there is published research to show that thick layers of arborist wood chip mulch enhance tree establishment and survival. And there are many poor planting practices that increase the likelihood of tree failure. But it’s easiest to blame the wood chip mulch, though it’s merely masking a multitude of planting sins.
Not interested in mulch volcanoes? Well, there are lots of other examples of garden and landscape management practices or phenomena that fall into the logical fallacy camp. I’ve linked to appropriate references, when available, that go into more detail:
and just about any gardening product you can think of where there is NO published evidence – or appropriate, established plant or soil science – that supports any causative, beneficial effect on plants or soils. Cornmeal, Epsom salt, gypsum, and kelp products are just some of these.
All of these products, practices or phenomena are correlated with some anecdotal observation (increased yield, healthier soil, plant failure, etc.) that elevates them to causative relationships. But no science.
I’d encourage you to think objectively about your closely held beliefs about your gardens or landscapes. Are you sure that what you’re doing is actually beneficial? How do you know there’s a cause-and-effect relationship? I’m not going to talk you out of your cherished beliefs – but if you are a science-based gardener, you might talk yourself out of them instead.
While most of the US is still seeing sweltering hot temps, the cool temps of fall and winter aren’t really all that far away for those of us unlucky (or lucky) enough to not live in a tropical climate. The tomatoes, peppers, cucumbers, and other warm-season crops planted back at the beginning of summer are still puttering along, even if they might be getting a little long in the tooth and starting to look a little worse for wear ( especially if disease has ravaged them). For those who aren’t quite done with gardening for the year or who want to reap the bounty of fall crops and get the most out of their production space, fall gardening can be a great tool to extend the garden season. But knowing when to plant what is tricky, especially when we are talking about different weather patterns and frost dates all around the country. So a bit of weather data, info from the seed packet or label, a touch of math, and a calendar can be great tools to figure out when you can plant no matter where you are. Of course if you do live in one of those warmer tropical areas your planting calendar is kind of turned on its head from what us more northern gardeners face. You may prefer to time your planting to avoid high heat.
The first thing to think about is what you can plant. Cool-season crops such as the Cole crops (cabbage, kale, broccoli, etc.), leafy greens (lettuce, spinach, Bok choi, etc.), root crops (radishes, beets, turnips, scallions), and some cool weather loving herbs like cilantro and parsley are all par for the course for a garden going into cooler fall and winter temps. Depending on when you have extra space in your garden to plant and how long your growing season is you can often sneak in a late planting of fast-growing warm season crops to mature before the last frost. Beans, cucumbers, and summer squash all have varieties that are fast maturing and can be started mid-summer for an early fall harvest. Unfortunately, as of this writing the window for those warm-season crops has passed for me, but others in warmer zones may still have time.
One question I get asked often is whether you should start indoors or out. I always tell folks that for things normally direct-seeded, like beans or lettuce, sow as normal. For things that are normally started indoors, the choice is yours. Cole crops are started indoors in spring because they need warmer temps to germinate. Since it is hot outside, you won’t need to grow them indoors for the heat (though it may be too hot outdoors if temps are over 85). You can start them in containers in a protected area outdoors instead of trying indoors. Theoretically you could direct seed them into the garden, but management is difficult to keep them watered, weed-free, and alive out there in the cruel garden world.
To know what you can plant and when, the first bit of info you’ll need is from the seed packet or label (or do some research if you know the cultivar/variety). You’ll want to know the “days to maturity”, which is an estimate of how long it will take to go from seed (or transplant) to edible crop. For those warm season crops, you might want to shop around because those days to maturity can be wildly variable – you can find beans that mature in 60-65 days and some that take 100+. You’ll want to choose faster maturing varieties.
Assuming that you’ll want a harvest window longer than a day and given that plant growth slows down as temperature cools (respiration is temperature dependent so plant processes slow down as temperatures drop), you’ll want to add a few weeks to the maturity days to take that into account. This should be sufficient for cool season crops that will survive well past the first frost and freeze dates. The aim for cool season crops is to get them close to a mature size before cold weather sets in since their growth will slow down at that point. For warm season crops you’ll want to add a little more time to provide a cushion against frost which will kill the plants. For info on killing temperatures of certain crops, check out my previous article here.
For example, if I wanted to plant Asian Delight Bok choi (I
fell in love with it when I trialed it for the All-America Selections program)
I’d see on the packet that it has an average maturity time of 37 days (which is
pretty damn fast). My math would be:
37 days (to maturity) + 14 days (harvest period) + 14 days
(fall factor) = 65 days
Next, you’ll need to know a bit of weather data – more specifically the expected date of your first frost/freeze. You can find this on the website of your local National Weather Service office, or get an idea from the map below. (This data is usually updated every decade or so – you’ll want to check it every few years for updates as the dates have been changing due to climate change.) The date ranges given are usually a median, meaning that half of the frost days fall before and half fall after the given dates. Keep that in mind – sometimes frost will come earlier or be much later.
I live in Omaha, Nebraska so our median frost date (according to the map) is Oct 10. Now I know that I need to plant my Asian Delight Bok choi 65 days before Oct 10. I can grab a calendar and count backward from October 10 (or I can cheat and use an online date calculator like this one) and see that the suggested planting date is August 6. Since I missed it by a week I can decide if I want to gamble a little and still plant since I know that it could very well frost later than Oct 10 and that the Bok choi will survive much later into the season anyway. But it gives me an idea of what to expect.
Had I wanted to plant something like beans for a late crop, my calculation would have definitely shown me that it was too late, letting me know that I shouldn’t waste my time. For example, Blue Lake beans take around 55-60 days to mature (almost twice as long as my Bok choi), plus I need to add that extra 14 days for the frost factor meaning that I would have had to plant 97 days before first frost, which would have been in early July for me.
You can extend the time you have for growing fall crops by using season extension techniques like row covers, low tunnels, cloches, etc. For row covers, the materials you buy such as the spun fabric row cover will offer a certain number of degrees of protection. For example, a medium weight row cover might offer 8 degrees of protection, meaning it will be 8 degrees warmer under the cover than the air temp. Keep those in mind when planning your fall garden. Perhaps we’ll have to talk about those in another article soon.
In this blog I continue to examine maladies caused by environmental conditions in the absence of a disease agent or insect.
Salinity Salt in soils or water is simply the presence of too many soluble ions in the soil-water solution. This tends to happen in dry climates where evaporation rates exceed precipitation rates. In these climates salts accumulate in soil when surface waters pick up minerals from soil that is high in precipitated salts. In wetter climates water leaches salts from soil so surface waters (rivers and lakes) have fewer dissolved salts. Also, in dry climates irrigation is often a must and irrigation sources usually have high amounts of dissolved salts. In high salt environments plants must use energy to increase their own salt balance at the root interface to make uptake of fresh water through their membranes possible. This energy is thus not available for growth. Salt affected plants are often smaller, even stunted depending on salinity levels and are more susceptible to root pathogens as their roots are more likely to be “leaky” giving pathogens chemical signals of their susceptibility. Salt damaged leaves often show “edge” necrosis or burning on the oldest leaves.
Salt affected soils should not be allowed to dry out as roots will be damaged. Leaching to dissolve salts and move them below the root zone is one approach to prevent further symptoms.
Soil compaction Soil compaction is the increase in soil bulk density beyond a point where roots function and grow. Bulk density is a measure of soil compactness and is calculated as the weight of dry soil per volume. Optimal and harmful bulk densities vary by soil texture. Sands have higher bulk densities than loams which are higher than clays in their growing range. Normal bulk density for a sand will be a compacted value for a clay. Values above 1.1, 1.4 and 1.6 g/cm3 can be restrictive for roots growing clays, loams and sands respectively. Compacted soils of any texture restrict plant growth. Stunting, poor growth and nutrient deficiencies due to loss of root function are common.
Extremes of light Light is necessary for photosynthesis but it is also a radiation source that can include damaging light energy when it reaches tissues that are not accustomed to it. This happens frequently on over-pruned or damaged trees, where the canopy is suddenly reduced and stem tissues receive intense sunlight. On thin or green barked trees this can cause sun scald. Apples are particularly sensitive and will develop large cankers on upper branch surfaces if too much light is allowed into the canopy during summer. Canopy loss compounds light injury because the tree is not cooling itself as efficiently with fewer leaves. Infrared energy (heat) builds up on branch surfaces and can kill underlying stem cambium layers.
Low light levels also harm plant productivity. All trees tend to lose interior branches as normal growth increases canopy density and light levels decrease in the innermost canopy. Inner branches store less and less energy and essentially die due to light starvation. The same thing can happen to entire trees if they are overgrown by vines, other trees or shaded by buildings. While canopy thinning will preserve inner branches, it is not absolutely necessary as branch dieback is a natural process in most trees.
Effects of Herbicides Sometimes herbicides cause damage to non target plants. This happens when herbicides are applied unknowingly, such as residues in composts, drift from off-site applications, or by choosing the wrong herbicide to use in a garden setting. Herbicides affect plants in different ways: some only affect tissues they contact, others are systemic, and some affect seeds as pre-emergent herbicides and have activity in soil over time. Diagnosing herbicide damage often requires sleuthing and inquiry of what has happened in the past and what materials your neighbors may be applying. As with any pesticide, herbicides should be applied according to label instructions.
In conclusion…Disease diagnosis can be a challenge for the gardener and dysfunctions caused by abiotic factors are no different. Carefully considering the symptoms that the plant presents is the first step to recognizing an abiotic disorder. Uniformity of symptoms is often indicative of disorders not caused by biotic pathogens. As with any plant health issue, figuring out the cause is the first step in helping plants succeed.
Costello, L., Perry, EJ, Matheny, NP, Henry, MJ, and PM Geisel. 2003. Abiotic Disorders of landscape plants a diagnostic guide. ANR publication 3420 University of California, Communication Services, Oakland CA.
Manion, P. 1981. Tree Disease Concepts. Prentice-Hall Inc., 399pp.
Do you have a favorite kind of weather that you love to experience? For me, it’s the first warm evening of spring, when the air is just warm enough and the wind just strong enough for the air to feel as though it is dripping off my fingers. On nights like that, I can tell that the air really is a fluid, the topic of this week’s blog.
What causes atmospheric weather patterns?
You may have noticed this year that the weather patterns across both the United States and Europe have been very persistent. That has led to the occurrence of record-setting high temperatures in some locations like the Pacific Northwest and the central US and southern Europe as well as day after day of rain in the Southeastern US. Both of these weather patterns have caused no end of grief for gardeners and farmers, since weather is seldom stuck on day after day of “perfect” conditions (even if you could define what those are).
To understand how weather patterns get stuck, it helps to know how the air moves through the atmosphere. Wind is driven by differences in heating between two areas. That contrast leads to differences in density and pressure between the areas, and the air flows from higher pressure to lower pressure to try to equalize the amount of air between those areas. The large-scale weather patterns across the globe are caused by differences in the sun heating the spherical earth at the equator and at the poles at different angles due to latitude. There are also smaller-scale wind circulations due to differences in heating between land and water (oceans or lakes) that cause the same movement of air molecules. The earth is not uniform, with continents in some areas and oceans in others and has mountain ranges that also divert the flow of air. The earth is also rotating, and that makes the wind appear to be turning towards the right in the Northern Hemisphere (left in the Southern Hemisphere), which we call the “Coriolis force.” Friction can also slow down the wind and cause it to change direction near the earth’s surface, which adds to the calculation.
The net result of all these forces acting on the air are a series of atmospheric waves of high and low pressure around the earth that control where the weather goes (these are known as Rossby waves after the great meteorologist Carl-Gustav Rossby). A great website to view these waves is at https://earth.nullschool.net/, with a dynamic view of the air flowing around the earth. You can move the earth around with your mouse by clicking and dragging it, and you can change the size by pinching and dragging on your touchscreen if you don’t have a mouse with a wheel. If you click on “earth” on the bottom left of their map, it pulls up a menu that allows you to pick different heights in the atmosphere (1000 mb is closest to the surface, 500 mb is about halfway up in the atmosphere, and 250 mb (shown in this figure from July 29, 2021) is roughly the height that jets fly.
The image shows areas of strong winds (in red) and weak winds (in blue). The strong winds at 250 mb are called “jet streams”, and they push weather systems around. The strong jet stream over the upper Great Lakes in this image helps explain why the severe weather that moved through Wisconsin on July 29 moved from northwest to southeast. The jet streams outline the atmospheric waves that control the large-scale weather patterns. In the Northern Hemisphere, winds blow clockwise around areas of high pressure (“ridges”) and counterclockwise around low pressure (“troughs”). It is much easier to see on the website than on this static image, although traditional weather maps usually show the wind direction using arrows to help. As the image shows, often a ridge in one half of the continent is accompanied by a trough in the other half just like an ocean wave.
The large-scale weather patterns that are connected to atmospheric waves are constantly shifting as the air in the atmosphere tries to balance out all the forces that are pushing it around. (This 30-second video shows how the waves typically evolve over time.) Often the waves move, usually from west to east in the mid-latitudes, and so the weather at those latitudes also tends to move from west to east. But sometimes the weather patterns get stuck in one spot, and cause day after day of the same weather. We call these “blocking” patterns because they block the natural movement of the waves, locking the weather pattern in place for long periods. That is what we have seen this summer, with a very strong high-pressure center locked over the western U. S. and a trough of low pressure draped over the eastern U. S. for much of the last few months. In summer, high pressure usually causes sinking air, lack of clouds, warm temperatures, light winds, and no rain. In contrast, low pressure leads to rising air that forms clouds and rain and cooler temperatures due to the clouds.
What is happening this summer?
The blocking this summer has been more persistent than usual, with strong high pressure in the west causing a very hot and dry area to form (sometimes called a “heat dome”). Once you get a strong high like that to form, it can get anchored in place by the dry conditions and high temperatures at the surface, making it very hard to move. The length of time that this summer’s high has lasted has contributed to the string of record-setting temperatures and lack of rain and subsequent drought that region has experienced this year. By contrast, in the Southeast, low pressure has led to a stubborn low-pressure trough that has brought rain to the region almost every day, preventing farmers from doing field work or drying hay and increasing the occurrence of fungal diseases on their crops. If you know you are in a blocking pattern, that means you should prepare to experience the same weather for protracted periods of time and adjust your gardening schedule to accommodate the weather you are likely to see for the next week.
How will atmospheric patterns change in the future?
Some but not all atmospheric scientists think that as the earth gets warmer, the temperature difference between the equator and the poles will decrease and that this will lead to atmospheric wave patterns that are loopier, like winding rivers in an almost flat coastal plain. This could lead to both more extreme weather and potentially, more frequent blocking patterns. Gardeners need to prepare by designing their gardens to handle both more extremes of weather, including heat waves and floods, and periods of more persistent weather, which could lead to more frequent and longer droughts. That means making sure that you need to have good drainage for high-intensity rainfall events but also need to use methods like mulching with arborist chips to preserve the moisture in the soil for those times when no rain is in the forecast for extended periods.
Four years ago we moved to the family farm (where I grew up) and we’ve enjoyed restoring the 1 acre landscape around the farmhouse. Given that the residential part of this farm is surrounded by pastureland, there is a continual influx of weed seeds into our managed beds. While our thick applications of arborist wood chips have kept out many weeds, they still pop up where mulch hasn’t been applied yet or is too thin.
One of these weeds is Hypericum perforatum (also known as Klamath weed or St. John’s wort), a species native to Eurasia. The latter common name can confuse gardeners, as there are several ornamental species of Hypericum also called St. John’s wort, but H. perforatum is easily identified by the perforations in the leaf. This invasive species is a problem for our cattle, as Klamath weed causes photosensitivity when it’s consumed and can be toxic in large amounts.
In the last few years H. perforatum colonized our stockpile of native soil waiting to be used in our raised beds. It was a small enough infestation that we could pull it all up, but a closer look revealed that some shiny metallic beetles were already busy feasting on the leaves. Putting on my IPM hat, I first needed to identify these interesting beetles. It didn’t take long to find out they were a Chrysolina species.
Chrysolina hyperici and C. quadrigemina (or St. John’s wort beetles) are also native to Eurasia and are specialist feeders – they only feed on Hypericum species. They were imported as biological control agents several decades ago and have been effective in controlling dense populations of St. John’s wort. C. quadrigemina in particular has been reported to feed on both ornamental and native species of Hypericum but not to the extent of causing significant damage.
Both species of the St. John’s wort beetle feed on the leaves, where they also lay thousands of eggs. The larvae that emerge from the eggs are voracious feeders and can defoliate dense stands of St. John’s wort. Like other animals that eat Hypericumperforatum, the larvae become photosensitive and generally feed before sunrise to avoid damage.
Since biological control agents depend on the presence of their host, it’s important to retain a small population of the host. And because this particular beetle is a leaf feeder, one can remove the flowers of the plants to reduce reproduction, but maintain the plants to support the beetle.
Many other introduced, invasive weeds can be controlled using carefully researched microbes and insects. Some of these biocontrol agents may already be found in your area – so it’s important to avoid using insecticides and fungicides, in particular, to conserve these garden assets.
We all know that water is essential for life and that we have to ensure our landscapes, gardens, and houseplants all have a sufficient supply of the stuff. Forget to water your garden during a hot, dry spell and it could mean disaster for your plants. But water can also create issues for plants, usually when it is in an overabundance – water helps spread and develop diseases on foliage and excess soil moisture can damage roots, creating opportunities for root rots and other diseases. How do you meet the water needs of the plant while also avoiding issues associated water? Understanding how water affects disease organisms will help, along with some tried and true Integrated Pest Management Strategies.
Water and Pathogenic Microbes
Both bacteria and fungi require water to grow and reproduce. Most do not have a mechanism to actively take up and manage water, so they uptake water mainly through osmosis. This means there must be some form of water present for those microbes that are actively growing and especially for processes like reproduction which use not only a lot of energy but might also be required to carry spores in order to spread.
Both pathogenic microbes and beneficial (or neutral) microbes require water to thrive. It is one side of what we refer to as the disease triangle. Water (along with temperature) are major components of the “favorable environment” side of the triangle, with the other sides being a plant capable of being infected and a population of pathogens capable of infecting. Those last two sides meaning you have to have a population of the pathogen big enough to initiate or sustain an infection and a plant that can actually be infected by that pathogen. For example – one disease spore may or may not be enough to start an infection (depending on the pathogen), but several hundreds or thousands definitely can. And the pathogen has to be one that can actually infect the plant – it doesn’t matter if you have a million spores of Alternaria solani (one of two closely related fungi that cause early blight in tomatoes) on your cucumber plants, they likely won’t get a disease. But if there are spores of A. cucumerina, a different species, you’ll likely get leaf spot on those cucumbers. But it doesn’t matter if you have both a susceptible plant and a pathogen, there has to be a favorable environment (water and temperature) for there to be a disease infection.
As this paper points out, water in the form of liquid (rain, ground water, dew, etc) and vapor (air humidity, fog) can provide the environment for microbe development in the soil and on foliage. Microbes in the soil are ubiquitous as water is typically available in most soils (except in droughty or arid areas) , but excess soil moisture can create booms in populations for both the “good” microbes and the “bad” ones. Microbes that live on foliage (sometimes referred to as epiphytic since they rely on moisture from the atmosphere) are much more likely to be water stressed since they are exposed to the atmosphere. When there isn’t water available on the surface of leaves (from rain, fog, etc.) microbes tend to colonize around areas where water leaves the plant – stomata and to a lesser extent around tricomes and hairs.
The paper also points out high atmospheric humidity is positively correlated with the number of fungi on a leaf surface. It’s also a requirement for diseases microbe spores to germinate, for filamentous fungi to break dormancy, for pathogen survival, for microbe movement on the leaf surface, and for disease infections to be sustained. It is also shown that heavy precipitation increases water availability to these microbes thus hastening their growth. Precipitation also dislodges and disperses pathogen spores and cells to adjacent plant tissues, and to leaves of nearby plants. High humidity also makes leaf cuticles more permeable and promotes opening of the stomata, which can serve as an entry point for pathogenic infection.
Once inside the plant, microbes such as fungi and bacteria can
thrive on the aqueous environment inside a plant, moving easily between cells
or into the vascular tissue (depending on disease). Pathogens that thrive in wet conditions,
however, may initiate water soaked lesions on the plant to develop conditions favorable
to their growth.
Water, water everywhere – so is there anything you can do?
Of course, water is naturally occurring and in most places falls from the sky in some form or another. In some places very little precipitation falls, in others there’s a lot. And don’t forget about the humidity, dew, and fog (which are often more common in places that get more rain, but provide moisture even in dry climates). There are a few places where the atmospheric moisture levels are in that “just right” zone to support plant growth but not pathogen growth, which makes agricultural production of certain crops easier. You could consider these areas the “Goldilocks” zone for crop production. For example, a lot of seed crops are produced in the Midwest and arid north West, potatoes in Idaho, apples in Washington, etc. The conditions there mean that, at least when those crops were getting established (before the advent of modern pesticides) in those regions, disease pressure was low.
You can’t stop the rain, of course, if you’re in a place
both blessed and cursed with abundant rainfall or atmospheric humidity. But there are some things that you can do
reduce the likelihood of diseases spread or supported by that water and
Evidence shows that there is a positive correlation between the density of planting and disease incidence. Therefore, proper plant spacing and pruning can do at least three major things. First, having space between plants, especially in the vegetable garden, can reduce the splashing of pathogens from one plant to the next during a precipitation event. Second, it increases air flow through the plant, which can reduce the likelihood of pathogen spores that might float in and land on foliage. Third, it reduces humidity in the immediate microclimate around the plant. The increased air flow in addition to the reduced amount of foliage that is releasing water through transpiration can have a significant effect on the humidity, which can have a big effect on the germination, establishment, and survival.
Utilize diverse planting plans in the vegetable garden and the landscape. Research shows that while having a variety of plants increases the diversity of disease organisms, it actually reduces the infection rate possibly because pathogens splashing from plant to plant are less likely to find a host plant if they are surrounded by non-host plants. This practice is promoted in intensive vegetable plantings such as square foot gardening.
As stated earlier, precipitation can drastically increase the population of microbes on foliage. This also includes water from overhead irrigation. For example, this study found that overhead watering of cabbage led to significantly higher and faster rates of spread of the black rot fungus as compared to drip irrigation. Therefore, reducing or avoiding overhead watering can reduce the likelihood of disease incidence.
Timing of watering may also contribute to disease development. The dew point, which usually happens during the night time hours, is when the air is totally saturated at 100% relative humidity and therefore cannot hold any more water. This is the point where excess moisture is deposited as dew on surfaces (another source of water on the foliage) and little to no evaporation of water already on surfaces happens (learn more at weather.gov). As shared in this book chapter review, lower temperatures resulting in reaching the dew point can extend the time leaves are exposed to high moisture and result in higher disease incidence.
As our own GP Linda Chalker-Scott points out in this review, mulching not only retains soil moisture, reduces erosion and more but also reduces the incidence of disease in plants by reducing the splashing of soil or spores from rain or irrigation onto the plant. This drastically reduces disease spread from pathogens found in the soil or on plant debris. The organic matter from organic mulches also has the benefit of increasing the population of beneficial microbes, which out-compete the pathogenic microbes.
Crop rotation, where crops are not grown in the same soil or plot for a number of years, also reduces disease incidence by reducing pathogen loads in the soil or from crop residues left in the garden. This study shows significantly reduced disease incidence on potato and onion when a crop rotation plan of four years is utilized (meaning that either onions or potatoes are not planted in the plot for a minimum of four years, with other crops planted between those years).
If root rots and pathogens are a problem, try improving drainage around the garden. Adding organic matter can help with water permeability of the soil over time. Raised beds can also drain faster than in-ground gardens.
Of course, if you’re having lots of problems with certain diseases on your plants, these cultural controls may not be enough. Finding resistant varieties may be a necessary step in breaking the disease cycle in your garden.
While water is required for plant growth, it can cause issues with plant diseases if there is too much or if it lingers on the wrong parts of the plant for too long. Water from rainfall, irrigation, high humidity, fog, and dew can all lead to the initiation, development, and longevity of plant fungal or bacterial diseases. Reducing the amount, persistence of water or humidity on or around foliage can significantly reduce the likelihood of plant disease incidence. Methods such as reducing overhead irrigation, timing of irrigation, mulching, and crop rotation are key cultural methods in reducing diseases spread by water.
Insects and pathogens cause damage and disease in garden plants, but damage can also occur in absence of pests. We refer to these diseases as abiotic disorders. Plant pathologists consider abiotic disorders diseases because plants develop symptoms that reflect the changes in their physiology over time. Unlike outbreaks caused by insects or pathogens, abiotic disorders do not cause epidemics or as plant pathologists say “epiphytotics” because abiotic disorders do not spread the way insects and pathogens can. Like all diseases, abiotic disorders are a perturbation of plant physiology that show up as different or “not normal” appearance. Symptoms typically define most abiotic disorders since signs (of the actual thing causing the disorder) are not usually visible.
Since abiotic disorders do not require an organism to begin or complete a life history, they can occur at any time and are often of sudden onset. The reverse can also be true, depending on the agent causing disease symptoms which may not show for years in some disorders. Abiotic disorders are often associated with the degree to which a plant is adapted to its environment. Adaptation and establishment in an environment are different. New plantings (those not yet established) do not tolerate abiotic extremes as well as established plants. Plants poorly adapted to the climate, soils or water of a region may be prone to abiotic conditions while plants adapted to their planting site thrive among the same abiotic factors.
Plants require mineral nutrients (which arrive in the sap flow after intake by roots) from soil solutions. While carbon, hydrogen and oxygen come from air and water, virtually all the other elements plants need for their growth and physiology come through the root system. Minerals are dissolved in water as ions and are available at various pH levels depending on their solubility characteristics. In general, under alkaline conditions many minerals are held in soil as insoluble precipitates and are unavailable; under acid soil conditions some elements again become insoluble and many leach away from the root zone causing soils to become depleted, especially of metals. Since roots take up minerals as ions (charged molecules) roots must be alive to regulate osmotic potential and the charge balance of ions entering and leaving roots. Anything that compromises root function can lead to inability to take up nutrients and eventually symptoms of nutrient deficiency. Compaction, flooding, root injury, poor soil food web conditions, and pathogens can all impair root function. Plants can show nutrient deficiencies for the following reasons: • The minerals are missing from the soil • The pH is not favorable for absorption of the nutrient which is insoluble • The roots are not able to function and absorb nutrients • Lack of mycorrhizae in soil or poor conditions for microbial growth
A soils diagnostic lab can help identify soil conditions and nutrient content of your samples, and suggest methods to provide optimum plant nutrition. Several soil samples should be taken all through the areas where affected plants grow, combined and sent to the lab soon after collection. Soil pH is like blood pressure – you can’t tell when it is too high or too low – so you need to have it tested. Knowing the soil reaction (pH) is the first step in investigating nutrient issues. Mulched plantings (with coarse tree trimmings chips) usually have few deficiencies in a wide range of soil conditions because nutrients are slowly but constantly provided and beneficial microbes can assist roots in nutrient uptake.
Temperature extremes cause injury and may cause abiotic disease to landscape trees. As the climate continues to warm, extreme hot weather is increasingly likely. In California, we had record all time high temperatures in the last three consecutive years and this year in the Pacific Northwest. In some cases these temperatures were damaging to native tree species, suggesting that they are no longer adapted to the new normal temperature extremes. Years of record breaking freezing temperatures have declined, although cold temperatures can harm sensitive species if freezing occurs suddenly or for prolonged periods. Sunburn comes when temperatures exceed the ability of bark and leaves to adequately cool the tissue. Burned leaves fall from the tree and bark often splits, cracks and dies. This damage can become an important entry point for fungal pathogens such as Botryosphaeria spp. that cause canker diseases in most landscape trees. Planning for a warmer climate means selecting trees that can tolerate higher peak temperatures in summer while surviving the low temperatures of winter.
Another air pollutant is sulfur dioxide, which reacts in the atmosphere to form sulfuric acid, and may reduce the pH of surface waters. Acid deposition due to SO2 (the precursor) is an eastern US problem and often tied to coal use for electrical generation. Air pollution damage to plants depends on the specific air pollutant, its concentration, and the sensitivity of the plant species, with ozone the air pollutant in California having the greatest effect on plants.
Air pollutants originate from a variety of both natural and anthropogenic sources. Some, called primary pollutants, are released directly into the atmosphere. Others, called secondary pollutants, are formed via atmospheric reactions of precursors. Some air pollutants are both primary and secondary. Ozone is a principal air pollutant in California which also affects plants. It is formed in the lower atmosphere when volatile organic compounds (VOC), i.e., short-chain carbon-containing compounds, which are released from a variety of anthropogenic as well as natural sources, react in the presence of sunlight with oxides of nitrogen (NOx) which come from internal combustion engines.
Ozone is toxic to plant cells because it is very reactive and quickly binds to plant tissues causing damage. Note that ozone in the stratosphere is necessary and protective of life. It is the same molecule but has a different chemistry of formation. In urban areas, such as the Los Angeles Basin, pollutants may be held in the lower atmosphere by topography and meteorology, and ozone levels may exceed federal standards for air quality, although much progress has been made since the 1960s . Conifers are particularly sensitive to ozone. Needle retention is reduced and the trees thin and appear yellowed.
While all plant tissues are susceptible to abiotic disorders, stems are most resistant, while leaves, shoots and young roots are perhaps most at risk of environmental factors that cause these disorders. Like biotic diseases, plants with abiotic disorders may require time to develop symptoms. There is a progression from slight to severe symptoms depending on the intensity and duration of the environmental factor causing the disorder. Below are some of the most common causes of abiotic disorders.
Costello, L., Perry, EJ, Matheny, NP, Henry, MJ, and PM Geisel. 2003. Abiotic Disorders of landscape plants a diagnostic guide. ANR publication 3420 University of California, Communication Services, Oakland CA.
Manion, P. 1981. Tree Disease Concepts. Prentice-Hall Inc., 399pp.
Schumann, G.L. and C.J. D’Arcy. 2010. Essential Plant pathology. 2nd ed. APS Press The American Phytopathological Society. St. Paul, MN. 369pp
How do you plan your work in your garden? One of the things that is most likely to affect what you do is rainfall. But how do you know when and how much rain is likely to fall? One way to get an idea of the possibility of rain is to look at something called “Probability of Precipitation”, or as we call it, “PoP”. How often have you heard someone say that the weatherman (or woman) was wrong because they predicted 30 percent chance of rain and they did not get anything? Or someone else says there was only a 10 percent chance of rain and they got flooded? If you understand how these forecasts are made, it might help you plan your outdoor activities, including your garden work and when you water.
How is “PoP” defined?
According to the National Weather Service (NWS):
PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measurable precipitation, if it occurs at all. The forecast is what we call a “conditional” forecast—that means it depends on two different things, one of which requires the other to occur. It’s important to keep in mind that these forecasts are made for a particular period of time (often 12 hours) and for a particular area (the forecast zone). The first part of the calculation is whether or not it will rain at all anywhere in the forecast zone during the time that the forecast covers. The second is how much of the forecast zone will be hit by precipitation sometime during the forecast period.
How likely is it that precipitation will occur?
The first part of the equation above, “C”, is whether rain will occur or not in the forecast zone during the forecast period. Sometimes that is easy to determine if a big high-pressure center is over the area and no rain is expected anywhere in the region. That means that the first part of the equation is zero, and so the PoP forecast would be zero. If a strong front is moving through your area or a tropical storm is headed your way, the probability of rain somewhere in the area is probably close to 100%. But often, the likelihood of rain is not so clear. What if you are not sure about the timing of the front or the tropical storm? If it moves slower than expected, it might not make it to the forecast area before the clock ends for that forecast period. Or if you are not sure the conditions are going to be right for a rain shower to occur, then you might or not get precipitation, depending on the actual conditions. Then C becomes something between 0 percent and 100 percent, depending on how much you trust the computer models that produce the forecasts.
How much area will be covered by the storms if it does rain?
As I am writing this, it is raining at my house just southeast of Athens, GA (you can see the tiny yellow splotch on the radar map above). Sometimes it is clear that rain will cover the entire forecast area during the forecast period. But most of the time, we think it will rain in parts of the forecast area, but it will be “hit or miss” rainfall from discrete storms, not widespread coverage. The second part of the equation, A, is the forecaster’s estimate of how much of the area will be hit by rain sometime during the forecast period. It’s not as easy as you think, because those storms are moving, and they cover more of the region than you might expect. In my research I have found that often the PoP forecast is too low, and that rain as estimated by radar covers a wider area than you might expect based on the forecast. Fortunately, the NWS does provide radar estimates of precipitation that are calibrated by actual ground-truth rainfall data. The map below shows the 24-hour rainfall ending on the morning of June 27, 2021, including the rain you see in the maps above. For my CoCoRaHS rain gauge, the 0.05 inches I got correspond quite well to the light blue on the map.
Where do you get PoP forecasts?
So where do you get PoP forecasts and how do you use them?
Most meteorologists provide PoP forecasts on their broadcasts or written
forecasts. I tend to use the similar Precipitation Potential from the NWS
hourly forecasts because they go six days out, which allows a longer planning
horizon. You can see a simplified example of a forecast graph below. The “Rain”
categories of Slight Chance, Chance, Likely, and Occasional correspond to Precipitation
Potentials of roughly 5-25%, 25-50%, 50-75%, and 75-100%. The amount of rain in
inches is shown superimposed on the bars, so you get an estimate of how wet it
will be in that time period. You can find instructions for how to get your own
hourly forecasts at my
The hourly graph allows me to plan well ahead of whatever
outdoor activity I am doing. The forecasts show hour by hour how likely
precipitation is, including the chance of thunderstorms, and an estimate of how
much rain will occur. Keep in mind that the forecasts six days out are not
likely to be as accurate as the ones for tomorrow, but they are still useful
for planning purposes. Using this information tells me when and how much rain
How do you use PoP forecasts in your planning?
These forecasts are especially useful for determining when to irrigate or apply insecticides or fertilizers that require specific wet or dry conditions to work properly. If you can see rain will be starting soon, you might choose not to water your garden unless the rain amounts are likely to be small or the chance low. Or maybe you want to mow your lawn now before it gets wet. If you are planning to fertilize and if it needs to be watered in, now might be a great time! If you need to apply a treatment that requires wet leaves to be effective, then you might wait until after the rain is over rather than applying now and seeing the chemical wash away in the storm. These precipitation forecasts can help you make the best use of your time by providing targeted, timely information on when rain will occur and how much is likely to fall when it does.
The movie “Field of Dreams” is a family favorite – we love how baseball and the supernatural are interwoven to create a great story. If you haven’t seen the movie, you should – and for those of you that have, you know why it was important for Ray to build the baseball field. Like the magic that unfolded once that physical space was provided, botanical magic emerges from garden soils that support mycorrhizal life. Garden product peddlers have taken advantage of the scientifically-established relationship between plants and mycorrhizal fungi by selling inoculants. And gardeners tend to focus on which of the many brands of inoculants to buy, rather on questioning their efficacy.
I’ve attached a link to my peer-reviewed fact sheet on mycorrhizae for a more in-depth discussion about this symbiotic relationship, but the bottom line is this: inoculants don’t work. To understand why, we need to consider a modified version of the disease triangle. Many gardeners are familiar with this concept, which depicts the three criteria needed for plant disease to manifest: the presence of the pathogen, the presence of a host plant, and environmental conditions conducive to pathogen growth. Pathogen spores are EVERYWHERE in landscape and garden soils – they just aren’t activated unless their host is present and environmental conditions allow their germination. Likewise, mycorrhizal spores are EVERYWHERE in landscape and garden soils. We can make a mycorrhizal triangle to visualize the three criteria for needed for mycorrhizae to develop.
While our understanding of mycorrhizal relationships continues
to expand, we do know some of the environmental factors needed for successful
Soil oxygen. Mycorrhizal fungi are aerobes,
meaning they are active when sufficient oxygen is present.
Woody debris on the soil surface. Mycorrhizal
species are also decomposers of woody material. There is increasing evidence
that a natural woody mulch (not sawdust, not bark) is required for mycorrhizal
establishment. Fungal hyphae colonize the debris, extract nutrients, and
transport them to their host’s roots. Arborist wood chips are an ideal mulch in
this regard as they absorb water and provide an ideal substrate for hyphal
There is a robust body of peer-reviewed research conclusively demonstrating that commercial inoculants applied to plants in landscaped soils have no substantial effect on the development of mycorrhizae. This lack of efficacy has induced some inoculant manufacturers to add fertilizer, especially nitrogen, to increase plant growth and fool consumers into thinking the inoculant was responsible.
The image on the left is the label from a mycorrhizal inoculant. Close inspection (middle image) reveals addition of a fertilizer, which is identical in NPK content to a fish fertilizer (right image).
And here is the lesson “Field of Dreams” provides: if you build it, they will come. Build a healthy soil by mulching with a thick layer of arborist wood chips. Not only do they provide nutrients and absorb water, but their presence reduces soil compaction and increases aeration. You can be assured your plants will be successfully inoculated with your soil’s native mycorrhizal species.
There’s all kinds of maladies that can strike your garden plants throughout the season- diseases, insects, negligence, and more. But one common issue we are seeing more and more here in the corn belt and other places with lots of crop production is herbicide drift. Of course, you don’t have to have a corn or soy field nearby to have issues with drift – it can happen anywhere and anytime an herbicide is applied and proper precautions aren’t taken, even when you or a neighbor are just treating a small area in the yard. There are other avenues of herbicide damage on plants as well, such as using herbicide-treated grass clippings as mulch in the garden.
A wide variety of plants can be damaged by herbicide drift from a variety of different products – trees, shrubs, roses, vegetables, and more. The damage can be slight to severe, and unless the dose is large most plants will grow out of the damage. Vegetables and fruits, though, are of particular concern due to the potential food safety risk from residues of unknown herbicides on the plants. Therefore, it is especially important to be able to identify signs of herbicide drift and take the appropriate course of action which is usually and unfortunately removal of the plant from the garden.
I have to remove the plants!?!?
Yes, you read correctly, I said removal of the plant! I, along with many of my extension colleagues, encourage gardeners who have drift or herbicide damage on their plants to remove them from their gardens. Why take such a drastic measure, especially if the plant may actually recover and “grow out” of the damage? The answer is mainly one of safety. Since it is likely impossible to know exactly which chemical or product formulation was used there’s no way of knowing if the product is safe to use on consumable crops, whether its residue is safe, or whether it is systemic and has a residual effect. A gardener cannot know if there is a pre-harvest interval where the crop will be safe after a certain passage of time or if it will never be safe. And even if you do know the product (let’s say you were the one that used it or you know what is being used by the neighbors) it is likely that there won’t be safety information for use on fruit and vegetable crops, since we don’t typically apply herbicides to plants we want to keep growing. You should also remember that application of such herbicides to fruit and vegetable crops, even if accidental, technically constitutes an off-label (and illegal) application of an herbicide to a non-target crop or pest.
What are the most likely fruit and vegetable plants to be
damaged from herbicide drift?
While just about any plant can be damaged by herbicide drift if enough herbicide gets on the plant, there are a few plants that seem to be more susceptible to herbicide drift. This means that these plants exhibit damage with smaller doses of herbicides than others and will show damage while other plants nearby remain unfazed. The plant that we get the most calls about are tomatoes. This is the vegetable garden crop that is the most susceptible to herbicide drift and just so happens to be the most widely planted crop in the garden. The other edible crop that seems to be highly susceptible to herbicide drift is grape. While grapes aren’t nearly as common as tomatoes in home gardens, wineries in regions with high herbicide use rates are struggling to keep their vineyards going due to the damage.
I live nowhere near a big farm, how do I keep getting drift
Of course, drift can come from anywhere, even a small application of herbicide on a neighborhood lawn or garden. But under the right weather conditions (high temps and wind) some herbicides like dicamba can volatilize and drift for 2-3 miles or more. Even if you think you live nowhere near a farm or other area where herbicides might be used you can get drift from miles away. This makes it hard to pinpoint where the damage is coming from in order to sleuth out what exactly was used. This is especially tricky here in our area where the city of Omaha is surrounded on all sides by farmland, and even has pockets of productions fields sandwiched between residential areas. Unfortunately, one of the prime herbicide application times in our region is shortly after most gardeners plant their tomatoes so we get lots of calls and questions that end up being drift. Thankfully there’s usually still time to replant tomatoes, but it isn’t fun telling people that started plants of their favorite or special varieties that they’ll have to rip them out and go buy new plants.
The kicker is that drift can be random. It can be one or two plants out of a bed of
twenty, or one plant on one side of the garden and another somewhere else, or
an entire field full of plants. It just
really depends on the wind patterns and concentration of herbicide.
Is it drift? Or is
it something else?
At first glance it can be hard to tell if an issue is drift or something else since the signs can look like some other problem until you get up close. There are a wide variety of herbicides on the market and therefore there can be lots of different signs. The most common types of damage you’ll see are light/white colored and necrotic spots from exposure to broad-spectrum herbicides like glyphosate, and curling, twisting, stunting, yellowing, and epinasty from broadleaf herbicides like 2,4-D and dicamba. Epinasty is an unusual, twisting growth pattern on the leaves that result when one layer of the leaf (usually the upper layer) grows faster than the other. You can get weird strappy looking leaves, weird margins, and other irregular growth patterns. The damage from broadleaf herbicides can sometimes be mistaken for heat or drought damage, viral diseases, or even excess watering, all of which cause leaf curling of some sort. I’ll share a few tomato pictures below to demonstrate herbicide damage vs other types of leaf curling. For a great pictorial guide to herbicide damage symptoms, check out this resource form the University of Tennessee.
Can you avoid drift?
Unfortunately, you can only control drift from the herbicides you apply yourself. Pesticides such as herbicides can be used safely and effectively if used appropriately. Reading and following the label instruction is important and is the law, paying special attention to wind speed, temperature, and application equipment, e.g., how fine of a mist does the nozzle create. Drift from the neighbors’ lawn treatment or a nearby farm is really outside of your control, so being watchful for signs of drift is important. Sheltering susceptible crops, like tomatoes, using something as a windbreak might be helpful. As this journal article points out, a windbreak or vegetative buffer around wetlands offers some protection and I noticed a similar effect recently in one of our Master Gardener project gardens. Our Master Gardeners grow thousands of pounds of produce a year for local food banks, and on a recent visit I noticed that about 25 percent of their tomato plants were showing signs of drift (and they were removed and replaced). The pattern was interesting – the only plants damaged were the ones on the outside edge of the garden and the ones along wide walkways in the garden. But plants in the interior were spared. So perhaps planting less susceptible crops on the exterior of the garden and along walkways to act as buffers might work.
And while it isn’t useful for home gardeners, specialty crop
producers (like those all-important wineries) and beekeepers can register for a
program called DriftWatch where they can
be informed when spraying will take place on local farms.