Understanding mysteries of plant diseases

(Post 1 of 3 in this blog series)

Gardeners, especially those new to gardening may find they have a “black thumb.” Plants die for no reason! “Oh well chuck it in the greenwaste recycling can and start again.” Or… “Oh I can’t grow cyclamens!… They always die in my garden for some reason.” For many gardeners it is mysterious why some plants fail to thrive or die suddenly. Plant disease processes are complicated, and it requires some knowledge of botany (anatomy and physiology), genetics, and microbiology to really understand what is happening. Also, since microbes are microscopic and most pathogens are microbial we can’t always see them at work, especially before symptoms develop. Symptoms are plant responses to the action of a disease agent. In this post I will try to describe the different kinds of diseases, and where they come from.

There are two broad categories …
of plant disease possible in gardens: biotic  diseases and abiotic diseases. Biotic diseases have a disease agent called a pathogen. The pathogen can be microbial, or a nematode or a virus, or a parasitic seed plant. Bacteria and fungi are the most common microbes. It is debatable weather viral particles are living, so also debatable weather or not they are considered microbes. Of the biotic pathogens, fungi cause  most diseases in gardens.  Many pathogens rely on environmental conditions to favor their lifestyle, this is particularly true of bacteria which like moist, warm environments.

The other category of disease is the abiotic category. Abiotic diseases have no pathogen. An environmental condition such as an excess or lack of an environmental condition causes physiological changes in plants that develop symptoms. Extremes of temperature, light, humidity, soil or water chemistry, soil physical conditions, air quality, and pesticide residue can all lead to abiotic diseases. Since there is no pathogen there is no epidemic, and abiotic diseases are not infectious. So spread, occurrence and movement of abiotic diseases are usually different than biotic disorders

So how does disease happen?
I have heard many gardeners make sweeping statements like “overwatering killed my plant” or “It just died of neglect” or “insects killed it”. Plant pathologists describe the disease process with a cartoon called the disease tetrahedron. It describes the interaction of four things: the pathogen, the environment, the host and time. Of course it is only a triangle for abiotic diseases since there is no pathogen.

For disease to occur there must be an active pathogen present that is virulent (has genes to cause disease). The pathogen must have enough inoculum present to begin the disease process. A single spore rarely leads to a successful disease (although it can in some systems).  Most importantly the pathogen must have the right genetics to recognize its host.

Next the environment must be conducive to the pathogen and its development and/or  harmful or stressful to the host. The environment can cause the host stress while favoring the pathogen.  An example would be oxygen starvation in flooded roots. The environment must favor the pathogen’s build up and dispersal of its inoculum (infective propagules such as spores, cells or seeds). Often splashing rain during the warming spring period is important for their spores to reach a susceptible host.

Finally for disease to happen, the host must be susceptible to the pathogen and possibly predisposed in some way to its attack.  Pathogens also have phenotypic synchronicity, that is the ability to produce inoculum at the same time as the host is producing susceptible plant tissues (leaves, buds or stems).

The final facet of the tetrahedron is time. Diseases do not occur instantaneously (even though we may only notice them instantly) – it takes time for them to develop.  Disease life cycles or life histories describe how pathogens survive, reproduce and disseminate themselves through the environment over time. The tetrahedron can be used to understand the factors that lead to disease but also can be used as a way to stop or control diseases (more on that in another post).

So where do diseases come from and where are they going?
Abiotic diseases are caused by environmental extremes.  Another way to look at them is that they occur when there is a violation of the adaptations of the host.  In this regard when we grow plants not well adapted to our climate or environment they can be harmed. A good example is my papaya tree. Right now it has been harmed by low temperatures. Growing a papaya in Ojai, CA is a violation of its adaptations.

Jim’s papaya tree is intolerant of Ojai winter temperatures–a violation of its adpatatons

If abiotic factors don’t cause actual symptoms, sometimes they are able to weaken the host so that a pathogen can enter, and begin disease formation.  So abiotic conditions are often predisposing factors for the development of biotic pathogens. Many of the root rot pathogens such as Phytophthora or Armillaria

Wet soils and fine texture (clay) predispose trees to root rot. Note the “root snorkels’ did not prevent the problem

require a predisposing abiotic factor such as drought, saturated soils, high salinity or compaction to facilitate disease development.

Many canker diseases such as those caused by Botryosphaeria are predisposed by drought conditions

So where do pathogens come from?

I like the hospital analogy. Where do you go to get sick? A hospital! They certainly have a difficult time controlling the spread of disease there because that is where sick people go. So where do sick plants come from? Often a nursery!  Nurseries import plants from wholesale sources, propagate from their own stock, sometimes reuse their container media, and grow many hosts in a concentrated place over time. There is no better place for diseases to occur than in nurseries.

This is especially true of root diseases because roots are inside the container and often not observed at the time of purchase

Inspect nursery stock for healthy roots before purchase

(but you always should inspect roots of all purchased plants from six packs of garden flowers to boxed trees). Also, some nurseries suppress diseases with fungicides that do not eradicate the pathogen, so when fungicides wear off (after you purchase your plant), disease can develop from now unsuppressed pathogens. Nurserymen relax! I’m not saying that all nurseries sell diseased plants (at least knowingly), but consumers should take extra care when selecting plants and when bringing new plants to their property.

While landscape mulches don’t likely spread viable pathogens they can change soil moisture status enhancing collar rots if irrigation is not adjusted. This tree was also planted deeply, another predisposing factor for disease

Once pathogens establish in the landscape, they may continue to harm new plants. Some pathogenic spores blow in on wind or inoculum moves in water courses along streams or other water paths. Animals, people and equipment can move infested soil onto a property.   Once diseases have run their course, pathogens often survive as saprophytes in the diseased tissues. They overwinter or over-summer in debris on the ground. So sanitation is critical in disease control (more on this in another post). Fruiting bodies can be moved in the greenwaste stream but there is very little research showing that disease is initiated by contaminated greenwaste, even though some pathogens may survive there. We do know that when greenwaste is chipped, it dramatically reduces pathogen and insect survival. Stockpiling wastes for as little as seven days will reduce chances pathogen survival by an order of magnitude. Certainly our favored arborist chips are very unlikely to have viable pathogens especially when sourced locally.

Understanding that diseases are not usually caused by gardening practices but by a pathogen or an environmental factor is the first step in diagnosis and control. In my next post I will talk about disease diagnosis and detection…

 

Tuning up for Pruning Up–Care, Maintenance and Utilization of Hand Pruning Tools

A. J. Downer

Fall is passing into winter and the bare sticks in my deciduous fruit orchard are calling to my annual fruit tree pruning chores.  I can prune my entire orchard with very few tools: a good pair of bypass clippers, a similar set of loppers

Illustration 1. Tri-edge saw blades are made from stainless steel and are not easily sharpened. When dull or bent they should be replaced.

(optional) and a high quality “razor” or “tri edge” saw.  Most hand tools require some maintenance especially the clippers and loppers.   Clippers are easily sharpened but modern saw blades can not be sharpened by gardeners. I usually just buy a new saw, replacing the old one when blade eventually dulls or bends from over zealous use (illustration 1).

Illustration 2. To sharpen bypass clipper blades follow the angle of the bevel. Do not sharpen the flat side of the blade

Before using your pruning tools inspect them for signs of damage. Blades should be sharp and straight.  Loppers should have their rubber “bumpers” intact otherwise your knuckles will be smashed after exerting force on a difficult branch.  Sharp tools offer less resistance and actually decrease injury to users. One exception here is with the modern “tri-edge” or “razor” saws. These saws can cut so quickly that you may pass through the branch you are cutting and continue on to some part of your anatomy quickly ripping your flesh open. I have suffered more cuts (some serious) from these saws than from any other gardening activity (although I was recently impaled by a frog metal art sculpture!).  They should be used with careful precision, not with the wild abandon and pruning fervor of the craven academic desperate for real world pruning experiences.  A thick long sleeved shirt and gloves will also help prevent cuts from hand pruning equipment.

Bypass clippers are so termed because the blade passes by the hook. To sharpen these, find the bevel on the edge of the clippers and align a small file to the same angle of this bevel, and file the bevel until you can feel the sharpness with your finger (Illustration 2).  Never sharpen the back side of the bevel—this will create a gap, and every time you cut, a flap of tissue will remain. Back bevel sharpened clippers will require blade replacement or grinding until the back bevel is gone. The hook does not require sharpening, do not attempt to file it. Repeat this process with lopper blades.

When you are done pruning for the day, wipe the blades of your clippers and loppers with an oil soaked rag or apply a few drops of oil and rub it into the blade. Most modern saws blades are made from stainless steel and require no oil protection.

As a Cooperative Extension Advisor, one of the most common questions I receive is: “Should I sanitize my clippers between cuts or between uses on various plants?”.  Indeed, many publications, extension leaflets, gardening columns, and other sources make broad recommendations to sanitize clippers after every cut. Some articles even compare various products for their killing efficacy.  Blind recommendations are often made to sanitize clippers when the pathogen is not  known or specified.  It is not necessary to sanitize your clippers when pruning most garden plants and fruit trees.  There are a few pathogens that are spread by moving plant debris, but published evidence that they are spread by hand pruning equipment (especially clippers) is nil. One exception is palm wilt caused by Fusarium oxysporum f.sp. canariensis which is easily spread by saws. Some of the canker fungi caused by Botryosphaeria can also be spread by pruning equipment. With many of these pathogens, a wound is required for infection so it may not be that the clippers are spreading disease so much as providing an entry point (infection court) so that pathogens have a way to enter.

If diseases are present in or near the plant already, sterilizing pruning equipment will simply provide a clean entry port for the pathogen—infection can still follow after the cut is made with a sanitized tool.

In my garden, I never sanitize clippers between cuts.  However, conditions vary across the US, and in some places rain, humidity, and temperature are more favorable for disease development.  If you have concern about spreading pathogens, prune during the dormant season, when the likelihood of pathogen activity is lowest. Apply dormant sprays containing copper to limit the onset of new fungal diseases that may enter pruning wounds.  If you still feel you need to protect wounds from dirty clippers I like to use the flame from a plumber’s torch to sanitize.  A few seconds along the cutting edge front and back kills all pathogens (Illustration 3).  The process is similar for a saw but efficacy is increased if the saw gullets are wiped clean with a cloth and then the flame applied. The only time I take these measures is when I know I am working with plants that can be inoculated by pruning (which is rare).


Illustration 3. A plumber’s torch will rapidly sanitize saws and blades when pathogens are present in plant tissues.

When pruning garden plants, there are a plethora of recommendations on how to make cuts. Rose experts have extolled the virtues of an angled cut so water runs away quickly, flush cuts used to be recommended by arborists as the highest quality cut. These examples are without research foundation. Cuts on woody plants should made to create the most circular exposure that leaves the smallest surface area possible. We abandoned flush cuts many years back because they cut into protective zones that limit decay in trees. Some gardeners feel compelled to cover their cuts with a pruning paint and there is a similar paucity of research to support this practice. Leave pruning wounds unpainted.