Watering can and vegetable garden

Water: Garden Friend….and Foe? – Water, Relative Humidity, and Plant Diseases

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.

File:Septoria lycopersici malagutii leaf spot on tomato leaf.jpg -  Wikimedia Commons
Septoria leaf spot, a common fungal disease of tomato that requires water for initiation and development.

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 humidity.

  • 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. 
Mulching and drip irrigation can both significantly reduce disease incidence in gardens.
  • 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. 


Aung, K., Jiang, Y., & He, S. Y. (2018). The role of water in plant–microbe interactions. The Plant Journal, 93(4), 771-780.

Burdon, J., & Chilvers, G. A. (1982). Host density as a factor in plant disease ecology. Annual review of phytopathology, 20(1), 143-166.

Café-Filho, A. C., Lopes, C. A., & Rossato, M. (2019). Management of plant disease epidemics with irrigation practices. Irrigation in Agroecosystems, 123.

Chalker-Scott, L. (2007). Impact of mulches on landscape plants and the environment—a review. Journal of Environmental Horticulture25(4), 239-249.

Krauthausen, H. J., Laun, N., & Wohanka, W. (2011). Methods to reduce the spread of the black rot pathogen, Xanthomonas campestris pv. campestris, in brassica transplants. Journal of Plant Diseases and Protection, 118(1), 7-16.

Rottstock, T., Joshi, J., Kummer, V., & Fischer, M. (2014). Higher plant diversity promotes higher diversity of fungal pathogens, while it decreases pathogen infection per plant. Ecology95(7), 1907-1917.

Wright, P. J., Falloon, R. E., & Hedderley, D. (2017). A long-term vegetable crop rotation study to determine effects on soil microbial communities and soilborne diseases of potato and onion. New Zealand Journal of Crop and Horticultural Science, 45(1), 29-54.

Published by

John Porter

John Porter is the Urban Agriculture Program Coordinator/Educator for Nebraska Extension. His specialties are fruit and vegetable production, small space intensive production, plant propagation, and general plant science (botany, physiology, genetics). He has a BS in Botany/Biology from Marshall University and an MS in Horticultural Science from West Virginia University.

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