What Diseases to Expect in a Drought Year?- An Interesting Read

Wednesday, May 17, 2023

Perennia's Horticulturalist and Plant Pathologist, Dustin MacLean, wrote this article: What Diseases to Expect in a Drought Year? This is an interesting read and will give you different aspects of disease infection.

It makes more sense to us when we say a wet season would bring more disease infection, but a dry season could also bring other challenges in terms of plant disease infection.

What Diseases to Expect in a Drought Year?

Dustin MacLean, M.Sc., P.Ag,  Perennia Horticulturalist and Plant Pathologist


In some cases, drought conditions directly impact disease by making the environment more favorable for pathogen infection, disease development and disease spread on a number of crops. In other cases, drought may not directly impact the rate of infection, however, the impact of the disease may be greater due to the plant being weakened from the effects of water stress.

Damage from many of the diseases causing yield loss in dry years is not visible initially, only appearing mid-way through the growing season or later, in some instances. Most importantly, damage from diseases in dry years is often mistaken for water stress. Before applying fungicides for what is believed to be disease in the field and potentially using up what could be a very important fungicide application with a chemical that may have a very limited number of applications, you may consult with one of our crop specialists or send a suspected disease sample to the Plant Health Lab at: Plant Health Lab – Perennia.

Below, we have provided some information on what diseases to look for during a drought or dry year:

Root rots: Some true fungal pathogens (such as Fusarium and Rhizoctonia) do not need much water to cause root rots, and when they do occur in dry years, their impact on yield is often more severe because the crop is already water stressed. While some damage is evident shortly after emergence, much of the damage is not noticeable until mid-way through the growing season. Alternatively, fungus-like root rot pathogens (such as Phytophthora and Pythium) need lots of soil moisture to cause infection and are usually much less severe in dry years. The exception to this rule are diseases that need sufficient moisture for only a very short period of time, such as downy mildew, which only needs wet soils after planting to cause systemic infection that may result in plant death regardless of environmental conditions.

Stem diseases and wilts: Stem diseases caused by soil-borne pathogens (like Fusarium and Rhizoctonia) may be unaffected by drought and are likely to cause high levels of damage and wilting to water stressed plants. Typically, the greatest damage isn’t seen until later in the season, and again, may be mistaken for water stress and premature senescence. However, a number of the most common and devastating stem and wilt diseases (Verticillium and Sclerotinia) are heavily dependent on ample moisture for infection to spread.

Fungal Leaf diseases: The most dramatic shift in diseases that occur in wet and dry years is with regards to leaf diseases. Pathogens causing rusts and powdery mildews need only brief periods of free moisture (such as dew) to infect plants, and are dispersed by wind, without the need for water. Although these diseases may not be more common in drought years per se, the damage they cause to crops experiencing water stress are significantly worse. These diseases have the ability to desiccate a water-stressed plant very rapidly. However, leaf spotting and diseases that require rainfall for infection and spread are far less common in dry years.

Head/flower/fruit diseases: The vast majority of these diseases need water for dissemination and are far less severe in droughts (Monilinia, Anthracnose, Apple Scab, Fusarium Head Blight, Sclerotinia Head Rot, Stemphylium Blight).

Bacterial diseases: Most bacterial diseases are heavily dependent on plant wounding and water splashing for infection and spread, so we would generally expect bacterial diseases to be less prevalent during dry years. While this is generally true, severe thunderstorms can still occur in a dry year, thus allowing bacterial pathogens to infect crop tissue. An example would be Fire Blight of Apple, where trauma blight events may cause rapid infection of trees when major weather events (such as hailstorms or thunderstorms with high winds) cause injury to the plant tissue.

Nematodes: Dry soils may provide a favourable environment for nematodes, as plant parasitic nematodes gravitate towards root tissue while any predatory organisms retreat deeper into the soil in search of water, as well as the plant being weakened, allowing for infection to more easily occur. Additionally, damage caused by nematodes in drought years is often more severe than when water is abundant. Nematodes may also exacerbate the impact from other root and stem diseases by weakening the plant and providing wound openings for other pathogenic organisms to invade. Like many of the root and stem diseases mentioned previously, damage caused by nematodes is often mistaken for water stress and premature senescence and may require the discerning eye of a specialist to properly identify.