Sunday, June 8, 2014

Did the Previous Cold Winter Hinder Crop Disease?

Recently, there have been many articles addressing how the winter will affect populations of overwintering insect pests, but how did the recent cold winter affect the incidence of crop disease (the fungus among us)? It, of course, will depend on the type of fungus, but typically most fungi require some basic items: respiration, nutrients (nitrogen or other minerals), water, light (but not in most cases), and specific temperatures. 

In general, fungi can tolerate the range of temperatures that typically occur in the place where they have taken up residency. So, fungal disease pathogens that have made a home in the Midwest, generally have developed the ability to tolerate our cold, winter temperatures. Fungi have figured out several ways to survive during cold temperatures, and many produce special survival structures that are thick walled, which can survive extended cold periods as well as extremely dry conditions, within residue or in the soil. After the winter, the fungal pathogens wait until just the right temperature to become active. Plant pathologists will always refer to the plant disease triangle when it comes to plant disease infection on your crops.

This picture is courtesy of

The Disease Triangle: a plant pathological paradigm revisited

A certain environment is needed for a plant disease to become active. Most plant pathogens that cause disease require an even more narrow range of specific temperatures, before they will sporulate (produce spores) or initiate infection. Fungal spores, of course, are one of the main ways that fungal pathogens spread among crops. The bad news is just because fungal pathogens are not able to produce spores or infect your plants at a particular time, because of unfavorable conditions, does not mean they will not be a problem in the future. Spring and summer bring warmer temperatures for fungi to become active and produce spores once again. Many of our plant disease pathogens are lurking on diseased plant material or trash on the soil surface in a dormant state, just waiting to get the chance to attack your crops.
Don't forget that some fungal disease pathogens do NOT overwinter here in Illinois. Rust spores blow up from the South and under the right conditions, can infect plant hosts during our growing season. Some examples of rust diseases that infect corn are common rust and southern rust. Soybean rust can be an issue on soybeans, but, fortunately, this disease has not been a yield reducer in the Midwest. Soybean rust has infected soybeans in the Midwest, but historically has been found late in the season, which is not cause for alarm at this time. 

Up close picture of soybean rust (Phakopsora pachyrhizi)  pustules on a soybean leaf. Picture taken by Mike Meyer.
Most rust disease pathogens require a living plant for survival, and these are more abundant in the South during the winter. The good news is that a severe winter in the South, where rusts overwinter, could decrease the amount of spores that are blown into "our neck of the woods". This is because the colder winter temperatures during last winter in the South most likely decreased the amount of living plant material, where these rusts are overwintering, which in turn, causes the demise of the rust pathogens. A Tweet on Twitter stated that plant pathologists report “little to no soybean rust overwintering on kudzu,which is good news for soybean growers!” Reduced amounts of corn and soybean rust disease in the South will most likely mean less rust spores will make their way North. However, it is hard to predict the future, and rust disease scouting should continue throughout the growing season. Track the movement of both Southern rust on corn and soybean rust as they infect and spread on crops in the Southern US to the Northern US at the IpmPIPE website:
Southern rust on corn:
Soybean rust:

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