As Dr. Scott Stewart pointed out in a previous article, “Cold Weather Effects on Insect Populations?”, the warm environment in the South promotes more insects, similarly the warm and humid conditions of the South also contribute to the amount of plant diseases. Slightly different from the insect side of things, the majority of diseases aren’t affected by cold winters, although there are some exceptions. Most pathogens that cause disease overwinter in crop residue either on the soil surface or in the soil and some can develop specialized ‘survival’ structures such as sclerotia (hard masses of mycelium) that can lay dormant in the soil for years. For example, Cercospora sojina (cause of frogeye leaf spot in soybean) and Cercospora zeae-maydis (cause of gray leaf spot in corn) overwinter in crop debris as mycelium, Macrophomina phaseolina (cause of charcoal rot in soybean and corn) can survive 2 or more years as microsclerotia in soil or host residue; and some seedling disease pathogens (i.e. Rhizoctonia, Fusarium, Pythium) are able to live in the soil indefinitely as saprophytes. These pathogens and the diseases they cause will not be significantly impacted by the cold weather.

Unlike the pathogens described above, rust pathogens can be affected by cold weather. Rust pathogens are obligate parasites which mean they have to survive on living tissue. This limits rust pathogens to southern areas of North America each year due to winter temperatures and this year is no exception.  Rust diseases in field crops include, stripe rust and leaf rust of wheat, southern corn rust and common corn rust, and soybean rust. While rust pathogens have most likely taken a hit this winter, they are among the most destructive plant diseases. Under the right conditions, rust diseases can explode in a relatively short time, and spores can blow into new areas quickly given the right wind current or tropical storm.

In summary, the cold winter will not change the need to scout for diseases in field crops in 2014. Current status of rust disease in the U.S. are as follows: active leaf rust of wheat has been found in Baton Rouge, LA (3/19) and Marianna, AR (3/20); and both leaf and stripe rust in Texas (3/4), no reports on corn rust, and soybean rust has only been reported in Puerto Rico and Louisiana, with 15 to 40% viable spores from coastal parishes and 15% from St. Tammany Parish (north of New Orleans) as of Feb. 26, 2014. In 2013, by the end of March soybean rust had survived at low levels as far north as Montgomery, AL and was reported in 8 Parishes in Louisiana, 4 counties in Alabama, and 3 counties in both Georgia and Florida.

Continue to follow the UTcrops blog at news.utcrops.com for status updates of rusts.