Rain makes diseases feel right at home, and we are clearly seeing that in soybeans this week. Across the central part of the state, especially, Phytophthora has reared its head. Nancy Pataky, director of the University of Illinois Plant Clinic, reports that several recent samples have tested positive for the "fungus-like organism" that causes the root rot Phytophthora sojae.
Yes, I said, "fungus-like organism." I suppose the acronym for that would be FLO. The fungi (former) that we have always referred to as the water molds, such as Pythium sp. and Phytophthora sp., have been reclassified by plant pathologists into the new "fungus-like organism" classification category. For those aficionados of plant pathology terminology, you may recall that for nearly 30 years plant pathologists had a class of disease-causing organisms that they referred to as "mycoplasma-like organisms," or MLOs. That class of organisms was then, without the fanfare you would expect after 30 years, named "phytoplasmas." This is fascinating information for taxonomists; the rest of the population probably isn't quite as thrilled by it. However, I want you to be aware that, regardless of how a taxonomist may classify Phytophthora sp. or whether it is a fungus or just fungus-like, it is still the same pathogen.
This disease is a tricky one. It doesn't have a limited season of infection like many of our key pathogens. Phytophthora sojae can attack and kill soybeans throughout the season. What we are seeing now is the phase of the disease that attacks mature plants. Typically, an infection of a mature or nearly mature plant is expressed as wilting and death of the top of the plant, significantly rotted roots, and a diagnostic dark brown or black canker extending from the soil line up the stem. The symptoms so far this season are may-be just a bit different from what you are used to seeing, though. The canker seems to be missing, and the roots aren't quite as badly rotted as one would expect on the affected plants we've seen so far this season. Additionally, foliar symptoms on the lower leaves of these affected plants look typical of very early SDS symptom expression. Odd. Now it could be that the plants were caught early in Phytophthora symptom development, and, additionally, the unfortunate plants could potentially have early SDS; but the symptoms are remarkable, and we need to keep an eye open for this somewhat nontypical symptom development when scouting.
Wilting of Phytophthora sojae–infected soybean and unusual foliar symptomatology. (Photo courtesy of Stephanie Satterlee.)
Management of Phytophthora relies on several factors, as Dean Malvik pointed out in issue no. 12 (June 13, 2003) of the Bulletin. For future seasons, drainage needs to be addressed in problem fields. But our primary method of managing the disease is with use of resistant varieties. Soybean varieties with specific resistance to Phytophthora should be selected to grow. The major resistance genes Rps1c or Rps1k should be effective in most fields. However, Dean noted that there are resistance genes no longer effective in some parts of Illinois. Races (pathotypes) of Phytophthora occur in some parts of Illinois that kill soybeans with Rps1a, Rps1c, and Rps1k. Many of the Phytophthora isolates from Illinois soybean fields can defeat Rps1a, and a smaller number can defeat Rps1c. A smaller number can defeat all of the common resistance genes (Rps1a, Rps1c, and Rps1k) available in commercial varieties for Illinois. So far, research shows that the aggressive isolates are causing damage but do not seem to be widespread in Illinois.--Suzanne Bissonnette