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Kentucky soybean update: Rust, no. Sudden death syndrome: Yes
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By Donald Hershman, Plant Pathologist
University of Kentucky
8/9/2005 -- As of August 4, there is no confirmed soybean rust anywhere near Kentucky. However, the University of Kentucky Plant Disease Diagnostic Lab at Princeton has received numerous samples of soybean sudden death syndrome (SDS).
I have also gotten about a dozen phone calls recently (about SDS), and I saw a report today where the disease is developing very significantly in some fields in southern Illinois. Point is, it looks like SDS may be a significant player this year.
By way of background, SDS is a root disease caused by the fungal pathogen Fusarium solani f.sp. glycines. The roots of plants are infected and become diseased during the vegetative stages of plant development. Then, typically as plants enter the pod development stages, foliar symptoms are expressed as a result of foliar sensitivity to one or more plant toxins produced by the fungus in diseased root tissue.
In extreme cases, plants can die prematurely, with yields being dramatically impacted. In some years (like 2004), plants will show symptoms early and later recover, with no impact on crop yield. In most years the response is somewhere in the middle of these two extremes.
SDS is characterized by a rotting of the primary and secondary root system and a subtle brown discoloration of the stem tissue immediately inside the green exterior. Leaf symptoms that start as yellow spots and blotches between the veins of upper leaves (usually), leading to large areas of brown tissue between the veins, and eventual defoliation of the leaflets, but not the petioles.
Abortion of flowers and young pods is common. Symptoms usually first develop in hot spots in fields of varying shapes and sizes. Entire fields may eventually become involved in severe instances, but even then, there are typically areas in fields where the disease is more severe than in others.
At this time, there is nothing that can be done to make the disease less or more serious. The die is cast, as it were. SDS is in a race against time with the crop. If the crop reaches the R5-R6 stage before the disease is severe, then the yield prognosis will be excellent. Serious disease prior to the R5 stage, however, can result in serious yield reductions. Typically the earlier the onset of serious SDS, the greater the impact will be on crop yield.
Initially, I was surprised we are seeing so much SDS, considering the predominately dry conditions we have had in June and July. Then I recalled the cool conditions that lingered in April and early May, and the fact that many growers planted earlier than normal in an attempt to escape soybean rust. Both the cooler soils and early planting dates may have resulted in more root infection by the SDS pathogen than is "normal". Then, the later season moisture stress conditions probably encouraged foliar symptom expression of SDS.
As I said, there is nothing one can do now to ameliorate the effects of SDS; the disease will have to run its course. However, the disease can be drastically reduced next season by planting a resistant variety coupled with avoiding late-April and early-May planting dates.
Source: Article in Kentucky Pest News, No. 1068 August 8, 2005.
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