An Example of the Disease Triangle at Work

In the last few days, I have been in contact with Drs. Tom Allen and Billy Moore who regularly scout Miss. soybean fields and research plots for the presence of diseases. They provide invaluable information about disease presence and severity in the Mississippi soybean crop during the growing season.

Their latest finding has been the unusually high incidence of southern blight, caused by the fungal pathogen Sclerotium rolfsii. I say unusual because estimates of disease effects on soybeans in the southern US indicate that this disease is rarely and/or only barely present on soybeans in the Midsouthern states. It has been estimated to occur to a minor extent in some of the southeastern US states along the Atlantic Coast in the past several years, but not in the Midsouth. So it comes as a surprise to see it in such high incidence in our region in 2018. Click here for the MCS blog article by Dr. Allen that describes these findings and gives a detailed oral and pictorial description of southern blight symptoms in 2018 soybean fields.

The basics of southern blight disease in soybeans are presented by Dr. Travis Faske of the Univ. of Arkansas. Information in his article provides the following pertinent points about this disease and its management.

•   Southern blight is considered a minor disease of soybean in Arkansas–rarely does yield loss from this disease exceed 1% in affected Arkansas soybean fields.

•   Disease symptoms can occur from the seedling stage to mature plants.

•   Disease development is favored by hot, humid weather conditions.

•   The disease pathogen overwinters as sclerotia in the soil, and these sclerotia can remain viable for 3 to 4 years.

•   Crop rotation with a grain crop for 2 years can be beneficial for reducing sclerotia survival and buildup in the soil.

•   All soybean cultivars are susceptible to southern blight.

The increased incidence of southern blight in Midsouth soybeans in 2018 is a perfect example of how the three points (host, pathogen, environment) in the disease triangle work together to result in increased incidence of a heretofore minor disease. The components of the disease triangle and how they interrelate to create a disease environment to limit crop productivity are presented in an MCS blog article by Dr. Allen. Pertinent points from that article follow.

•   Pathogens that cause soybean diseases are usually present in Miss. soybean fields.

•   The general information contained in the disease triangle is important in determining why a specific disease (e.g. southern blight) has or has not occurred in a specific setting.

•   A period of time is normally required between when a host (e.g. soybean) comes in contact with a pathogen (e.g. southern blight pathogen) and when favorable environmental conditions for disease development occur.

•   Environment is likely the single most important factor for the development of a plant disease; i.e., without an environment that is conducive for disease development, a plant disease will not occur or will occur only to a minor extent.

•   For effective or yield-limiting disease pressure to occur, the conducive environment must remain so for a specific amount of time. In other words, disease(s) does not occur instantaneously even when a conducive environment occurs.

So, the case of the increased incidence of the southern blight disease that is being documented by Dr. Allen in 2018 is an example of the disease triangle at work. The host or susceptible soybean cultivars and the pathogen in the form of sclerotinia in the soil as indicated by Dr. Faske above were present. This then means that this more severe outbreak of this disease was supported by an extended period of environmental conditions that favored disease development.

There are few options to manage southern blight in soybeans. Currently, there are no fungicides with efficacy against the causal pathogen. There are no resistant soybean varieties. That leaves rotation with a grain crop as indicated by Dr. Faske above as the only viable option if this disease is deemed a future major threat to Midsouth soybeans.

There are several conclusions that can be surmised from this increased incidence of southern blight on soybeans in 2018.

•   This may be only an isolated year when southern blight reared its ugly head. This has happened with other soybean diseases in the past; i.e., other minor diseases have infrequently caused economic loss to soybean producers.

•   Since all soybean cultivars are susceptible to this disease, that means they do not contain genes that confer resistance. Thus, there are no short-term control options for this disease since there are no efficacious fungicides.

•   Regrettably, it is not likely that the environmental conditions that favored the heavier outbreak of southern blight in 2018 can be exactly pinpointed as to when or how long they occurred. This means that prediction of a southern blight outbreak in future years cannot be made. However, we now know that two thirds of the disease triangle–i.e., host plant and pathogen–will be in place in future years.

•   If, as Dr. Allen surmises, this disease will become increasingly important in monocropped soybean systems in the Midsouth, then plant breeders must be enlisted/encouraged to identify genetic sources that can be incorporated into soybean breeding lines so that resistant cultivars can be developed.

•   There is a perceived association between the presence of southern blight and root knot nematode (RKN) infestations of soybean; i.e., southern blight is perceived to be more severe when RKN is present. Since RKN is becoming an increasingly important nematode pest of soybean, it is plausible that this may contribute to an increased incidence of southern blight as a fungal pest. This will be worth watching and documenting in future soybean crops that are infested with these pests.

Composed by Larry G. Heatherly, Sept. 2018, larryheatherly@bellsouth.net