St. Paul Campus White Mold Outbreak

September 30, 2019

by Jim Kurle

It’s not often that the campus becomes a demonstration of plant disease principles. This year is a spectacular exception. Most of the campus production soybean fields, especially those in the A series south of Larpenteur, are severely infested with white mold. Why? Considering the disease triangle, environment, susceptible host and pathogen, will explain a lot. White mold, caused by Sclerotia sclerotiorum, has been unusually severe and widespread this year because of ideal environmental conditions, planting of susceptible soybean varieties, and buildup of inoculum.  

Pathogen: White mold inoculum persists as sclerotia that accumulate and persist in the soil.  Unless favorable conditions occur to stimulate germination or parasitism by soilborne fungi or bacteria, inoculum will accumulate. White mold is spread as airborne ascospores produced on apothecia emerging from sclerotia. Apothecia eject ascospores which are capable of infecting the soybean plant whenever flower petals are present.

Environment: This year environmental conditions required for white mold have been ideal. Abundant precipitation has stimulated germination of sclerotia to form apothecia throughout the growing season. As a result, ascospores have been available whenever flower petals were present. In addition, since the beginning of flowering, all reproductive (R) stages have been accompanied by moderate temperatures and high relative humidity, ideal conditions for disease development following infection.

Host: This year’s white mold outbreak seems to be especially severe in newly released varieties. There are a couple of possible explanations. The varieties are GMO varieties containing multiple herbicide resistance genes. The genes could be associated with increased susceptibility to white mold. This should be investigated. However, there is another possible explanation. in 1996 and 1997, when the first Roundup Ready varieties were released there were outbreaks of white mold similar to those occurring this year. Supposedly, the Roundup Ready gene increased susceptibility. Two things had happened. Growers created ideal conditions for white mold by planting high populations in narrow rows because cultivation was no longer necessary. In addition, further observation and a little self-criticism revealed that disease resistance had not even been considered in the rush to release the new GMO varieties. In this current outbreak on campus, in addition to white mold, brown stem rot and stem canker are present. It appears that disease resistance was given low priority when varieties were chosen for release.

Look at the campus soybean fields. If humidity is high you may see the fluffy white mycelium produced by the white mold fungus, Sclerotia sclerotiorum. At any time the necrotic stems and leaves caused by white mold are present throughout fields A1, A2, and A3.