Agronomic Bulletin


White Mold Resurgence

With moderate temperatures and ample rainfall for many growers providing near ideal conditions for crop growth, these same conditions are also providing the necessary environment for the rapid development of Soybean White Mold.

Sclerotinia White Mold or White Mold is a disease which thrives under moist and cool environment (less than 84 degrees) and where soybeans have developed a thick, dense canopy.

The mold itself results from spores which are released from fruiting apothecia found on the soil surface and resemble small clay pots.

The spores enter the soybean plant through blossoms and quickly develop within the stems of soybeans and produce small black chunks called sclerotia. The White Mold fungus quickly starts to restrict water and nutrient flow to upper stems and leaves resulting in dead tissue and pods. Infected plants will exhibit dying leave and pods on the upper part of the plant and looking lower into the canopy, one can see the fuzzy white growth of the fungus itself. Unfortunately very little can be done when plants have become infected and yield and grain quality can be drastically reduced.

The single best way to combat White Mold is to begin planning early on by selecting varieties with highest White Mold rating as possible. Fields with a history of manure or other high yield environments can be planted to wider rows to allow more air flow or in some cases planted later to reduce the height of the plants.

Lowering the plant population is another option but still does not guarantee the severity of the disease will be lessened.

Finally crop rotation may help as the sclerotia will germinate and produce spores and lacking a suitable host will not reproduce. Unfortunately there are a large number of host plants such as ragweed, pigweed, lambsquarters and velvetleaf, along with most bean crops that will support White Mold. Even a seven year continuous corn field near Galesburg IL had fruiting bodies present and with the number of blossoms present could very easily infect these fields.

A fungicide treatment like Domark may be an option, but fungicides must be applied before any sign of spore activity is present to have some level of control.

Tillage affects the survival of white mold, but there are no good recommendations for disease management because of this disease's unique features. Sclerotia can survive in deep soil up to seven years. Only sclerotia within 2 inches of the soil surface can germinate and release spores to infect soybeans. Burying infested residues by mold-board plow can prevent germination of the sclerotia. But the effectiveness in controlling white mold by burying sclerotia is affected by cultivation and tillage in following seasons. For example, if you cultivate, disk, or use a chisel plow in corn the following season, you bring some sclerotia to the soil surface. Further, if soybean fields have already been infested by white mold in past seasons, the sclerotia may be mixed in the soil. Fall tillage of these fields would bury new sclerotia in deep soil, but uncover old sclerotia.

Growers in Wisconsin and Minnesota have been exposed to White Mold for years and have adapted their practices to alleviate some of the potential yield losses associated with White Mold. Growers who have not experienced need not to panic, but to plan early and smart to deal with this disease.

Click the thumbnail images below to see full size picture that show the progress of the disease from the mold emerging on the stem, to sporadic plants dying in the field to the sclerotia in the stem that will be the White Mold vector in the soil for future infections.

Credits
  1. Pictures courtesy of Iowa State University

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