Variety selection is the foundation of a management plan to minimize yield loss to white mold. Management options that can be used to minimize white mold are determined by the specific soybean variety selected for a problem field.

One or a combination of traits appears to be involved in how a soybean variety reacts to the pathogen, Sclerotinia sclerotiorum. Possible traits governing variety reaction are inherent physiological traits which determine the rate and severity of symptom development, plant architectre, lodging traits, maturity group, and management practices that govern canopy develpment. Although all plant parts are infected, lesions on the main stem frequently are lethal and result in low seed number.

Variety reaction types
The reaction of soybean varieties to the white mold pathogen is expressed as a percentage of the plant population which express symptoms, and the severity of symptoms within the symptomatic (diseased) plants. Within the population of symptomatic plants, some plants may reach maturity (R8), but usually a high percentage will die by growth stages R6-R7. Yield of diseased plants is governed by how quickly individual plants die in relationship to reproductive development.

Grain yield in the presence of white mold and plant mortality can be combined to characterize variety reactions to the white mold pathogen. Four general "reaction types" are used to describe soybean variety performance in the presence of white mold and serve as guidelines for selecting soybean varieties for fields with high white mold potential (Table 1).

The rate and severity of stem lesion development and subsequent reproduction of the plant are governed by inoculum density of the pathogen, climatic conditions, microclimate of the crop canopy, agri-chemicals, and the inherent traits of the variety. Long-term field trials in Wisconsin indicate that for each 1% increment of plant mortality at the R6-7 stages, soybean yield is reduced 0.25-0.50 bushels per acre. For example, a variety that expresses 20% plant mortality is losing 5-10 bushels per acre of yield potential to white mold. 

Soybean varieties differ in the percentage of symptomatic plants and plant mortality that develops by late pod development (R7). Varieties within maturity groups 0 through late II typically range from 15% to 80% plant mortality in studies conducted at sites with a high white mold potential. The correlation between plant mortality and yield has been typically 60 to 70%. This level of correlation is evidence that plant mortality is an acceptable component of a method to characterize soybean varieties for reaction to the white mold pathogen.

Some varieties within the susceptible reaction group express levels of plant mortality that greatly vary from trial to trial compared to other varieties within the group. Varieties that vary in reaction from trial to trial likely express traits that allow plants to escape high levels of infection. Examples of traits linked to white mold escape are plant architecture, lodging, variable expression of flower formation, and relative maturity. This type of variety reaction type is an example why a variety must be evaluated in several locations to accurately characterize its reaction type to white mold. No soybean variety has yet been identified that consistently expresses a high level of resistance to S. sclerotiorum.

When reviewing test data, consider a variety relatively susceptible if it expresses greater than 25% plant mortality. Yield performance must be reviewed in conjunction with plant mortality data, especially if a tolerant variety would be acceptable. Relative maturity of soybean varieties has been suggested as a criterion to select varieties for problem fields. Trials conducted in Wisconsin since 1992 indicate that relative maturity is not a reliable trait to use to select soybean varieties for fields with a history of white mold. All white mold reaction types have been identified in maturity groups 0-III.

Northrup King (Novatis) S19-90 is a soybean variety which has expressed relatively low plant mortality and acceptable yields in numerous trials in Wisconsin and other states. Because of its relatively high and consistent performance, it is used as a standard for the moderately resistant reaction type to compare the relative performance of all other varieties. Sturdy, in contrast, is used as a susceptible standard because of its consistent high plant mortality and poor yield performance in the presence of white mold.

Inconsistent variety reaction to white mold and yield performance from trial to trial has been a reoccurring problem in trials to intended to evaluate soybean varieties for reaction to white mold. A variety must be evaluated in several trials before being characterized for white mold reaction and suitability for problem fields.

Each year, the performance of soybean varieties in the presence of white mold are included in Wisconsin soybean variety trials. Data is published each year in the Wisconsin Soybean Variety Test Results (UW Extension Bulletin A3654). The publication is available free from your Wisconsin county Extension office or from the Department of Agronomy, 1575 Linden Dr., Madison, Wisconsin 53706. Phone (608) 262-1390. Or view online at WI Soybean Variety Trial Results (link to UW Agronomy Extension). Data from 1997 to the present year are available online.