Managing sclerotinia stem rot, or white mold, takes more than just the right variety, tillage system, rotation or even the right weather. It takes all four.

So say researchers studying the potential for lowering white mold losses by variety selection, crop rotation and tillage.

Three Wisconsin plot sites, Sharon, Janesville and Waunakee, were moldboard plowed, chisel plowed or no-tilled. Subplots were planted to: 1) continuous soybeans, 2) a one-year planting of a non-host crop followed by two years of soybeans, 3) a three-year rotation of soybeans/non-host crop/soybeans and 4) a non-host/non-host/soybean rotation.

Three varieties were planted within those subplots: the moderately resistant Novartis S19-90, the moderately susceptible BSR101 and Sturdy, a susceptible variety.

In the third year, rainfall during the growing season was above normal in Waunakee, near normal at Janesville and below normal at Sharon. No white mold was observed at Sharon, resulting in the highest yields, 55 bu/acre. But rainfall timing and amount favored white mold development at the other two sites. Yields were 32 bu/acre at Waunakee and 46 bu/acre at Janesville.

Variety selection was the most important management factor influencing crop yield and white mold incidence. S19-90 produced the highest yield, at 43 bu/acre, while sustaining the lowest white mold infestation of 14%. Both BSR101 and Sturdy yielded 37 bu/acre and sustained an average disease incidence of 62%.

No-till plots produced the highest yield, at 41 bu/acre and 30% disease severity. Moldboard-plowed beans gave the lowest yields and sustained the most severe disease (36 bu/acre and 58%).

Differing plant populations and canopy density among tillage systems affected disease severity. Moldboard-plowing resulted in significantly higher plant populations (172,000 ppa) than did no-till (145,000 ppa). Greater plant populations and canopy density favored disease development.

Yields and disease severity of the susceptible variety, Sturdy, were highly variable compared to the more resistant variety, S19-90. Averaged over crop sequences, S19-90's yield was 49 bu/acre; its disease severity, 11%.

(J.E. Kurle, University of Minnesota; C.R. Grau and E.S. Oplinger, University of Wisconsin)