As many of you know, the Iowa Soybean Promotion Board supports testing by the ISU Department of Plant Pathology of varieties tolerant to soybean white mold. Following preliminary experiments in 1995, we tested 63 entries from private and public sectors in the 1996 growing season.

Although results have been presented on various occasions, we continue to receive numerous requests from soybean producers for this information. Here is a portion of the data. Full results can be found on our World Wide Web site at this address: http://www.exnet.iastate.edu/Pages/plantpath/whitemold.html

The experiments were conducted at three locations and results at the following locations are presented: the Humboldt research plot in Humboldt County and the ISU Kanawha Research Farm in Hancock County. The Humboldt field has been naturally infested with soybean white mold for years and has the highest disease pressure among all testing sites. The Kanawha field was uniformly infested with a high sclerotia density in the fall of 1994 and again in the fall of 1995 using infested soybean debris. We also inoculated the plants with white mold culture at flowering time. Data of control plots showed that inoculation at flowering time was not effective and infections were from spores produced by sclerotia in the soil.

Table 1 and Table 2 (below) list the varieties whose tolerance was ranked in the top 30 at each location. Two susceptible varieties, A2242 and Williams 82, were grown as checks, and their results also are provided.

Tolerance of each variety was measured as the percentage of plants killed. The fewer plants killed for a variety, the higher the tolerance and the higher the variety's rank. The number of plants killed differed between locations because of varying disease pressure. The Humboldt site had higher disease pressure than the Kanawha site. For all data presented, there were four replications of each entry at each location. Maturity information is provided by individual companies or from 1995 ISU uniform yield tests.

To select a variety, compare the percentage of dead plants with that of the most susceptible varieties and look for consistency in tolerance rankings. Also, select varieties with higher yields. Keep in mind that these two locations had high disease pressure, which may not be present in your fields. In fields with mild disease pressure, varieties that did not receive high ranks in these two fields may do well.

Table 1. White mold resistance at the Humboldt plot

Table 2. White mold resistance at the Kanawha plot