In 1996, the Iowa State University (ISU) Plant Pathology Department conducted tolerance tests of soybean varieties to white mold. The tests were conducted again this year, with funding support from the Iowa Soybean Promotion Board. Sixty-four entries were used from public and private sectors.

The experiments were conducted in field sites in Ames, Kanawha, and Mason City that had severe disease in 1996. Because of drought or hail injury, white mold incidence was light in Ames and Kanawha, and only the site at North Iowa Community College in Mason City had severe disease symptoms.

The field plots in Mason City were planted in mid-May. Each variety was replicated four times in plots four rows wide and 12 feet long with a row spacing of 15 inches. Plots were separated by a border row of the soybean variety Kenwood. Disease and yield data were collected on September 12 and 19, respectively. Maturity information was obtained from various publications, including Soybean Disease-Resistant Varieties for Iowa (Pm 1649).

See the table below for a list of the 1997 varieties and tolerance results. We used a few varieties as checks for tolerance, susceptibility, and moderate susceptibility/tolerance. Tolerance of each variety was measured as the percentage of plants killed. The fewer plants killed for a variety, the higher the tolerance. Keep in mind that the number of plants killed differed from location to location and from year to year because of variation in environmental conditions and farming practices.

This year, the results show greater variability in the relationship between plants killed and yields than did our 1996 results. When choosing a soybean variety, try to select one that is consistent over years and locations, or that seems to perform well for growers in your area. So far, we have noted the failure of two varieties under field conditions, although these varieties did very well in ISU and company tests.

Keep in mind that yield losses are not significant when the percentage of plants killed is less than 20 percent. Depending on how conservative you want to be, varieties with less than 30 percent plants killed can be considered as having acceptable tolerance. In fields with mild disease pressure, varieties that did not receive a high tolerance rating in our test may do well, because our fields had higher disease pressure than did most production fields.

It is important to note that varieties in later maturity groups often have greater disease levels because our test was done in northern Iowa, a region that is outside of their range of normal adaptation. These varieties should have higher yields and less disease when grown in southern Iowa. See the web page at http://www.exnet.iastate.edu/pages/plantpath/whitemold.html for our 1996 results.

Each cultivar was replicated 4 times with plots of 4 row wide and 12 feet long. Row spacing was 15". Yield data were taken from center rows of each plot.

This project was funded in part by the Iowa Soybean Promotion Board.