Prevalence of Sclerotinia stem rot of soybeans in the north-central UnitedStates in relation to tillage, climate, and latitudinal positions

Since the early 1990s, Sclerotinia stem rot, caused by Sclerotinia scleroti orum, has caused considerable damage to soybean production in the north-cen tral United States. To determine the extent of its distribution and associa ted factors, investigations were conducted in 1995 and 1996 in Illinois, Io wa, Minnesota, Missouri, and Ohio. Investigations also were conducted in 19 97 and 1998 in Iowa, Minnesota, and Missouri. In each state, soybean fields were randomly selected in collaboration with the National Agricultural Sta tistics Service. From each field, 20 soybean stems 20 cm long (from the bas e) in 1995 and 1996 and full-length stems in 1997 and 1998 were sampled in a zigzag pattern. During the 4-year period, stem samples were collected fro m 1,983 fields and assessed for the presence or absence of the disease. Of the five states, Sclerotinia stem rot was most prevalent in north-central I owa and southern Minnesota. Sclerotinia stem rot was not detected in Missou ri during the 4-year investigation period. The disease was most prevalent i n 1996 and least prevalent in 1995, The prevalence of the disease was stron gly related to cumulative departures from normal maximum and minimum temper atures in July and August. The disease was more prevalent when yearly tempe ratures were below normal than when they were above normal. In 1996, a year with a cooler-than-normal summer, the disease was detected farther south t han in 1995. In both years, the prevalence of the disease was exponentially related to latitudinal positions of the fields (R-2 = 0.93 and 0.83 for 19 95 and 1996, respectively) reflecting the effect of the north-south variati ons in temperature. During the 4-year period, there was no relationship bet ween precipitation and the prevalence of the disease. The lack of relations hip may suggest that there was no shortage of moisture since it is one of t he primary factors for disease development. The prevalence of Sclerotinia s tem rot was less in no-till than in minimum-till or conventional-till field s (P = 0.001 and 0.007, respectively) and greater in minimum-till than in c onventional-till fields (P = 0.07). Fields that had Sclerotinia stem rot, h owever, did not differ in incidence of the disease regardless of the tillag e system.