Survey of Heterodera glycines Population Densities and Virulence Phenotypes During 2015-2016 in Missouri.

The soybean cyst nematode (SCN), Heterodera glycines, is one of the most important pathogens of soybean. Periodic monitoring of SCN population densities and virulence phenotypes is necessary for developing management strategies utilizing resistant cultivars, the primary strategy used to combat this pest. Therefore, we conducted a statewide survey of Missouri to determine SCN population densities and virulence phenotypes during 2015-2016 and compared these results with a similar survey conducted in 2005. SCN population densities were determined for 393 soil samples representing 74 soybean-producing counties across eight geographical regions of Missouri. Eighty-eight percent of samples tested positive for SCN, up from 50% in 2005, and population densities ranged from 125 to 99,000 eggs per 250 cm3 of soil. The virulence phenotypes of 48 SCN populations also were determined. For this, female indices (FI) were calculated by dividing the mean number of females that develop on the roots of a set of resistant soybean indicator lines by the mean number of females that develop on the roots of susceptible cultivar Lee74 after 30 days in the greenhouse then multiplying by 100 to obtain a percentage. Notably, all SCN populations evaluated during 2015-2016 had a FI > 10 on PI 88788, the most widely used source of resistance in Missouri, in contrast to 78% in 2005. Moreover, 50% of these populations had a FI > 50 on PI 88788, up from 16% in 2005. Forty-three percent of populations tested also had a FI > 10 on Peking, the second most common source of resistance by farmers. Our results show that over the last decade, SCN has become more prevalent in Missouri fields. Additionally, the percentage of individuals within SCN field populations that are virulent on PI 88788 and Peking has markedly increased. The results stress the importance of rotating cultivars with different types of resistance when using resistant cultivars to manage SCN.