Relationship between soybean pubescence density and soybean mosaic virus field spread

An important factor in the natural spread of soybean mosaic virus (SMV) is that viruliferous aphid landing must be followed by probing. Previous studies showed that increased pubescence density of soybean provided resistance to SMV by inhibiting the probing activities of aphids. In this study we compared the incidence level of SMV infection in six 'Clark' isolines with different pubescence densities and fit the disease progress curves (increase with time in the percent of plants infected with SMV) in three Clark isolines with normal, dense, and extra-dense pubescence to compare patterns of SMV spread in the field. Our objectives were (1) to corroborate the effects of dense pubescence on the incidence level of SMV infection and extend this to soybean with extra-dense pubescence; (2) to characterize the effects of pubescence density on occurrence of SMV epidemics. Each increase in pubescence density reduced the incidence level of SMV infection. The pubescence density of soybean affected the disease progress curves of SMV. The dense pubescence isoline had a lower upper asymptote parameter, and the extra-dense pubescence isoline had both a lower upper asymptote parameter and a larger delay parameter than the normal pubescence isoline. On the other hand, denser pubescence did not significantly affect the rate of SMV spread. Therefore, dense pubescence provided resistance to SMV by reducing the maximum incidence of SMV infection while extra-dense pubescence provided resistance by both reducing the maximum incidence of infection and delaying the time at which maximum SMV spread occurred.