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 studi es showed that increased pubescence density of soybean provided resistance to SMV by inhibiting the probing activities of aphids. In this study we com pared the incidence level of SMV infection in six 'Clark' isolines with dif ferent pubescence densities and fit the disease progress curves (increase w ith time in the percent of plants infected with SMV) in three Clark isoline s 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 t o soybean with extra-dense pubescence; (2) to characterize the effects of p ubescence density on occurrence of SMV epidemics. Each increase in pubescen ce density reduced the incidence level of SMV infection. The pubescence den sity of soybean affected the disease progress curves of SMV. The dense pube scence isoline had a lower upper asymptote parameter, and the extra-dense p ubescence isoline had both a lower upper asymptote parameter and a larger d elay parameter than the normal pubescence isoline. On the other hand, dense r pubescence did not significantly affect the rate of SMV spread. Therefore , dense pubescence provided resistance to SMV by reducing the maximum incid ence of SMV infection while extra-dense pubescence provided resistance by b oth reducing the maximum incidence of infection and delaying the time at wh ich maximum SMV spread occurred.