Brazilian bud blight, a soybean disease caused by the tobacco streak virus, is prevalent in a few areas in the States of Paraná and São Paulo. Although the virus is seed-transmitted, outbreaks of the disease are related to the presence of virus in donor plants in the weed vegetation and not to seed sources. Species of Frankliniella act as vectors. Thirty six soybean genotypes, about half of them known to have varying degrees of resistance to insects, were evaluated when exposed to natural infection by bud blight in five replicated plots sown in an area where medium to severe outbreaks of the virus had occurred in previous years. Incidence of bud blight was high in the experiment and reached 90% infection for some of the genotypes. Best genotypes for resistance to bud blight were PI 227687 (13% infection), IAC 73-228 (25%), IAC 80-1177 (36%), IAC 80-1191 and IAC 84-20-1 (both 38%), and PI 274453 (40%). The best five most resistant genotypes mentioned above plus seven others were also screened by mechanical inoculation of greenhouse plants with five isolates of the soybean bud blight virus. The results were not parallel to those read in the field exposure test. Thus, the response of PI 227687, IAC 73-228 and probably those of some of the other best genotypes are not considered the result of plant resistance to the virus, but rather as field resistance related to the interaction plant/thrips vector.
Glycine max (L.) Merrill; resistance to insects; thrips; Frankliniella