REDUCTION OF DISEASE INCIDENCE IN SMALL-FIELD PLOTS BY ISOLATE-SPECIFIC RESISTANCE TO BARLEY YELLOW DWARF VIRUS

Epidemics of two isolates, RMV and MAV, of barley yellow dwarf virus ( BYDV) were studied in three spring oat genotypes. Two of the genotypes previously were found to possess varying levels of resistance express ed as a reduction in the accumulation of virus antigen titer in plants . The reduced virus antigen titer contributed to reduced virus acquisi tion and transmission efficiencies by aphid vectors. The third genotyp e was susceptible to the two BYDV isolates. Resistance was expressed a t high levels against the RMV isolate; and in 1990 and 1991, the final disease incidence of RMV and rate of disease progress were significan tly reduced in the resistant genotypes relative to the susceptible gen otype. The resistance reduced both rate of disease progress and final disease incidence of the MAV isolate during both years of the study. T he magnitude of the reduction in final MAV incidence relative to the s usceptible genotype varied between the 2 yr and could be related to ve ctor population intensity and environmental factors. Resistance expres sed as reduced or suppressed virus accumulation in plants can be easil y identified by serological or nucleic acid-based assays. The lower vi rus titer may reduce disease impact on plant growth and can reduce the efficiency with which aphid vectors acquire and transmit virus among resistant plants.