EFFECT OF SOWING TIME ON BARLEY YELLOW DWARF VIRUS-INFECTION IN WHEAT- VIRUS INCIDENCE AND GRAIN-YIELD LOSSES

In 4 field experiments at 2 sites in Western Australia, wheat was sown at different times in autumn and early winter. Within each sowing, in cidence of barley yellow dwarf luteovirus (BYDV) was monitored by ELIS A tests on leaf samples, using antiserum specific to BYDV serotype PAV . Delaying sowing decreased BYDV incidence in all 4 experiments. At th e higher rainfall site, decreased BYDV incidence due to delayed sowing was associated with increased grain yield and size, and in 1 experime nt also with a decrease in the proportion of shrivelled grain. In cont rast, at the lower rainfall site, grain yield penalties derived from l ate sowing negated the yield increases associated with decreased BYDV levels. When imidacloprid seed-dressing followed by foliar sprays of a lpha-cypermethrin were applied to control aphids at the lower rainfall site, within each sowing BYDV incidence and the proportion of shrivel led grain decreased and grain yield and size increased. Applications o f pirimicarb at the higher rainfall site also decreased BYDV incidence and the proportion of shrivelled grain within each sowing, and grain yield and size were increased. In all experiments, the only, or predom inant, colonising aphid species was Rhopalosiphum padi. At flag leaf e mergence, delayed sowing decreased the number of aphids on plants.In h igh BYDV risk zones, when an appropriate insecticide is not applied to control aphid vectors in autumn-sown wheat, delaying sowing may be ad visable to minimise BYDV-induced grain yield losses. However, the yiel d benefits from decreased BYDV infection need to be balanced against p ossible yield penalties due to late sowing.