Disease is more severe in cool, moist weather. In the Willamette Valley, cool, moist conditions in spring or fall favor epidemics. In the cereals, symptoms develop 2 to 4 weeks after infection when temperature is 68 to 77°F and there is ample light. No symptoms develop in barley when temperature is greater than 86°F. Symptoms are not expected to develop on grass seed crops during the winter months because low temperature and light conditions inhibit symptom development. Plants become systemically infected, therefore, once perennial grass plants are infected, symptoms can reappear during subsequent growing seasons.

Damage is not always severe, it is influenced by the growth stage at infection; more severe symptoms occur when plants are infected as seedlings. Virulence of the viral strain and dosage (aphid levels), amount of primary and secondary spread, and susceptibility of the cultivar/grass species all affect the level of damage that occurs on a grass seed crop. Research in the UK shows that sometimes rye-grass is asymptomatic but that a reduction in perennial ryegrass height and sward growth can still be detected when plants show little foliar symptomology. While some ryegrass cultivars may exhibit a decrease in leaf production, others may have a marked increase in foliage. Infection of ryegrass by BYDV can increase the proportion of vegetative to fertile tillers, and the pattern of seasonal produc-tivity may change. Root development of ryegrass is negatively affected by BYDV in and even cultivars that gave increased shoot growth exhibited reduced root growth. Whether variation exists among cultivars grown in Western Oregon grass seed fields is un-known and little is known about the impact of barley yellow dwarf on grass seed crops.

1. Sow new plantings in late fall or early spring so seedling stage is before or after peak aphid flight.
2. In affected plantings, reduce stress on plants by providing a balanced fertility and avoid drought stress.