Winter wheat cv. Avalon was sown in autumn 1981, 1982 and 1983 on a clay loam soil following two cereal crops. Multifactorial experiments tested the effects of combinations of the following eight factors, each at two levels: rotation, sowing date, timing of nitrogen, amount of nitrogen, growth regulator, pesticide, spring fungicide and summer fungicide.The best 16-plot mean grain yields in 1982-4 were respectively 8-7, 10-2 and 11-1 t/ha. Rotation had the largest effect on grain yield. Wheat following barley was severely infected with take-all and yielded, on average over 3 years, 2-2 t/ha less than wheat following oats. Take-all was more severe on wheat sown in mid-September than in mid-October; its effects on yield were lessened by early timing of N in 1982. Take-all decreased growth and N uptake mainly after anthesis, and also number of ears and dry weight per grain. Sowing in mid-September compared with mid-October decreased yield of wheat after barley by an average of 0-8 t/ha because take-all was more severe. Early sowing had negligible effects on grain yield of wheat after oats, but increased straw dry weight by 1 • 1 t/ha. Spring fungioide increased yield by an average of 0-3 t/ha. Effects were larger after barley than after oats, associated with a greater incidence of eyespot after barley. Summer fungioide increased yield by an average of 0-3 t/ha. Foliar diseases were slight in all 3 years. Fusarium ear blight and sharp eyespot were prevalent in 1982 and were not well controlled by the fungioide treatments. Fungicide temporarily decreased the incidence of some components of the mioroflora on the ears. Pesticide increased grain yield of wheat after oats only in 1984, when aphids on the ears were numerous. Aphids were present on early-sown plots in all three autumns but there was little barley yellow dwarf virus infection even without pesticide. Pesticide always decreased the number of nematodes after harvest to fewer than present before sowing. Populations never approached levels expected to affect yield.Early N application (main application early March) resulted in a larger grain yield in 1982 than N applied a month later. In 1983 and 1984 grain yield and N uptake by the grain were greater with the late application, especially when wheat was sown early. The soil contained more mineral N in the autumn of 1982 and 1983 than in 1981. Straw weight was always greater with early than with late application. Increasing the amount of N applied from 163 to 223 kg/ha increased N uptake by 40 kg/ha and grain yield by 0-5 t/ha after oats and by 0-6 t/ha after barley. N uptake in grain plus straw by the best yielding crops ranged from 205 kg/ha in 1982 to 246 kg/ha in 1984.Chlormequat applied at the start of stem extension shortened the stems at maturity by 2 cm each year. In 1984 it inoreased yield of early-sown wheat by 0-3 t/ha and also decreased lodging, which did not occur in the first 2 years.

Suction trapping data indicate three periods of migration of Rhopalosiphumpadi in spring, summer and autumn. Four alate morphs are present at different times during the year. A comparison of data from suction traps operating at 12.2 and 1.5 m suggests a different behaviour of females in autumn with more being recorded at 12.2 than 1.5 m. Males, which are only present in autumn, were also more numerous at 12.2 m. During tests to measure barley yellow dwarf virus (BYDV) infectivity, only 9% of female R. padi reproduced on oat seedlings in autumn compared with 74% in summer. Tests on alate female R. padi trapped alive showed that in summer all were exules, but during the first half of September these were largely replaced by gynoparae so that in autumn only 5% of all R. pad; trapped at 12.2 m were alate exules. The aerial densities of gynoparae and males were 10 times greater at 12.2 than 1.5 m while densities of alate exules were similar at both heights. It is suggested that gynoparae and males fly higher to increase the chance of finding a taller dispersed host plant.The implications for BYDV epidemiology of the behaviour and presence of the various R. padi alate morphs indicate that autumn-sown cereals emerging before mid-September are particularly at risk from colonisation by alate exules before the transition to a mainly sexual migrant population is complete. Alate exules introduce BYDV from comparatively local sources. The ratio of total R. padi to Sitobion auenae in suction trap samples in autumn usually exceeds 100 : I , but on crops it was only 10 : I . The ratio of alate exule R. padi to S . avenue in suction traps in autumn was only 12 : I , similar to that observed on crops.

No abstract