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.

Multi-factorial experiments on winter wheat cv. Hustler in autumn 1979 and 1980 sown on a clay loam soil following potatoes tested the effects of combinations of the following eight factors, each at two levels: sowing date; amount of nitrogen; division of nitrogen; timing of nitrogen; irrigation; autumn pesticide (aldicarb); summer aphicide (pirimicarb); and fungicide (carbendazim, tridemorph, maneb and captafol).The mean grain yields of all plots in 1980 and 1981 were respectively 9-6 and 8-3 t/ha; the best eight-plot means were 11-2 and 9-9 t/ha. Fungicides had the largest effect on grain yield, increasing it by 0-8 and 1-7 t/ha in 1980 and 1981, mainly by increasing grain size. Effects were greater with earlier sowing and the larger amount of nitrogen. Benefits from fungicide were well related to the control of leaf diseases, mainly Septoria spp., which became severe after anthesis. Fungicide temporarily decreased the number of saprophytic fungi on the developing ears. Aphids that appeared in autumn on plots sown in mid-September were controlled by autumn pesticide, which also prevented the spread of barley yellow dwarf virus that occurred only in 1981. Consequently, yield of early-sown plots in 1981 was increased by autumn pesticide, but only when the severe infection with leaf diseases was controlled by fungicide. Autumn pesticide decreased nematode populations. Aphid populations in summer were small and yield was unaffected by the decrease in numbers that followed application of an aphicide.Sowing on 20 or 15 September, as compared with 19 or 30 October, caused faster growth and development and greater uptake of N from the soil early in the season. Effects were smaller after April: earlier sowing increased total dry weight by 2-5-3' 0 t/ha and, when leaf diseases and barley yellow dwarf virus were controlled, increased yield by 0-9-l-0t/ha. Increasing the amount of N applied by 70 kg/ha (from 105 or 80), increased yield only in 1980 and then only when fungicide was used. Extra N decreased yield in 1981 in the absence of fungicide. Extra N always increased N uptake and decreased grain size. Applying mostoralloftheNon4orl9 March instead ofl5or23 April resulted in less uptake of N from anthesis onwards and smaller yield, especially in 1980. N in three applications instead of one had negligible effect. Trickle irrigation decreased yield slightly, despite delaying leaf senescence and increasing straw weight.Attributes of wheat on best yielding plots differed little between years. Average values were: 534 ears/m 2 ; 40-7 grains/ear; 40-4 mg/grain; 18-8 t/ha total dry matter and 214 kg N/ha uptake by grain plus straw.

A multifactorial experiment sown to winter wheat cv. Hustler in autumn 1978 tested the effects of combinations of the following eight factors, each at two levels: drill type, sowing date, amount of nitrogen, division of nitrogen, irrigation, autumn pesticide (aldicarb), summer aphicide (pirimicarb), and fungicide (carbendazim, maneb and tridemorph).The mean grain yield of all plots was 9-7 t/ha and the best eight-plot treatment mean was 11-3 t/ha. The factors that had the greatest effect on yield were aphicide and fungicide, mainly from the control of Metopolophium dirhodum and Septoria spp. respectively. Both factors increased grain size and their effects were more than additive and greater with 250 than with 160 kg N/ha. Aphicide and fungicide also temporarily decreased the numbers of microbes on the developing ears. Autumn pesticide gave good control of aphids in the winter and some control in the summer: it also decreased nematode populations and slightly increased yields.Precision sowing compared with random distribution of seeds along the row had little effect on growth or yield. Sowing on 21 September compared with 13 October greatly increased growth early in the season but had less effect after anthesis; it was the only factor that increased yield when aphicide and fungicide were applied. There was negligible infection by barley yellow dwarf virus in crops sown on either date. The amount and division of N fertilizer affected N uptake early in the season and had small effects on the production and survival of tillers. Three N applications instead of one slightly increased grain yields but did not affect total N uptake by grains plus straw, which averaged 190 kg N/ha. The larger amount of N always increased N uptake but decreased yield in the absence of aphicide and fungicide. Irrigation slightly decreased yield despite prolonging the duration of green leaf area. benefits to be expected from the components of high-input cereal-growing systems. Much research has been done on the effects of This paper describes the first of a series of various agricultural practices, including the control experiments designed to investigate the effects and of pests and diseases on yield of winter wheat. The interactions of some factors that are thought to different factors have usually been examined in limit the yield of winter wheat. Treatment effects separate experiments, resulting in little informa-on yield and the mechanisms involved were studied tion about the interactions between them. More-in order to identify likely causes of variation in over, many of the experiments used cultivars and yield between fields. The upper level of each factor agronomic practices that result in much smaller and the basal treatments were intended to give a yields than those now common. Much of the large yield from the best combination of treatments information available about modern high-yielding so that the results would be relevant to modern crops relates to packages of treatments; these agriculture (Dawson, 1980). The multi-disciplinary,...

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.

SUMMARYBarley yellow dwarf virus is persistently transmitted by a number of aphid species of which three, Rhopalosiphum padi, Sitobion avenae and Metopolophium dirhodum, are common in most years. Other aphids may be locally important. Isolates of the virus differ in their virulence and geographical distribution and are not transmitted equally well by all aphid vectors. Isolates with similar properties are grouped into strains according to their transmission by vectors and their severity. Changes in strain and aphid occurrence from year to year alter the incidence of virus and its effect on yield. These changes emphasize the need for detailed knowledge of cereal aphid biology and epidemiology of BYDV before effective control can be used.

SUMMARY Suction traps operating at low level (1 5 m) were used to catch live alate Rhopalosiphum padi, Macrosiphum (Sitobion) avenae and Metopolophium dirhodum which were tested for transmission of barley yellow dwarf virus (BYDV). The first species caught and infective was R. padi, followed by M. (S.) avenae infective some 2–3 wk later and M. dirhodum 3–4 wk later still. Never more than 11‐5% of the annual catch of any species transmitted BYDV and the proportion fluctuated from week to week and between seasons in different years. The relative abundance of infective vectors of ths three species varied; annual numbers of infective M. (S.) avenae and M. dirhodum varied inversely with infective R. padi, the latter also usually transmitted severer virus. The results of the infectivity tests have been compared with the catches of these aphids by the Rothamsted Insect Survey and show that numbers of alate aphids do not necessarily indicate the likely incidence of BYDV.

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