Eyespot (Cercosporella herpotrichoides) and other factors influencing yield of wheat in the six-course rotation experiment at Rothamsted (1930-60)

SUMMARY Yeoman wheat, in the six-course rotation experiment (potatoes, rye, sugar beet, barley, clover, wheat) at Rothamsted between 1937 and 1960 averaged 34% of straws at harvest infected, 19% severely, with Cercosporella herpotrichoides, the cause of eye-spot. In different years infected straws ranged from 2 to 99%, which included 1?81% with severe lesions. Straws severely infected increased by about 7% in each succeeding 6-year cycle, presumably because the amount of infectious material steadily increased. The small amount of eyespot at the beginning of the experiment can be explained by the few cereal crops taken on the land before the experiment began. The fungus survived 5 years between wheat crops partly on spring-sown barley and autumn-sown rye, which (1952-60) each averaged 8% infected straws, only 2% severely, while wheat had 45 % infected, 27 % severely. Other factors that probably influenced yield in some years include lodging (which occurred in 10 years, when straw yield or severe eyespot were above average), other diseases and pests, notably wheat bulb fly, weeds and differences in cultivations and treatments. Mean yield of grain in the 30 years was 28·5 cwt./acre, the maximum 41·1, minimum 18·7. In a neighbouring field, Yeoman wheat, unharmed by soil-borne diseases and pests and given the same amount of nitrogen (0·6 cwt./acre) yielded about 40 cwt./acre with a year-to-year variance of only 5 cwt./acre compared with a range of 19·1 cwt./ acre in the six-course wheat in the same period (1952-58), the same weather having different effects on healthy and diseased wheat. The number of straws per foot row (adjusted to 7 in. spacing) averaged annually 21·3 (maximum 28·0, minimum 14·8). Regression analyses showed that 18% of the variance in grain yield was accounted for by the number of straws and that when the number with severe eyespot lesions was included as a depressing factor, together they accounted for 48%. These figures further indicate that severe infection of all straws would almost halve the yield, and that the mean loss attributable to severe eyespot lesions at harvest (1942-60) was about 12%.