Effects of sowing date and volunteers on the infectivity of soil infested with Gaeumannomyces graminis var. tritici and on take-all disease in successive crops of winter wheat

in a field experiment on winter wheat, take-all on plants and the infectivity of the soil were studied in crop sequences with different combinations of sowing dates. Take-all was negligible in the first wheat crop, but thereafter the mean disease intensity (measured using a take-all rating, TAR, with a maximum of 300) was 108, 190, 118 and 251 in the second to fifth successive crops. In each growing season, the disease differed amongst sequences and built up more rapidly and was more intense on plants sown in mid-September than on plants sown in mid-October. In late-sown plots, where volunteers had been present during the mid-September to mid-October period, take-all reached an intensity intermediate between that in early-sown plots and that in late-sown plots that had been kept free of volunteers. Volunteers, therefore, partially offset the expected beneficial effect of decreased disease with later sowing. Differences in take-all amongst sequences were most pronounced in the second wheat crop and early sowing of the previous wheat increased intensity of disease. In the following (third) crop, differences in disease intensity amongst sequences were smaller. Soil infectivity (measured by seedling bioassay after harvest) built up progressively from a low level after the first crop to peak after the third crop. In this build-up phase, soil infectivity estimates were always numerically greater after harvest of early-sown treatments than after later-sown treatments, although never significant at P = 0.05. The greatest difference (P = 0.06) was recorded in October before sowing of the third crop, where the comparison was between soil after two previous early sowings and soil after two previous later sowings and control of volunteers. In the same autumn, presence of green cover (i.e. volunteers) was associated with a smaller loss of soil infectivity between harvest and later sowing than occurred in an absence of green cover. In 2(nd)-4(th) crops, where comparisons were available and mean TARs indicated moderate levels of take-all, sowing later had no yield benefit, despite more take-all and greater soil infectivity associated with early sowing. Important considerations for the management of crops at risk of take-all are 1) choosing appropriate sowing dates to minimize take-all or to encourage take-all decline and 2) controlling volunteers and weed hosts where crops are sown late to minimise take-all.