Analysis of disease-progress curves for take-all in consecutive crops of winter wheat

Disease-progress curves of take-all, caused by Gaeumannomyces graminis var. tritici, were analysed for consecutive crops of winter wheat. Comparisons were made over 9 years amongst a sequence of consecutive wheats and first and second successive wheats grown after non-susceptible break crops (spring beans). The principal objectives were to identify differences in the shapes of the disease-progress curves that could be attributed to the rotational treatments. Shapes of disease-progress curves were summarized by average rate of disease increase and components for curvature together with the mean amount of disease. Analyses were done for curves based upon percentages of diseased plants and of diseased roots. Mean levels of disease in second and continuous wheats rose from the start of the experiment (1979) to reach a maximum in the middle years (1982-84) and thereafter declined. This pattern was absent in first wheats in which disease progress within years was rectilinear and comparatively slow. In second and continuous wheats, years of high disease were characterized by more marked curvature than years of low disease. For proportions of diseased plants, the rate of disease increase in second and continuous wheats slowed as the season progressed. Differences in shapes of disease-progress curves for second and continuous wheats were apparent when proportions of diseased roots rather than plants were considered. The average linear rate of increase in the proportion of diseased roots in continuous wheats was intermediate in magnitude between those of first and second wheats. Epidemics in second wheats were initially slow and accelerated as the season progressed, whilst in continuous wheats, an early faster rate of increase in disease subsequently slowed. Some epidemiological consequences of these effects are discussed in relation to the phenomenon of take-all decline, which is associated with the suppression of the disease in cereal monoculture. The effects of cropping history on the relative importance of infections arising from soil-borne inoculum and infections arising from the spread of disease from neighbouring infected roots are inferred and epidemiological hypotheses for take-all decline are advanced. Methods involving the use of weighted linear functions were used to overcome statistical problems of repeated observations within plots as well as variable sampling frequencies and intervals within and amongst years. These methods, which have general applicability for epidemiological work, are described.