The effect of incorporating straw or manure into the soil on the natural microflora of winter wheat

Over 3 years, five crops of winter wheat with different fertilization or pesticide treatments were grown in replicated plots in which straw, cattle manure or nothing was incorporated into the soil. It was previously shown that severities of several foliar diseases were reduced by the treatment with straw. Treatments with straw slightly increased leaf microbial numbers at growth stage (GS) 10, and significantly but slightly decreased numbers at GS 60 and 70, when averaged over all crops. There was no evidence of differences in potential for biological control between plots, because no differences were demonstrated between treatments in the proportion of phylloplane residents able to produce chitinase, siderophores or antibiotics. There was no link between soil and leaf nitrogen and microbial populations or soil amendment. The two youngest leaves of straw‐treated plants remained green longer than leaves from untreated or manure‐treated plants. Microbial numbers differed between crops, but within crops were more stable on leaves and ears than in soil. Soil populations were larger before the crops were sown. Bacteria predominated over fungi in all habitats throughout the growing season. The ratio of Gram‐negative to Gram‐positive bacteria recovered was similar on leaves and in soil, but not on ears. Soil and leaf microbes utilized similar carbon sources. More leaf epiphytes than soil microbes produced siderophores. The majority of identified organisms belonged to only a few species. Chromogenic organisms were more abundant on aerial surfaces. A fungicide, chlorothalonil, was applied in one crop; it reduced the microbial population but did not alter its composition (without affecting the reduction in disease due to straw). It is postulated that straw acts in the field by altering plant physiology and thus altering both microbial colonization and disease.