Methods of investigating populations of the take‐all fungus(Ophiobolus graminis)in soil

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Abstract
SUMMARY: Attempts to isolateOphiobolus graminisdirectly from infested soils failed, so host‐infection techniques were used to study soil‐borne populations of the fungus. Extracting organic debris from soils and grading it by wet sieving through standard meshes concentrated the fungus. Fractions were tested for infectivity either as layers in pots of sand or by packing into short lengths of polyvinyl chloride tubing, through which wheat seedlings were grown. Coarse debris (retained by 420 μ aperture sieves) was most infectious and usually caused lesions within 3 weeks; whole soil and especially fine debris (< 420 μ) caused fewer and less severe infections, which often became apparent only after 3 weeks. Slight infectivity of the sediment fractions was attributed to imperfect separation of debris. Soil sampled in crop or stubble rows caused more infections than soil from between rows.Usually seedling infection was made manifest by root lesions and runner hyphae, but these symptoms were not always plain or conclusive. Many seedling roots that rotted when kept moist and in the light produced perithecia within 6 weeks. Although perithecia formed on some roots where neither lesions nor hyphae were found, they did not form on all diseased roots. More needs to be known about the factors influencing perithecial formation before it can be used as a reliable confirmatory test.