Decrease of take-all by a transmissible factor in take-all decline soils

Irrespective of type and cropping history, soils with take-all decline (D soils) contained a take-all-decreasing factor that could be transmitted to cereal soils in which take-all had not declined (U soils). As little as 0.001 % by weight of D soil decreased take-all by 50 % in some seedling assays, but in a longer, outdoor test the decrease proved to be temporary and within 10 wk disease increased to equal that in unamended U soil. In two tests in which D soil was amended with U soil there was an unexpected increase in disease in seedling assays. Particulate fractions (0.2-2.0 μm) from leachates of D soils, but not the coarse fractions (> 1 mm, dry sieved; > 150μm, wet sieved) of these soils, decreased disease in assays. Although moved downwards by water, the transmissible factor in D soil did not spread laterally into adjacent unamended U soil and D soil amendments of U soil were most effective as top dressings and least effective when initially placed below assay seedlings. D soil was slightly less effective when added 14 days after planting wheat in U soil, but incubating soil mixtures for up to 4 wk before planting gave variable results, although there was always some decrease in take-all. The transmissible factor could be eliminated by heating moist soil for 30 min at 70 ^oC (but not 60 ^oC). The evidence suggests that the factor is biological, but is insufficient to suggest that it is the cause of take-all decline.