SUMMARY Several lines of circumstantial evidence collectively indicated that poor early growth of apple (‘replant disease’) might be associated with the effects of soil‐borne pythiaceous fungi. This hypothesis was supported by pathogenicity tests. All isolates tested of P. sylvaticum and certain isolates of seven other Pythium spp. significantly reduced the growth of apple seedlings. The growth reductions caused by certain Pythium isolates were of comparable magnitude to the growth increases occurring after chloropicrin‐fumigation of apple orchard soils. The Pythium isolates most virulent to apple were of low virulence to a clonal cherry rootstock. Reappraisal of the nature of the disease as a non‐specific soil malaise is consistent with established features of the pathology of Pythium spp. The disease, however, is an ill‐defined ‘poor growth phenomenon’ with no diagnostic symptoms and conclusive evidence that Pythium spp. are widely causal is likely to be elusive.

During routine, annual, field wilt‐resistance trials of hop progeny, ‘breakdowns’ of resistance occurred in control varieties. Pot‐plant pathogenicity tests of isolates from these trials, and other isolates obtained during farm surveys, confirmed the existence of apparently new strains potentially highly pathogenic to resistant cultivars. Results of the tests provided little evidence that enhanced pathogenicity was associated with specificity. The assumption of essentially host nonspecific pathogenicity was consistent with other observations on the inheritance of resistance. Highly pathogenic strains were identified among isolates from nine farms in Kent. The locations and ownership of these farms, and their wilt histories, provided strong circumstantial evidence that these strains had spread from a single focus, as did progressive wilt originally. Legislation, introduced to restrict further spread, may have been too late.

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In two large-scale observation trials on commercial farms inorganic nitrogen fertilizer was applied at 70 lb. N/acre (N I), 140 lb. N/acre (N2) and at a normal commercial rate of 210 lb. N/acre (N3). Mean wilt incidence was 60 and 25% less with N I and N 2 respectively than with N3 during 5 years at one site and 3 years at the other. During these periods wilt declined progressively and this was tentatively attributed to the reduced intensity of host colonization, and to progressive declines in soil infectivity resulting from effects on the quantity, quality and longevity of annually produced inoculum.Marked annual fluctuations of wilt incidence and yields occurred at one site and these were associated with weather in the spring and early summer: wilt incidence was inversely related to soil temperature, and yield was directly related to rainfall. Yields were not diminished by the low-N treatments, but clear correlations between yields and wilt incidence were probably obscured by the differential effects of weather conditions at the three levels of N application.The results emphasized the importance of reducing traditionally high N applications and, on farms where fluctuating wilt is severe, of applying the minimum levels of N commensurate with the maintenance of satisfactory yields.

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