Isolates of Helminthosporium solani were obtained from potato tubers and cultured on agar containing thiabendazole. Some grew on agar containing 100 mg/l and these were classified as resistant, whereas sensitive isolates produced no mycelial growth on agar containing 5 mg/l. Isolates were also found that were intermediate in their sensitivity to the fungicide. All isolates from Scottish virus tested stem cutting stocks (VTSC) on two farms were resistant whereas those from another farm were sensitive to the fungicide. The proportion of resistant isolates from eight English once‐grown seed stocks ranged from 0 to 96%. There has been no previous report of fungicide resistance in H. solani and the possible reasons for these results and their implications are discussed.
Fusarium species obtained from stored potato tubers affected with dry rot were grown on agar containing thiabendazole. All 40 isolates of F. coeruleum and 60 isolates of F. avenaceum tested were sensitive to the fungicide, but 68% of the 85 isolates of F. sulphureum and one isolate of F. culmorum were classified as resistant. When isolates were made from dry rots on tubers that had been treated with thiabendazole during loading into store, all 81 isolates of F. sulphureum were resistant, whereas all the isolates of F. coeruleum (25), F. avenaceum (4) and Phoma foveata (10) were sensitive. Resistance was not found in five isolates of Cylindrocarpon destructans. All the Fusarium spp. were sensitive to imazalil and were pathogenic when inoculated into potato tubers. Resistant and sensitive isolates of F. sulphureum caused rots of similar size.
SummarySeed tubers of cvs Désirée and Pentland Crown with different severities of black dot were planted in 1988 and 1989 at Rothamsted in fields in 4– or 7‐course rotations, respectively. Tubers treated with prochloraz (1988) or imazalil (1989) were planted in some plots, and in others Colletotrichum coccodes inoculum was added to the soil at planting. In further experiments at Mepal, Cambridgeshire in 1989 and 1990 and at Rothamsted in 1990 on sites where potatoes had not been grown for more than 15 years, large amounts of inoculum were added to the soil around disease‐free seed tubers of two (1989) or three (1990) cultivars at planting. In all experiments plants were sampled during the season and the effects of treatments on disease development, growth and yield were recorded.Disease on roots, stem bases and tubers was found early in the season and was more severe on Désirée than on Pentland Crown plants from fields in 4– or 7‐course rotations. Severity increased throughout the season and with increasing amounts of disease on the seed tubers, especially with Desiree. Disease was also found on plants from disease‐free tubers and was more severe in 1988 than 1989. At harvest, black dot on tubers was significantly more severe from severely affected than from disease‐free seed, and was most severe where inoculum, especially large amounts, had been added at planting. Fungicide treatment decreased disease early in the season but had no effect on tuber infection at harvest. In 1989 the weight loss of seed tubers during sprouting increased with increasing amounts of black dot, but the disease had little effect on plant size through the season. At harvest the yield of ware tubers (>50 mm) decreased with severe disease but total tuber yields were not significantly affected. However, at harvest in 1988 severely affected seed yielded significantly less than healthy seed.Plants grown from mini‐tubers were free from disease on sites where potatoes had not been grown for at least 15 years. Inoculum applied at planting caused severe disease on all cultivars in both years, whereas disease was slight on uninoculated plants. Inoculated plants senesced early at Mepal in 1990, but there were no significant differences in total tuber yield in any experiment. However, yields of ware tubers (>50 mm) were sometimes decreased and the total tuber number per plant increased.
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