Dr. Y.S. Parmar University of Horticulture and Forestry, Scab Monitoring and Research Laboratory, Kotkhai, Distt., Shimla (H.P.) 171202
Four sterol-inhibiting fungicides, viz. bitertanol (0.075%), hexaconazole (0.03%), myclobutanil (0.04%) and penconazole (0.05%) were evaluated to determine their curative (after-infection) and eradicant (post-symptom) activity against Venturia inaequalis (Cke.) Wint. in apple orchard. When applied within 72 h of start of infection period, all of these fungicides completely suppressed the appearance of scab lesions on tagged young leaves of apple. These fungicides also depicted an excellent pre-symptom activity when applied after 168 h of the infection period; penconazole (0.05%) being the most effective with 83.9 per cent disease control followed by bitertanol (0.075%) hexaconazole (0.03%) and myclobutanil (0.04%). The treated leaves produced atypical reddish brown lesions. When applied on apple leaves having well established freshly sporulating lesions, the test SI fungicides reduced the conidia formation significantly, the effect being more pronounced after seven days. By the day nine, conidia formation again increased in the lesions and was found to be suppressed in the leaves of the trees which received a second spray at seven day interval. Penconazole (0.05%) and myclobutanil (0.04%) were the most effective with 90 and 85 per cent reduction, respectively. Two consecutive sprays of these fungicides resulted in 89–93 per cent reduction in the conidia production by the 14th day. A marked decrease in conidia germination was observed and the effect was highly conspicuous after seven days of spray. Per cent reduction ranged from 59.6 to 93.6 in myclobutanil and bitertanol treated lesions, respectively. The second spray further reduced the conidia germinability after day 14 which ranged from 91.4 per cent in hexaconazole to 100 per cent in bitertanol treated lesions.
