Scheduling of Oxadixyl-Based Fungicide Against Late Blight in Potato and Development of Oxadixyl Resistant Strains in Phytophthora infestans
Three sets of experiments were conducted at Shimla/Kufri during 1988–92 to i) Standardize oxadixyl doses and spray intervals against late blight, ii) develop oxadixyl-based fungicide schedules for management of late blight and iii) study development of resistance in Phytophthora infestans against oxadixyl. Optimum fungicide dose varied from cultivar to cultivar, being 0.25% for cv. Kufri Chandramulchi and 0.2% for cvs. Kufri Jyoti and Kufri Badshah respectively. Fifteen days spray interval was ideal for all the cultivars. Cultivars differed in their response to oxadixyl application, cvs. Kufri Badshah and Kufri Jyoti being more responsive than cv. Kufri Chandramukhi. 0.2% and 0.15% cone, of oxadixyl were enough to complement the polygenic resistance in cvs. Kufri Jyoti and Kufri Badshah respectively. For getting optimum disease control, two sprays each of oxadixyl and mancozeb were required on cv. Kufri Chandramukhi (AUDPC 245); one spray of oxadixyl + 3 sprays of mancozeb on cv. Kufri Jyoti (AUDPC 388) and four sprays of mancozeb on cv. Kufri Badshah (AUDPC 446). Oxadixyl-based fungicide schedules were more beneficial to cv. Kufri Chandramukhi and Kufri Jyoti (C/B ratio 2.8 - 5.2) than cv. Kufri Badshah in which they were either at par or, even inferior, to mancozeb (C/B ratio 1:2.4 -1:2.9 as compared to 1:2.9 in mancozeb). Buildup of resistance in Phytophthora infestans to oxadixyl was slow. Five years of continuous application resulted in 4% isolates having developed resistance upto 5 ug/ml cone. only.