Assessment of seed tuber, in‐furrow, and foliar chemical treatments for control of powdery scab (Spongospora subterraneaf.sp.subterranea) of potato

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Abstract
Two field trials were carried out (at different sites at Lincoln, Canterbury, New Zealand) to test the efficacy of chemicals for control of powdery scab (caused by Spongospora subterranea (Wallr.) Lagerh. f. sp. subterranea Tomlinson) of potato (Solanum tuberosum L.). In Trial 1, at a site where potatoes had not been previously grown, 17 seed tuber treatments and eight in‐furrow treatments were applied just prior to, or at, planting, to seed tubers of ‘Agria’ infected with powdery scab, and six foliar treatments were later applied to plants grown from infected tubers. In Trial 2, seven in‐furrow treatments were appl ied at the time of planting healthy seed tubers of ‘Rua’ or ‘Agria’ into a field site heavily infested with S. subterranea, and one foliar treatment was later applied to plants of ‘Rua’. Both trials were irrigated to provide soil conditions likely to encourage development of powdery scab. The trials were harvested at crop maturity, when all tubers were assessed for powdery scab infection, and yield parameters were determined. In Trial 1, treating scab‐infected seed tubers with fluazinam, mancozeb, dichlorophen‐Na, dichlofluanid, or a mixture of fluazinam + mancozeb before planting reduced the incidence of powdery scab in harvested tubers. All of these chemicals, except dichlofluanid, increased the yield (kg/plot) of “marketable” tubers (those with S. subterranea may have affected plant growth and tuber development. In‐furrow treatment with zinc oxide and foliar treatments with phosphorous acid did not control the disease. A low rate in‐furrow mancozeb treatment increased powdery scab incidence. These results confirm that chemicals from several different classes can control powdery scab of potatoes. In‐furrow chemical treatments, both for infected seed tubers planted into disease‐free soil and for healthy seed tubers planted into soil infested with S. subterranea, may provide an alternative to seed tuber treatment for applying chemicals to control the disease. Minimum rates of fluazinam and mancozeb for effective control of powdery scab were: for seed tuber treatment, 25 g fluazinam/100 kg seed tubers or 40 g mancozeb/ 100kg; and for in‐furrow application, 2 kg fluazinam/ ha or 7.5 kg mancozeb/ha. Chemical treatments are recommended as part of an integrated disease management strategy for powdery scab, along with the use of resistant potato cultivars and appropriate cultural practices.

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