Control of powdery scab of potatoes with chemical seed tuber treatments

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Abstract
Powdery scab, caused by Spongospora subterranea (Wallr.) Lagerh. f. sp. subterranea Tomlinson, has become an increasingly important problem on potatoes grown in New Zealand in recent seasons. During the 1992/93 season, the efficacy of 25 chemical seed tuber treatments against tuber‐borne inoculum of S. subterranea was tested in a field trial. Four treatments (stored treatments) were applied 6 weeks before planting whereas 21 were applied within 24 h of planting. Treatments were applied to tubers of cultivars ‘Agria’ (which had severe powdery scab infection) and ‘Rua’ (light infection). Plant emergence was assessed and tubers from the subsequent crop were harvested and scored for powdery scab infection. Yield parameters were also determined. Mancozeb, formaldehyde solution, fluazinam, propineb, dichlorophen‐Na, and mixtures of maneb + zinc oxide, mancozeb + zinc oxide, propineb + zinc oxide, fluazinam + zinc oxide, and tolclofos‐methyl + cupric hydroxide, applied to ‘Agria’ tubers just before planting, reduced the proportion of diseased tubers from 95% at planting to an average of 29% in the subsequent crop compared with the mean for controls of 70%. Inclusion of zinc oxide in these mixtures did not improve disease control. Stored treatments of zinc sulfate and zinc oxide reduced infection in the subsequent crop compared with untreated controls. For ‘Rua’, only dichlorophen‐Na, maneb + zinc oxide dust, fluazinam + zinc oxide, and the stored zinc oxide treatment reduced the proportions of diseased tubers. Phytotoxic effects were observed with: formaldehyde solution (reduced plant emergence, tubers harvested per plot and total tuber yield); zinc sulfate stored (reduced emergence); tolclofos‐methyl + cupric hydroxide (reduced emergence and tubers per plot); tolclofos‐methyl + zinc oxide (reduced tubers per plot); and dichlorophen‐Na (reduced tubers per plot).

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