Chemical control of onion white rot (Sclerotium cepivorumBerk.) in the Pukekohe district of New Zealand

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Abstract
The fungicides iprodione, vinclozolin, and procymidone were compared for the control of onion white rot in two trials in the Pukekohe (South Auckland) district of New Zealand during the 1984–85 and 1985–86 crop years. Neither iprodione nor vinclozolin was effective as a seed treatment or as a combination of seed treatment and foliar sprays. Procymidone was highly effective when used in a combination of seed treatment (5.0 g a.i./kg seed) and foliar sprays (0.75 kg a.i./ha monthly × 4). In the year of higher disease incidence (1984–85) plant losses were reduced from 58 to 20% using a combination of procymidone seed treatment and foliar sprays. In-furrow applications of procymidone (3.0 kg a.i./ha) at planting were highly effective in controlling the disease but were severely phytotoxic to young plants. Highest yields were obtained in procymidone-treated plots. Seed treatment with iprodione gave higher yields than did untreated controls (captan 8 g a.i./kg seed) despite the failure to control white rot. The failure of both iprodione and vinclozolin to control disease in these trials, whereas both were effective in previous trials, suggests that these chemicals may have succumbed to the phenomenon of enhanced degradation in the soil.