Evaluation of fungicides for control of Alternaria leaf spot of Pseudopanax

Alternaria tenuissima causes a leafspot disorder of Pseudopanax 'Gold Splash', a New Zealand native shrub with export potential. A range of fungicides viz., bitertanol, captafol, chlorothalonil, dichlofluanid, iprodione, mancozeb, prochloraz, procymidone, propineb, and vinclozolin were tested for efficacy against the pathogen. Three techniques were used based on growth on: (1) fungicide amended media; (2) leaf disc; and (3) intact plants. The efficacy of fungicides in agar was not directly related to their effectiveness on leaf discs or on intact plants. In contrast, all fungicides effective on leaf discs were also effective on intact plants. The leaf disc technique showed that bitertanol, chlorothalonil, iprodione, prochloraz, and vinclozolin became less effective 4 weeks following application. In contrast, captafol, dichlofluanid, and propineb were effective inhibitors throughout the 4 weeks tested by the leaf disc technique. On pot plants in a greenhouse, mancozeb was the most effective protectant followed by propineb, prochloraz, dichlofluanid, and captafol.