Scab (Sphaceloma batatas) a new disease of sweet potatoes in Australia: fungicide and cultivar evaluation

Abstract

Since 1982, stem and foliage scab caused by Sphaceloma batatas has occurred in crops of sweet potatoes cv. 'Porto Rico' on the Atherton Tableland in North Queensland. This is the first report of the occurrence of this disease in Australia. We developed a natural substrate agar suitable for sporulation of S. batatas and demonstrated that a suspension of conidia was pathogenic, producing lesions on the stem, petioles and leaf veins of sweet potatoes. Nine fungicides were tested for control of stem and foliage scab in a field experiment. Benomyl (400 g a.i./ ha), chlorothalonil (1300 g a.i./ha), captafol (1520 g a.i./ha), fentin hydroxide (300 g a.i./ha), copper oxychloride (1500 g a.i./ha) and mancozeb (1 500 g a.i./ha) reduced the disease at the earliest assessment, 55 days after planting (P= 0.05). However, at 82 days after planting, copper oxychloride and mancozeb were no longer controlling stem and foliage scab, and, at the third assessment (after 111 days) only benomyl and chlorothalonil reduced the disease (P = 0.05). A field rating after 195 days confirmed that benomyl provided significantly better disease control (P= 0.05) than any other treatment throughout the season. There was a negative correlation between yield and disease levels at 55 and 82 days (r= -0.39 and -0.36, P= 0.05), but not at 111 and 195 days (r= -0.23 and -0.10. n.s.). Twenty sweet potato cultivars were evaluated for disease resistance. Of the major commercial cultivars grown in Queensland, Centennial '83 was highly resistant, Beerwah Gold was moderately resistant, while Red Abundance, NC-3, Lo 323 and Porto Rico were susceptible to stem and foliage scab.