Virulence of Botryosphaeria dothidea and Botryosphaeria obtusa on apple and management of stem cankers with fungicides

The virulence of isolates of Botryosphaeria dothidea and B. obtusa was compared on apple fruit, trunks of 2-year-old apple trees, and twigs of mature apple trees. In general, B. dothidea isolates were more virulent than B. obtusa isolates. There was no correlation between virulence on fruit and virulence on 2-year-old trees. Several compounds were tested as topical wound treatments to control stem cankers caused by B. dothidea and B. obtusa. Benomyl, kresoxim-methyl, and trifloxystrobin, when applied at rates recommended for foliar application, consistently reduced the incidence and size of cankers compared with the water control. Clove oil, garlic extract, and neem oil did not reduce the incidence and size of cankers compared with the water control. Kresoxim-methyl was also effective when applied with an air-blast sprayer, despite incomplete spray coverage. Although fungicides reduced external canker symptoms, B. dothidea and B. obtusa were frequently isolated from xylem tissue, suggesting that fungicides might not provide long-term control of these pathogens.