Cultivation of English walnut (Juglans regia L.) has considerable economic potential in South Africa. In February 1999, die-back of walnut trees was observed on ≈30% of trees in an orchard in Gauteng Province, South Africa. Die-back was associated with characteristic cankers and discoloration of the cambial tissue of twigs and young branches. Fifty pieces of discolored tissue from the margins of cankers of five trees were transferred to potato dextrose agar (PDA) after surface-sterilization with 0.5% NaOCl. Fungal colonies emerged from all 50 pieces of diseased tissue. Fungi recovered included Fusarium solani (74.5%), Alternaria tenuissima (17.3%), and a Phoma sp. (6.2%). Small pieces of sterile cheesecloth (10 × 5 mm) were cultured on PDA with an isolate of F. solani from infected walnut trees. Colonized pieces of cheesecloth were applied to stems (≈5-mm diameter) of nine potted J. regia plants in a glasshouse. Each stem was wounded by lightly scraping off a length of bark (5 × 5 mm). Colonized cheesecloth was wrapped around the wound and covered with Parafilm to prevent desiccation of mycelia in the cheesecloth. Nine walnut stems were similarly treated with sterile cheesecloth pieces to serve as control treatments. After 4 weeks, die-back of foliage was observed in all plants artificially inoculated with F. solani. Cheesecloth pieces and surrounding bark were removed, and the length and width of each cambial lesion was measured. The mean area of cambial lesions (length by width) resulting from artificial inoculations was 27.3 × 8.5 mm. None of the control plants developed any symptoms. Koch's postulates were confirmed by consistently reisolating F. solani from inoculated plants. A similar disease of black walnut (J. nigra) was reported by Carlson et al. (1) from five central states in the United States. Our findings represent the first report of F. solani as a causal agent of stem canker of English walnut in South Africa.
Reference: (1) J. C. Carlson et al. N. J. Appl. For. 10:1, 1993.
