The cashew plant (Anacardium occidentale L.) (family Anacardiaceae) is native to Brazil. It was introduced in East Africa by the Portuguese in the 16th century where it is now widely cultivated, especially in Tanzania, Kenya, and Mozambique. The processed kernels are the most important product derived from the plant, although in Brazil and India, juices, jam, and alcoholic and soft drinks are also made from the pear-shaped edible receptacle. The plant is currently being evaluated in South Africa for commercial production. During May 2002, at least 25% of 5-year-old cashew trees grown from seed in the northern KwaZulu-Natal Province of South Africa were infected with powdery mildew. Signs included extensive growth of white, superficial mycelium bearing upright conidiophores on young shoots with tender leaves, inflorescences, and young receptacles. In severely affected trees, approximately 35% of young shoots and 45% of young receptacles displayed signs of powdery mildew. Severely infected young leaves were brown and deformed in contrast to older leaves that were unaffected. Microscopic examination of diseased tissue revealed hyaline, cylindrical-to-slightly doliform, single-celled conidia (10 to 17.5 × 2.5 to 5 μm) borne in chains. The pathogen was subsequently identified as Oidium anacardii Noack on the basis of morphology (1). No other species of powdery mildew fungi have been reported on cashew. A pathogenicity test was conducted by gently pressing a heavily diseased leaf onto two healthy leaves of each of 10 cashew plants maintained in pots on open benches in the glasshouse at 22 to 25°C and mean relative humidity of 65%. Control treatments entailed pressing an asymptomatic leaf onto each of two healthy leaves per plant. The experiment was conducted three times. After 14 days, at least one powdery mildew colony had developed on 80% of inoculated leaves but were absent from all replications of the control treatment. The source of inoculum for this reported outbreak is unknown, although O. anacardii is known to occur in southern Mozambique less than 100 km from the infected site. Cashew powdery mildew was first officially reported in Tanzania in 1979 where significant crop losses, partially attributable to the pathogen, have been recorded since (3). No significant damage to production has been recorded in Brazil (2). To our knowledge, this is the first report of O. anacardii occurring on cashew in South Africa.
References: (1) E. Castellani and F. Casulli. Rivista di Agricoltura Subtropicale e Tropicale 75:211, 1981. (2) F. C. O. Freire et al. Crop Prot. 21:489, 2002. (3) P. J. Martin et al. Crop Prot. 16:5, 1996.