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Disease Note.

Bark Canker of Avocado, a New Disease Presumably Caused by Pseudomonas syringae in South Africa. L. Korsten, Department of Microbiology and Plant Pathology, University of Pretoria, South Africa. J. M. Kotzé, Department of Microbiology and Plant Pathology, University of Pretoria, South Africa. Plant Dis. 71:850. Accepted for publication 22 April 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0850F.

In 1980, a bark canker was first observed on 8-yr-old avocado (Persea americana Mill.) plantings in South Africa. The disease is characterized by cankerous, necrotic, watery pockets under the bark on trunks and branches, with brown streaks in the vascular tissue extending from the pockets. Infected areas are slightly sunken and darker than surrounding healthy bark. During spring and autumn, a white exudate surrounds the lesions. Leaves and fruit are not affected. The disease occurs mainly on older trees but has also been observed on young trees and new nursery plantings in the orchard. Heavily infected trees show poor growth and defoliation. The disease spreads erratically and occurs predominantly on the cultivars Hass and Edranol and to a lesser extent on Fuerte. The disease has been observed in all major avocado-growing areas of South Africa but is of no economic importance to the industry at present because of the small percentage of trees infected. Pseudomonas syringae van Hall was isolated from the periphery of the thin, brown streaks in the vascular tissue, although not consistently. When artificially inoculated into 2-yr-old Hass seedlings, this P. syringae caused necrosis and browning of the vascular tissue and could be readily reisolated from the seedlings. Comparison with two P. s. pv. syringae isolates from apricot and cherry cankers, P. s. pv. savastanoi and P. aeruginosa, indicated that the P. syringae is probably a new pathovar.