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A New Canker Disease of Apple, Pear, and Plum Rootstocks-Caused by Diaporthe ambigua in South Africa. W. A. Smit, Department of Plant Biotechnology and Pathology, INFRUITEC (Stellenbosch Institute for Fruit Technology), Agricultural Research Council, Private Bag X5013, Stellenbosch 7599, South Africa. C. D. Viljoen, B. D. Wingfield, and M. J. Wingfield, Department of Microbiology and Biochemistry, University of the Orange Free State, Bloemfontein 9300, South Africa; and F. J. Calitz, ARC - Agrimetrics Institute, Private Bag X5013, Stellenbosch 7599, South Africa. Plant Dis. 80:1331-1335. Accepted for publication 6 August 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1331.

Diaporthe ambigua was found to be the cause of a newly recognized disease of apple, pear, and plum rootstocks in South Africa. The fungus was isolated from margins of cankers on rootstocks and branches of diseased trees, and from spores taken from perithecia and pycnidia imbedded in cankers. Characteristic symptoms included sunken, pointed lesions with marginal longitudinal cracks. Key identifying characters were perithecia, separate or in groups, with elongated necks protruding from the bark under moist conditions, and stromata delimited at the outer margins by broad, blackened zones. Pathogenicity tests were conducted on 3-year-old apple, pear, and plum rootstocks. Vegetative compatibility (VC) groups were identified by pairing isolates on oatmeal agar, and the sexual system was studied by inoculating single ascospores onto sterile apple twigs on water agar medium. D. ambigua was consistently associated with cankers on apple, pear, and plum rootstocks, and testing of Koch’s postulates demonstrated its pathogenicity conclusively. The fungus was found to be homothallic. In addition, isolates from one rootstock tended to be of the same VC group, whereas those from adjacent rootstocks usually represented different genetic entities.

Keyword(s): Malus domestica, Prunus salicina, Pyrus communis