A NEW CANKER DISEASE OF APPLE, PEAR, AND PLUM ROOTSTOCKS CAUSED BY DIAPORTHE AMBIGUA IN SOUTH-AFRICA

Diaporthe ambigua was found to be the cause of a newly recognized dise ase of apple, pear, and plum rootstocks in South Africa. The fungus wa s isolated from margins of cankers on rootstocks and branches of disea sed 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 per ithecia, separate or in groups, with elongated necks protruding from t he bark under moist conditions, and stromata delimited at the outer ma rgins by broad, blackened zones. Pathogenicity tests were conducted on S-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 st erile apple twigs on water agar medium. D. ambigua was consistently as sociated 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 ro otstock tended to be of the same VC group, whereas those from adjacent rootstocks usually represented different genetic entities.