Scytalidium wilt of citrus

In July 1998, a sudden wilt of 'Star Ruby' grapefruit (Citrus paradisi Macf .) occurred in Israel in a region with a warm Mediterranean climate. The wi lt of the middle and upper canopy of main limbs was accompanied by gum oozi ng from the affected branches. The bark of these branches attained a dark c olour and the epidermis sloughed off easily revealing a mass of black powde r, resulting from copious sporulation of dark conidia. Both the bark and th e xylem were intensively colonized with mycelium. The fungus was identified as Scytalidium lignicola Pesante, based on the characteristic mixture of s ome colourless and dark conidia, produced in branched chains by conversion of the vegetative hyphae. Artificial inoculations induced typical disease s ymptoms and the fungus was reisolated, thus confirming Koch's postulates. S imilar symptoms appeared in 'Star Ruby' interim segments which had been cut and top grafted with various citrus cultivars. This is the first detailed report of pathogenicity of S. lignicola to citrus trees. It is postulated t hat the disease developed after predisposition of 'Star Ruby' grapefruit tr ees by comprehensive pruning followed by extremely hot conditions.