Fusarium wilt of field-grown muskmelon in British Columbia

Abstract

A wilt disease of field-grown muskmelon (Cucumis melo L. var. cantalupensis) was observed during the 1999 and 2000 growing seasons on three farms in the semi-arid interior region near the Okanagan Valley of British Columbia. Initial symptoms were yellowing and wilting of leaves, which sometimes occurred unilaterally; in some cases, this was followed by plant death prior to flowering. The crown area of diseased plants had dark brown lesions that progressed into lateral branches for distances of 45—75 cm. Some lesions also extended into the developing fruit, which frequently became infected and rotted. Stem lesions were accompanied by dark brown exudate droplets and necrosis extended into the cortical and vascular regions. Plants with fruit collapsed when temperatures exceeded 25-s30°C in mid-July and early August. White mycelium developed on the infected crown area and on fruits collected from mature plants in the field, after incubation under high humidity conditions, and macroconidia and microconidia of Fusarium oxysporum Schlechtend.:Fr. were produced. A number of muskmelon cultivars were affected, and the most susceptible were Galia and Castella, while Athena appeared unaffected in the field. Pathogenicity tests demonstrated that muskmelon was the only susceptible host and no disease symptoms developed on cucumber (Cucumis sativus L.), watermelon (Citrullus vulgaris Eckl. & Zeyh), or squash (Cucurbita pepo L.). Based on symptomology and host range, the pathogen was identified as F. oxysporum Schlechtend. f. sp. melonis (Leach & Currence) W.C. Snyder & H.N. Hans. Inoculation of a set of differential muskmelon cultivars revealed that the pathogen was F. oxysporum f. sp. melonis race 1. The pathogen was recovered from diseased transplants in a propagation greenhouse and from seed recovered from naturally infected fruits on mature plants in the field. This is the first report of fusarium wilt of muskmelon in British Columbia, and the first report of race 1 of F. oxysporum f. sp. melonis in Canada.