Development of fusarium root and stem rot, a new disease on greenhouse cucumber in British Columbia, caused byFusarium oxysporumf. sp. radicis-cucumerinum

A root and stem rot of greenhouse cucumber was first observed in the Fraser Valley of British Columbia in 1994 and has since increased in frequency and severity. Affected plants wilted at the fruit-bearing stage, especially at temperatures over 27°C, and mycelial growth and orange spore masses developed on the crown and stem. The pathogen was isolated from roots and crowns, and from cortical and vascular tissues up to 75 cm from the crown. Reactions of 25 cucumber cultivars ranged from highly susceptible to moderately resistant; the widely-grown long English cultivars Flamingo, Mustang, and Serami were all highly susceptible. The pathogen was identified as Fusarium oxysporum Schlechtend.:Fr. forma specialis radicis-cucumerinum D.J. Vakalounakis (F.o.r.c), which has not previously been reported in Canada but occurs in Greece and the Netherlands. Muskmelon (Cucumis melo L.), squash (Cucurbita pepo L.), watermelon (Citrullus vulgaris Schrad.), and gourd (Luffa aegyptiaca Mill.) developed root and stem symptoms similar to those on cucumber when inoculated using a root dip method. Random amplified polymorphic DNA analysis using eight primers revealed that the pathogen was distinct from Fusarium oxysporum f. sp. cucumerinum J.H. Owen (F.o.c, cause of fusarium wilt). Pectolytic enzyme production in vitro was greater in F.o.r.c. than F.o.c. The optimal temperature range for growth of F.o.r.c. on potato dextrose agar was 24-27°C. Disease developed at 17 and 24°C but not at 32°C. The pathogen was recovered on Komada's medium at a frequency of 105 colony-forming units (cfu)/cm3 from the growing substrate and run-off water, but at a low frequency from the air. The initial sources of inoculum may be contaminated seed or growing medium, and infection of seedlings can occur within the first 4 weeks of growth. Artificial seed inoculation caused damping-off on cucumber and muskmelon seedlings. Wounding of roots and low temperatures during seedling development and fruit load on mature plants enhance disease severity. Fusarium root and stem rot has the potential to spread to other regions of Canada.