Development of fusarium root and stem rot, a new disease on greenhouse cucumber in British Columbia, caused by Fusarium oxysporum f. sp radicis-cucumerinum

A root and stem rot of greenhouse cucumber was first observed in the Eraser Valley of British Columbia in 1994 and has since increased in frequency an d severity. Affected plants wilted at the fruit-bearing stage, especially a t temperatures over 27 degreesC, and mycelial growth and orange spore masse s 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 moder ately resistant; the widely-grown long English cultivars Flamingo, Mustang, and Serami were all highly susceptible. The pathogen was identified as Fus arium oxysporum Schlechtend.:Fr. forma specialis radicis-cucumerinum D.J. V akalounakis (F.o.r.c.), which has not previously been reported in Canada bu t occurs in Greece and the Netherlands. Muskmelon (Cucumis melo L.), squash (Cucurbita pepo L.), watermelon (Citrullus vulgaris Schrad.), and gourd (L uffa aegyptiaca Mill.) developed root and stem symptoms similar to those on cucumber when inoculated using a root dip method. Random amplified polymor phic DNA analysis using eight primers revealed that the pathogen was distin ct from Fusarium oxysporum f. sp. cucumerinum J.H. Owen (F.o.c., cause of f usarium 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 potat o dextrose agar was 24-27 degreesC. Disease developed at 17 and 24 degreesC but not at 32 degreesC. The pathogen was recovered on Komada's medium at a frequency of 10(5) colony-forming units (cfu)/cm(3) from the growing subst rate and run-off water, but at a low frequency from the air. The initial so urces 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. Woundi ng 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.