First Report of Fusarium Stem and Root Rot of Greenhouse Cucumber Caused by Fusarium oxysporum f. sp. radicis-cucumerinum in Ontario.

Fusarium stem and root rot on greenhouse long English cucumber (Cucumis sativus L.) cvs. Bodega and Gardon was observed at four commercial greenhouses in Leamington, Ontario, Canada. Losses of 25 to 35%, representing 2.5 ha, were noted. The greenhouse cucumber industry in Ontario comprises 119 ha, with sales in 1999 of $78 million (Canadian). Some foliar chlorosis developed slowly on the lower foliage of affected plants. Basal stem tissue developed a yellow buff discoloration with superficial rot, followed by advanced stages of stem disintegration, which were accompanied by the production of white buff fungus mycelium and orange spore masses externally, and yellowish or reddish brown discoloration of vascular tissue that extended for 5 to 6 cm. Yellowish brown to brown external discoloration extended throughout the affected roots. In contrast, the main symptoms on cucumber infected by F. oxysporum f. sp. cucurbitaceae are wilt, yellowing, and vascular discoloration. The shape of macroconidia, the presence of microconidia on short lateral phialides, and the occurrence of chlamydospores and sporodochia on acidified potato dextrose agar (aPDA) were used to identify the isolates as F. oxysporum. To confirm pathogenicity and formae specialis designation, the roots of a range of 19-day-old host plants of the family Cucurbitaceae and some of the family Solanaceae were clipped and inoculated with the use of a root-dip method (1), with 5 × 10 spores per ml from 6-day-old aPDA cultures of three isolates and a water check with six plants per isolate. Plants were subsequently grown in a greenhouse soil mix (3:2 Fox sandy loam-peat moss, vol/vol) at 22°C day/19°C night, under environmental conditions similar to those reported elsewhere (1). A 0 (healthy) to 5 (dead) scale was used to rate plants after 30 days. In preliminary studies, cucumber cv. Corona, inoculated with various isolates and grown at 26 to 28°C for 30 days, remained asymptomatic, similar to the results of Vakalounakis 1996 (2) at 30°C and Punja and Parker 2000 (1) at 32°C. Muskmelon (Cucumis melo L. cvs. Early Dawn and Summet), watermelon (Citrullus vulgaris Schrad. cv. Yellow Doll), and cucumber cvs. Tasty Green, Odessa, Mustang, and Orient Express were the most susceptible and had ratings of 3.7, 3.0, 2.3, 3.8, 3.3, 1.1, and 1.6, respectively, with stem and root symptoms similar to those we observed in naturally infected plants and reported previously in other work (1). Cucumber cvs. Calypso, Slicemaster, Flamingo, and Marketmore 76 were less susceptible, with ratings of 0.6, 1.0, 0.1, and 1.0, respectively. No symptoms were observed on tomato (Lycopersicon esculentum Mill. cv. Trust), pepper (Capsicum annuum L. cv. Cubico), or squash (Cucurbita pepo L. cv. Taybelle) in our trials, which is in agreement with other work (1). Symptom development on a range of hosts and the cultural and morphological characteristics of the imperfect state of the fungus in vivo and in vitro confirm the identity of the fungus. To our knowledge, this is the first report of F. oxysporum f. sp. radicis-cucumerinum in Ontario. References: (1) Z. K. Punja and M. Parker. Can. J. Plant Pathol. 22:349, 2000. (2) D. J. Vakalounakis. Plant Dis. 80:313, 1996.