First report of Fusarium oxysporum f. sp. radicis‐cucumerinum on cucumbers in Turkey

Abstract

During spring in the years 2007, 2008 and 2009, a root and stem rot disease was observed on greenhouse-grown cucumber plants (Cucumis sativus) in the Kumluca district of Antalya, Turkey. In 2009, about 10% of plants were lost due to the disease. The symptoms were severe wilting of the above ground parts associated with yellowing and drying of the lower leaves. A yellowish brown discolouration of the vascular tissues and rotting of the roots and stem were also observed. A fungus was isolated from the discolored vascular tissue of the lower plant stems on potato dextrose agar (PDA). The fungus produced salmon-pink colonies on PDA and light colored colonies on sucrose nutrient agar. On both media orange sporodochia formed. The fungus produced abundant microconidia in false heads on simple phialides (5·00–10·75 × 1·75–3·25 μm). Typical macroconidia were mostly three-septate (31·25–42·50 × 3·00–4·50 μm). Chlamydospores were observed. Based on the morphological characteristics, the fungus was identified as Fusarium oxysporum. Pathogenicity tests were conducted on Cucumis sativus, Luffa acutangula, L. aegyptiaca, Cucurbita maxima × C. moschata, Cucurbita ficifolia, Cucumis melo and Citrullus lanatus plants. An isolate of the fungus was grown on PDA at 23°C for 7 days, with a 12 h photoperiod under fluorescent lamps. The seedlings at the first true leaf stage were inoculated by dipping the roots in a spore suspension produced from PDA cultures (3 × 106 spores per mL) for 10 min. The plants were kept in a growth chamber at 25°C with a 12 h photoperiod. Control plants were treated with sterile distilled water using the same technique. Three weeks after inoculation, cucumber plants presented severe root and stem rot symptoms and died. Luffa aegyptiaca, Cucumis melo and Citrullus lanatus plants showed the same symptoms, whereas Luffa acutangula and Cucurbita species were healthy. Fusarium oxysporum was reisolated from diseased plants to fulfil Koch’s postulates. Two formae speciales of F. oxysporum infect cucumber; f. sp. cucumerinum causes vascular wilting, whereas f. sp. radicis-cucumerinum causes wilting accompanied by root and stem rot. In addition to cucumber, the latter fungus can also infect melon (Cucumis melo), watermelon (Citrullus lanatus) and sponge gourd (Luffa aegyptiaca). Based on the disease symptoms on cucumber plants and differing pathogenicity on different plant species of the Cucurbitaceae, this fungus was identified as F. oxysporum f. sp. radicis-cucumerinum (Vakalounakis, 1996; Punja & Parker, 2000). Wilt disease of cucumber caused by F. oxysporum f. sp. cucumerinum has been recorded in Turkey for a long time (Yıldız & Delen, 1977), but this is the first report of F. oxysporum f. sp. radicis-cucumerinum in this country.