Occurrence of Fusarium Wilt on Canola Caused by Fusarium oxysporum f. sp. conglutinans in Argentina.

Abstract

Canola (Brassica napus) is a developing oleaginous crop grown commercially in Argentina, primarily in the southeastern region of Buenos Aires Province. Since 2002, plants exhibiting symptoms of wilt and xylem discoloration were observed in canola plants in experimental field plots located at the University of Buenos Aires, Buenos Aires, Argentina. Average disease incidence in 5- to 6-month-old canola cultivars developed in different countries was 18% (range = 9 to 27%). Disease symptoms that included yellowing, wilting, stunting, and necrosis of leaf tissue and suppressed root development appeared in irregular-shaped patches following the rows of plants. The first symptom observed was leaf yellowing followed by an irregular, brown necrosis of the leaf margins. Lesions coalesced to form large necrotic areas that led to severe defoliation beginning with the lower leaves. As the disease developed, a pale brown discoloration girdled the stems that progressed from the basal tissues to the apex. Affected plants were stunted and had small pods with no seeds. Diseased plants eventually collapsed and died. From June to July 2003, six samples consisting of five affected plants per sample were randomly collected from experimental field plots. Pieces (1 cm long) of disease basal stem tissue were thoroughly washed, surface sterilized in 1% sodium hypochlorite for 1 min, rinsed in sterile distilled water, blotted dry on sterile Whatman's filter paper, and incubated on potato dextrose agar in the dark at 26°C for 10 days. Ten resulting colonies were examined microscopically and identified as Fusarium oxysporum Schlechtend.:Fr. f. sp. conglutinans (Wollenweb.) W.C. Snyder & H.N.Hans. (3). Pathogenicity tests for three single-spore isolates of the fungus were performed on 6-week-old canola plants of cvs. Impulse, Master, Mistral, Monty, Rivette, and Trooper. Koch's postulates were completed for each isolate by dipping the roots of seedlings in a conidial suspension (2 times; 105 conidia per ml) for 15 min. Plants were repotted in a sterilized soil mix (soil/sand, 2:1). The experiment, which included five inoculated plants and three noninoculated (roots dipped in sterile distilled water) control plants for each cultivar, was conducted in a greenhouse at 23 to 25°C and 75% relative humidity with no supplemental light. Characteristic symptoms, identical to the original observations, developed within 14 days after inoculation on 100% of the inoculated plants for all three isolates. The pathogen was successfully reisolated from internal diseased stem tissue in all instances. Symptoms included stunted seedlings, leaf necrosis, and external stem discoloration. None of the control plants developed disease. The experiment was repeated once with similar results. F. oxysporum f. sp. conglutinans, which has been reported to cause disease in canola in Canada (1) and the United States (2), represents a serious threat to the main canola cultivars grown in Argentina. To our knowledge, this is the first report of canola wilt incited by F. oxysporum f. sp. conglutinans in Argentina.