April 13, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

Occurrence of Fusarium Wilt on Canola caused by Fusarium oxysporum f. sp. conglutinans in Argentina
S. A. Gaetan, Catedra de Fitopatologia, Facultad de Agronomia, Universidad de Buenos Aires, Avda. San Martin 4453 (1417), Buenos Aires, Argentina. Plant Dis. 89:432, 2005; published on-line as DOI: 10.1094/PD-89-0432C. Accepted for publication 13 Jan 2005.

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 percent (range = 9 to 27 percent).

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, 6 samples consisting of 5 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 percent 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 deg C for 10 days.

10 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 (200 000 conidia per ml) for 15 min. Plants were repotted in a sterilized soil mix (soil/sand, 2:1). The experiment, which included 5 inoculated plants and 3 noninoculated (roots dipped in sterile distilled water) control plants for each cultivar, was conducted in a greenhouse at 23 to 25 deg C and 75 percent relative humidity with no supplemental light.

Characteristic symptoms, identical to the original observations, developed within 14 days after inoculation on 100 percent of the inoculated plants for all 3 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 1st report of canola wilt incited by _F. oxysporum_ f. sp. _conglutinans_ in Argentina.

References:
(1) D. Bernard et al. Can. Plant Dis. Surv. 81:102, 2001.
(2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989.
(3) P. E. Nelson et al. Fusarium species. An Illustrated Manual for Identification. Pennsylvania State University Press. University Park, PA, 1983.

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ProMED-mail <promed@promedmail.org>

[Fusarium wilt of canola is a disease that has only recently been discovered in western Canada, and little is known about the host/pathogen relationship. Canola varieties differ in disease susceptibility, and one of the first priorities has been to determine the amount of genetic resistance currently available in commercial canola cultivars. Varieties, inoculated with _Fusarium oxysporum_ f. sp _conglutinans_, at the seedling stage in a greenhouse were classified as susceptible or resistant. A majority of varieties are resistant. Management of this disease should be possible by simply screening susceptible cultivars. In addition, field results from locations with natural incidence of fusarium wilt have correlated well with results from this artificial inoculation procedure,
providing some validity to this method of testing. The fact that infected cultivars can be readily detected in fusarium-infested field plots will assist research in disease management.

Link: <http://www.usask.ca/agriculture/plants/symposium.pdf> - Mod.DH]