March 18, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First outbreak of blackleg Caused by Phoma lingam in commercial canola fields in Argentina
S. A. Gaetan, Facultad de Agronomia, Universidad de Buenos Aires, Avda. San Martin 4453 (1417), Buenos Aires, Argentina. Plant Dis. 89:435, 2005; published on-line as DOI: 10.1094/PD-89-0435B. Accepted for publication 24
Jan 2005.

Canola (_Brassica napus_) was introduced as an alternative crop to wheat in Argentina. During 2004, typical symptoms of foliar blackleg disease were observed on canola plants in commercial and experimental fields in southern Buenos Aires Province. Average disease incidence was 32 percent on 2- to 4-month-old plants of canola cvs. Impulse, Master, Mistral, and Teddy. The range of incidence on these cultivars was 21 to 43 percent.

Foliar symptoms were randomly distributed at seedling, rosette, and flowering stages. Symptoms included necrosis and chlorosis of the affected leaf tissue and defoliation. Foliar leaf spots were circular to irregularly
oval, 5 to 12 mm in diameter, pale brown in the center, and grayish green at the margin. Small, black pycnidia formed in the center of the adaxial surface of diseased foliage. Under favorable temperature and moisture conditions, lesions enlarged and coalesced. Older lesions appeared chlorotic and desiccated with shredded tissue at the center. A severe defoliation of the lower leaves was observed. As the disease developed, basal stem cankers formed on these plants, although disease incidence in this phase was low.

Pieces (0.5 cm long) were taken from leaves and stems of diseased plants, dipped in 70 percent ethanol, surface sterilized with NaOCl (1 percent) for 2 min, and rinsed in sterile water. Each segment was blotted dry and placed
on potato dextrose agar (PDA). Plates were incubated in the dark at 26 deg C for 4 days, and then plates were incubated under 12 h of NUV light and 12-h dark for 7 days. 4 fungal isolates were obtained. The fungi were
examined microscopically and confirmed as _Phoma lingam_ (Tode:Fr.) Desmaz. (_Leptosphaeria maculans_ (Desmaz.) Ces. & De Not) (3,4). _P. lingam_ was the only fungus isolated from the infected tissue. The teleomorph stage was not observed.

Koch's postulates were completed for 2 isolates by spray-inoculating foliage of 3-week-old canola plants of cvs. Impulse, Master, Mistral, and Teddy with a conidial suspension (1 million conidia per ml). The experiment, which included 4 inoculated plants per isolate and 2 noninoculated control plants for each cultivar, was performed in a
greenhouse at 22 to 24 deg C and maintained at 75 percent relative humidity with no supplemental light. Inoculated and control plants were covered with polyethylene bags for 48 h after inoculation.

Plants developed small, pale brown lesions on leaves within 11 days for both isolates, and the pathogen was reisolated. Control plants, inoculated only with sterile distilled water, remained symptomless. The experiment was
repeated once with similar results.

Blackleg is the most important fungal disease affecting canola with a worldwide distribution (1,2). In 1995, the disease was first observed on canola in Argentina in northern Buenos Aires Province, but only in experimental field plots with a low disease incidence. Since that time, it has not been found in other areas where canola is produced. The results emphasize the importance of this pathogen in Argentina, since at the current time most commercial cultivars were susceptible to _P. lingam_.

To our knowledge, this is the 1st report of an outbreak of _P. lingam_ infection in commercial crops in the main canola-production region in Argentina.

References:
(1) H. A. Lamey and D. E. Hershman, Plant Dis. 77:1263, 1993.
(2) G. A. Petri. Can. Plant Dis. Surv. 65:43, 1985.
(3) E. Punithalingham and P. Holliday. No. 331 in: Descriptions of Pathogenic Fungi and Bacteria. CMI Kew, Surrey, UK, 1972.
(4) B. C. Sutton. Fungi imperfecti with pycnidia, acervuli and stromata. Pages 386-388 in: The Coelomycetes. CMI, Kew, Surrey, England, 1980.

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[The source of the infection is likely contaminated canola seed or crop residue in the field. Disease management of canola requires annual monitoring of the crop for disease severity, plant the most resistant cultivars suited to the rainfall and farming system, establish a barrier of at least 500 meters between the current crop and the previous canola stubble, and consider using a fungicide and/or destroying last year's canola stubble if warranted. - Mod.DH]

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<http://www.gov.on.ca/OMAFRA/english/crops/facts/85-043.htm>