First Outbreak of Blackleg Caused by Phoma lingam in Commercial Canola Fields in Argentina.

Abstract

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% 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%. 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% ethanol, surface sterilized with NaOCl (1%) 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°C for 4 days, and then plates were incubated under 12 h of NUV light and 12-h dark for 7 days. Four 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 two isolates by spray-inoculating foliage of 3-week-old canola plants of cvs. Impulse, Master, Mistral, and Teddy with a conidial suspension (1 × 106 conidia per ml). The experiment, which included four inoculated plants per isolate and two noninoculated control plants for each cultivar, was performed in a greenhouse at 22 to 24°C and maintained at 75% 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 first report of an outbreak of P. lingam infection in commercial crops in the main canola-production region in Argentina.