First Report of Gray Leaf Spot Caused by Alternaria brassicae on Canola in Argentina

Canola (Brassica napus) is a developing oleaginous crop grown commercially in Argentina. During 2003, typical symptoms of a foliar disease were observed on canola plants in experimental field plots in Buenos Aires. Average disease incidence across 14 6-month-old canola cultivars was 27% (range 12 to 42%). Climatic conditions in Buenos Aires during August 2003 included moderate temperatures and periods with high humidity, which were apparently favorable for disease development. Symptoms were observed on leaves, stems, and pods. Leaf symptoms were randomly distributed on the adaxial surfaces and consisted of zonate lesions of alternating light gray and dark brown areas that were 6 to 10 mm in diameter. Remaining leaf tissue was chlorotic and affected leaves abscised. Stem infections appeared as irregular and elongated black lesions, 0.7 to 1.2 cm long. Pods lesions were circular, 6 to 8 mm in diameter, gray in the center, and surrounded by a diffuse dark brown margin. The disease developed progressively from the lower leaves to the pods, resulting in premature senescence of the tissues, chlorosis, and defoliation. Conidiophores bearing conidia colonized the lesions as a dark gray growth of spore masses. Segments (0.5 cm long) taken from leaves, stems, and pods of diseased plants were 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. Plates were incubated in the dark at 25°C for 2 to 3 days, followed by incubation under NUV light and a 12-h light/dark photoperiod for 6 to 8 days. Six fungal isolates were obtained. Fungal colonies were pale gray with dark concentric rings. Conidia were yellow to pale brown, ellipsoid to ovoid, produced singly or in short chains, with 8 to 10 transverse septa and 2 to 6 longitudinal septa. The spore body measured 13 to 22 × 68 to 135 µm with a beak cell 42 to 101 µm long. On the basis of conidial and cultural characteristics, the fungus was identified as Alternaria brassicae (Berk.) Sacc (1). Koch's postulates were completed for three isolates by spray-inoculating foliage of 6-week-old canola plants of cvs. Caviar, Dunkeld, Eclipse, Impulse, Mistral, and Sponsor with a conidial suspension (1 × 10 5 conidia per ml). The experiment, which included four inoculated plants and two noninoculated control plants for each cultivar per isolate, was conducted in the 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. Within 12 days, inoculated plants developed small, brown lesions on leaves and stems for all three isolates; the pathogen was successfully reisolated in all instances. Control plants, inoculated only with sterile distilled water, remained symptomless. The experiment was repeated with similar results. The results suggest that A. brassicae may be a threat to the main cultivars being grown in Argentina. To our knowledge, this is the first report of A. brassicae causing gray leaf spot of canola in Argentina. Reference: (1) J. Joly. Le genre Alternaria. Recherches Physiologiques, Biologiques, et Systématiques. Paul Lechevalier, ed. Paris, France, 1964.