February 15, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of Gray Leaf Spot caused by Alternaria brassicae on canola in Argentina
S. Gaetan and M. Madia, Catedra de Fitopatologia, Facultad de Agronomia, Universidad de Buenos Aires. Avda. San Martin 4453 (1417). Argentina. Plant Dis. 89:207, 2005; published on-line as DOI: 10.1094/PD-89-0207A. Accepted for publication 28 Nov 2004.

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 percent (range 12-42 percent). 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 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 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. Plates were incubated in the dark at 25 deg C for 2 to 3 days, followed by incubation under NUV light and a 12-h light/dark photoperiod for 6 to 8 days.

6 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 x 68 to 135 micrometers with a beak cell 42 to 101 micrometers 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 3 isolates by spray-inoculating foliage of 6-week-old canola plants of cvs. Caviar, Dunkeld, Eclipse, Impulse, Mistral, and Sponsor with a conidial suspension (100 000 conidia per ml).

The experiment, which included 4 inoculated plants and 2 noninoculated control plants for each cultivar per isolate, was conducted in the 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. Within 12 days, inoculated plants developed small, brown lesions on leaves and stems for all 3
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 1st report of _A. brassicae_ causing gray leaf spot of canola in Argentina.

Reference:
(1) J. Joly. Le genre Alternaria. Recherches Physiologiques, Biologiques, et
Systematiques. Paul Lechevalier, ed. Paris, France, 1964.

[There are 2 types of canola, the short-growing season Polish type (_Brassicae rapa_ and _B. campestris_) and the longer-season Argentine type (_B. napus_). Canola is produced extensively in Europe, Canada, Asia, Australia, and to a limited extent in the United States. Gray leaf spot [also known as black spot], caused by _Alternaria brassicae_ and _A. raphani_, is present in all canola and rapeseed production areas. The disease overwinters on plant debris and seed. All above-ground plant parts are susceptible to infection from the spores
produced on plant debris or infected seed. Polish-type canola cultivars are more susceptible than Argentine-type cultivars. Argentine varieties have a higher yield potential and are also taller and have a higher oil content
than Polish varieties. Argentine varieties require about 95 days to reach maturity, while Polish varieties need approximately 80 days to reach maturity.

Disease management depends upon planting certified, disease-free seed, a rotation of 1-4 years, and control of
weedy mustards and volunteer [self-sown] canola. It is most likely that the introduction of grey leaf spot was the result of planting infested seed. Fungicides containing iprodione and azoxystrobin are registered for control of gray leaf spot on canola, but I do not know if they are recommended for use in Argentina. Using fungicide-treated seed may help to increase stand establishment when infected seed is planted.

Links: <http://res2.agr.ca/saskatoon/result/letter9706-lettre9706_e.htm>
<http://www.agf.gov.bc.ca/cropprot/alternar.htm>
- Mod.DH]