Canola (Brassica napus) is a developing oleaginous crop grown commercially in the Buenos Aires and Santa Fe provinces of Argentina. During the autumn of 2003, typical signs of powdery mildew were observed on canola plants in experimental field plots in Buenos Aires. Average disease incidence was 42% on 3- to 6-month-old canola cultivars developed in the following countries: Argentina (Eclipse, Impulse Master, Mistral, and Nolza); Australia (Oscar and Rainbow); Canada (Sentry); France (Cadillac, Camberra, and Capitol); and Sweden (Maskot, Sponsor, and Wildcat). The range of incidence on these cultivars was 35 to 93%. Other cultivars exhibited an apparent high level of resistance or escaped disease. These included: Charlton (Argentina); 46CO3, Dunkeld, Insignia, Mystic, Monty, Outback, Rivette, and Surpass 400 (Australia), and Caviar (France). Climatic conditions in Buenos Aires, especially rainfall, from March to May 2003 were apparently favorable for powdery mildew development. On susceptible cultivars, fungal growth was observed on leaves, stems, and pods that resulted in premature senescence of the tissues. The mycelium, with multilobed hausthoria, was white to gray, dense or fine, and in patches or covering the entire adaxial leaf surfaces. Appressoria were lobed and conidiophores were straight. Foot cells were cylindrical, straight, measured 35 to 42 × 7 to 10 μm, and were followed by two cells. Conidia were produced singly, cylindrical to ovoid, and measured 36 to 40 × 18 to 20 μm. The conidial length-to-width ratio was 2.0. No fibrosin bodies were observed in the conidia and conidia germinated at the ends. Cleistothecia were not observed. On the basis of mycelial, conidial, and hausthoria characteristics observed on six leaves for each affected cultivar, the fungus was identified as Erysiphe polygoni DC (1). Pathogenicity was confirmed on 5-week-old canola plants of cvs. Eclipse, Impulse, Master, Mistral, and Maskot by gently pressing (1 min) one adaxial infected leaf with abundant sporulation onto one adaxial healthy leaf. The experiment, which included five inoculated plants and three noninoculated control plants for each cultivar, was conducted 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. Powdery mildew developed on all inoculated plants of all cultivars after 12 to14 days. The control plants did not develop disease. The experiment was repeated with similar results. E. polygoni has a worldwide distribution (2); however, the results suggest that this fungus may be a threat to the main cultivars being grown in Argentina (Eclipse, Impulse, Master, Mistral, and Nolza), since high levels of disease incidence, as much as 70%, were observed. Under propitious environments, this pathogen could cause severe yield losses in commercially grown canola in Argentina. To our knowledge, this is the first report of canola powdery mildew caused by E. polygoni in Argentina.
References: (1) H. J. Boesewinkel. Rev. Mycol. Tome 41:493, 1977. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St.Paul, MN, 1989.