October 31, 2004
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited] <http://www.apsnet.org/>

First report of canola powdery mildew caused by Erysiphe polygoni in Argentina
S. Gaetan and M. Madia, Catedra de Fitopatologia, Facultad de Agronomia, Universidad de Buenos Aires, Avda San Martin 4453, 1417, Buenos Aires, Argentina. Plant Dis. 88:1163, 2004; published on-line as D-2004-0722-01N, 2004. Accepted for publication 13 Jul 2004.

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 percent 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 percent. 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 x 7 to 10 millimicrons, and were followed by 2 cells.

Conidia were produced singly, cylindrical to ovoid, and measured 36 to 40 x 18 to 20 millimicrons. 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 6 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 (one min.) one adaxial infected leaf with abundant sporulation onto one adaxial healthy
leaf. The experiment, which included 5 inoculated plants and 3 noninoculated control plants for each cultivar, was conducted in a greenhouse at 22 to 24 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. Powdery mildew developed on all inoculated plants of all cultivars after 12 to 14 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 percent, were observed. Under propitious environments, this pathogen could cause severe yield losses in commercially grown canola in Argentina.

To our knowledge, this is the 1st 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.

[_Erisyphe poloygoni_ [Eg] grows on the surfaces of leaves and stems. Older infected leaves yellow and wither, and growth of heavily infected plants is diminished. The disease is favored by moderate temperatures. Eg produces airborne spores that facilitate spread of the disease. Moisture is not necessary for germination and infection, and is actually detrimental to the fungus. Several powdery mildew fungicides are effective if applied in a
regular preventative program commencing with the 1st signs of the fungus. Disease management depends upon the use of resistant cultivars, applications of registered fungicides in rotation to delay evolution of resistant strains of Eg, crop rotation, and removal of old plant debris.

Links: <http://ag.arizona.edu/pubs/garden/mg/pathology/bean.html>
<http://www.ipm.ucdavis.edu/PMG/r280113211.html>
- Mod.DH]