Downy mildew

Downy mildew disease symptoms on a canola leaf Downy mildew disease symptoms on a canola leaf Downy mildew disease symptoms on a canola leaf Downy mildew disease symptoms on a canola leaf

Downy mildew is a plant disease caused by the pathogen Hyaloperonospora brassicae. Downy mildew is currently a minor disease that is not of economic concern for canola grown in Canada, as it has no known significant effects on seed yield and quality. Leaf lesions on the top surface of the leaf and mealy white conidiophores on the underside of the leaves can be used to identify downy mildew whenever scouting canola fields between seedling to podding stage, especially after high relative humidity and/or prolonged high moisture periods.1,2,3

Overview

Downy mildew disease symptoms on the upper surface of a canola leaf
Downy mildew disease symptoms on the upper surface of a canola leaf

Downy mildew disease in canola is caused by the pathogen Hyaloperonospora brassicae. It had previously been called Peronospora parasitica (which is now known to be the pathogen of shepherd’s purse), until it was updated to better align with its corresponding host species. Both H. brassicae (the pathogen that causes downy mildew disease) and the pathogen that causes white rust disease belong to group called oomycetes (or water moulds), which are not true fungi. H. brassicae is soil-borne and seedborne and may persist in the soil for five to 10 years.

Disease cycle

H. brassicae overwinters on infected crop residue as oospores. This pathogen then produces air-borne spores called conidia, which infect canola leaves and other above-ground tissues. These newly infected tissues produce lesions in which new conidiophores develop and release more conidia, which are a source of secondary spread and infection. H. brassicae prefer cool temperatures (8 to 24°C) and relative humidity above 80 per cent.1,2,3

Identification (disease symptoms)

Downy mildew on the underside of a canola leaf
Downy mildew disease symptoms on the underside of a canola leaf

Downy mildew disease symptoms can appear as early as the seedling stage or as late as the podding stage. Environmental factors, such high relative humidity and/or prolonged high moisture periods, can make the disease more likely to appear.

Symptoms of downy mildew disease in canola appear primarily on leaves and inflorescences (buds and flowers) and can become more evident as infected tissues age. Typical leaf lesions appear as discolored spots, sometimes chlorotic and/or necrotic, on the top surface of the leaf. The underside of the leaves will commonly have characteristic mealy white conidiophores present in the lesions. Plants infected at the cotyledon stage may die if cotyledon infection progresses into the hypocotyl. Canola pods can be infected late in the season developing lesions as well.1,2,3

When scouting for disease in canola crops, downy mildew symptoms can usually be differentiated from other diseases:

July 31 Quiz
Blackleg is on the left. Downy mildew is on the right.
  • Downy mildew does not create morphological changes as white rust (staghead) does.
  • While they can have leaf lesions, canola plants infected with downy mildew will not have the tiny black pycnidia spots that blackleg-infected plants will have.
  • Since the pathogen causing downy mildew needs a living host to survive, it likely won’t exhibit premature ripening and doesn’t severely degrade cellular tissue the way sclerotinia stem rot disease does (ex. causing bleached stalks that shred and shatter easily).
  • While powdery mildew disease also causes white growth on canola leaves, powdery mildew is best noticed at swath timing when white clouds of dust are released.
  • Similarly, alternaria disease tends to exhibit symptoms later in the season than downy mildew symptoms would typically be found, despite both plants diseases cause leaf lesions.

Management

Downy mildew mycelium canola close up
This photo of downy mildew mycelium in a canola leaf shows the spore-producing structures called conidiophores, which usually come out of stomata. Most fungal growth/mycelia is contained intercellularly within the cells/tissues for downy mildew, as shown.

There are currently no fungicides registered for control of downy mildew in canola 4. Using a crop rotation with non-cruciferous crops, ensuring control alternate hosts such volunteer canola, stinkweed and wild mustard is recommended for general canola disease management.

Resistance in B. napus, B rapa, and B. juncea to downy mildew disease has been reported in the scientific literature, but cultivar resistance in Canada is not tested or reported publicly. Currently downy mildew is not a disease of economic concern for canola grown in Canada, and has no significant effects on seed yield and quality.1,2,3,5

Footnotes

  1. Runno-Paurson, E., Lääniste, P., Eremeev, V., Kaurilind, E., Hõrak, H., Niinemets, Ü., & Metspalu, L. 2019. Evaluation of downy mildew (Hyaloperonospora brassicae) infection severity on different cruciferous oilseed crops. Proceedings of the 9th International Scientific Conference Rural Development 2019. http://doi.org/10.15544/RD.2019.047
  2. Mohammed, A. E., You, M.P., Banga, S. S., & Barbetti, M. J. 2019. Resistances to downy mildew (Hyaloperonospora brassicae) in diverse Brassicaceae offer new disease management opportunities for oilseed and vegetable crucifer industries. European Journal of Plant Pathology, 155, 915-929. https://doi.org/10.1007/s10658-018-01609-7
  3. Eastburn, D.M., & Paul, V.H. 2007. Compendium of Brassica Diseases. Downy Mildew.
  4. Manitoba Agriculture. 2022. Guide to Field Crop Protection. Available online.
  5. Bailey, K.L., Gossen, B.D., Gugel, R.K., & Morrall, R.A.A. 2003. Diseases of Field Crops in Canada, 3rd Edition. ISBN 0-9691627-6-6

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