Alternaria black spot, also known as grey leaf spot, is caused by three species of the Alternaria fungus:
- Alternaria brassicae
- Alternaria raphani
- Alternaria alternata
Why Does it Matter?
- Alternaria is widespread on a large variety of plants and attacks the stems, leaves and pods of canola.
- Low levels of fungi are found in most canola fields, but cause little damage unless moisture and temperature conditions are favourable for disease development.
- Alternaria black spot can strike at all growth stages in particular, at early stages when infection can quickly build up and result in severe disease levels and eventually plant death.
- Losses: Infection during podding and ripening can reduce yields by causing premature shattering, and reduce grades by causing higher green seed, chlorophyll content, shriveled seed and reduced oil content.
What are the Symptoms?
Early season:
- Infected seed will produce seedlings with small dark spots on the cotyledons.
Mid- to late season:
- Grey to black spots on stems, leaves and pods.
- Severe spotting causes leaves to wither and die and can also cause floret abortion.
- Premature ripening of heavily infected plants; pod splitting.
What is the Disease Cycle?
- Alternaria fungi overwinter on seed, infected crop debris and on related (cruciferous) weeds, such as flixweed, mustards and stinkweed.
- Spores are produced in the infected plants or crop debris and are spread by wind, rain and insects to other plants, where they remain until conditions turn warm (15o C to 20o C) and humid.
- When conditions are right, the spores germinate and penetrate the plant surface, causing lesions to form within a few days.
- These new lesions produce more spores for distribution by wind, rain and insects.
- Seed can become infected through lesions on the pods or through spore contamination in the combine.
How is it Controlled?
- Use clean, high-germinating, disease-free seed.
- Follow a minimum three-year canola rotation.
- Control volunteer canola and cruciferous weeds.
- Bury infected stubble.
- If the disease does occur, apply fungicides to protect yield potential.
How Does 20/20 Seed Labs Inc. Test for Alternaria Black Spot?
- 200 seeds are put on culture agar and incubated for 7 days.
- A qualified staff member examines the plates for the presence of Alternaria spp.; identification is made by the color and shape of the spores produced.
- This test tells you the percentage of infected seeds to 0.5%.
- If individual species of the fungus need to be identified, we can do that for an extra fee.
- This test takes one week to complete.
Please contact us for more information.
