Scouting for diseases: Downy mildew
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EmailEditor’s note: This article is from the archives of the MSU Crop Advisory Team
Alerts. Check the label of any pesticide referenced to ensure your use is
included.
Pathogen: Peronospora spp. and Plasmopara spp. (view images)
Hosts: Many fungi cause downy mildew. Each has a fairly limited host range. Common hosts include: Buddleia,
Delphinium, Dianthus, Dicentra, Geum, Helianthus, Iberis, Lamium,
Lathyrus, Oenothera, Papaver, Potentilla, Primula, Rudbeckia, Veronica
and Viola.
Symptoms: Fuzzy, gray to black mold develops on the undersides
of infected leaves. Chlorotic or necrotic lesions appear on the upper
surfaces of infected leaves. Lesions may have angular edges; some
lesions are bordered by veins. Infected foliage may be cupped, and new
growth may become distorted. Severely affected plants are stunted. Some
seedlings can be infected systemically, causing new growth to be stunted
and severely distorted.
Spread: Spores are readily released and carried by air
currents. Peak spore release often occurs when relative humidity rapidly
decreases, which typically occurs in the morning. Some downy mildews
are spread by contaminated seed; others are effectively spread on
vegetative cuttings and seedlings.
Management: Scout susceptible incoming plant material carefully
for signs of downy mildew paying careful attention to leaf undersides.
Remove and destroy infected plants. Do not compost the plant debris.
Warm days and cool nights with high humidity are favorable conditions
for downy mildew spore production. Maintain good air circulation and
increase night temperatures in greenhouses. Fungicides should be used
preventively on especially susceptible crops. Downy mildews are capable
of developing resistance to several effective systemic fungicides.
Rotate use of systemic fungicides with protectants to slow resistance
development.
