Downy mildew

Mildews<br />

<strong>Downy</strong> <strong>mildew</strong><br />

Pathogen: Peronospora spp. and Plasmopara<br />

spp.<br />

Hosts: Many fungi cause downy <strong>mildew</strong>. Each<br />

has a fairly limited host range. Common hosts<br />

include: Buddleia, Delphinium, Dianthus, Dicentra,<br />

Geum, Helianthus, Iberis, Lamium, Lathyrus,<br />

Oenothera, Papaver, Potentilla, Primula, Rudbeckia,<br />

Veronica and Viola.<br />

Symptoms: Fuzzy, gray to black mold develops<br />

on the undersides of infected leaves. Chlorotic<br />

<strong>Downy</strong> <strong>mildew</strong> on the underside of Rudbeckia foliage produces a<br />

thick, whitish, fuzzy growth. At right, lesions on the upper surface.<br />

Note the angular edges of lesions that are bordered by the leaf<br />

venation.<br />

D. Brown-Rytlewski, MSU<br />

D. Brown-Rytlewski, MSU

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<strong>Downy</strong> <strong>mildew</strong> – continued<br />

or necrotic lesions<br />

appear on the upper surfaces<br />

of infected leaves.<br />

Lesions may have angular<br />

edges; some lesions<br />

are bordered by veins.<br />

Infected foliage may be<br />

cupped, and new growth<br />

may become distorted.<br />

Severely affected plants<br />

Lesion on Potentilla<br />

foliage.<br />

are stunted. Some seedlings can be infected systemically,<br />

causing new growth to be stunted and severely<br />

distorted.<br />

Spread: Spores are readily released and carried by<br />

air currents. Peak spore release often occurs<br />

Mottling symptoms.<br />

Note the angular edges<br />

of the lesions.

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<strong>Downy</strong> <strong>mildew</strong> – continued<br />

when relative humidity rapidly decreases, which typically<br />

occurs in the morning. Some downy <strong>mildew</strong>s are<br />

spread by contaminated seed; others are effectively<br />

spread on vegetative cuttings and seedlings.<br />

Management: Scout susceptible incoming plant<br />

material carefully for signs of downy <strong>mildew</strong> paying<br />

careful attention to leaf undersides. Remove and destroy<br />

infected plants. Do not compost the plant debris.<br />

Warm days and cool nights with high humidity are<br />

favorable conditions for downy <strong>mildew</strong> spore production.<br />

Maintain good air circulation and increase night<br />

temperatures in greenhouses. Fungicides should be<br />

used preventively on especially susceptible crops.<br />

<strong>Downy</strong> <strong>mildew</strong>s are capable of developing resistance<br />

to several effective systemic fungicides. Rotate use of<br />

systemic fungicides with protectants to slow resistance<br />

development.<br />

Downward curling and yellowing<br />

of Veronica leaves caused by<br />

downy <strong>mildew</strong>.