Downy mildew

Margery Daughtrey, University of Cornell

Published April 05, 2021

Downy mildew diseases are caused by oomycetes, a group of fungus-like organisms that also includes Pythium and Phytophthora species. Downy mildew pathogens are very different from powdery mildews. They attack different plants under different environmental conditions. Also, they are controlled by different classes of fungicides.

Most of the downy mildew fungi are very host specific and infect only one plant family. Pathogens include species of Peronospora, Pseudoperonospora, Bremia, Plasmopara, and Basidiophora. Downy mildews infect almost all ornamental plants as well as some indoor plants. Perennial hosts include aster, buddleia, coreopsis, geranium (not Pelargonium), geum, gerbera, lamium, delphinium, veronica and viola. Downy mildew is also caused on rose by Peronospora sparsa.

Source: UMass Extension

facts about downy mildew

Symptoms: Downy mildews begin as yellow to light green, irregular leaf lesions that may become purple to dark brown and are delimited by leaf veins. Under humid conditions, the fungus sporulates on leaf undersides, producing a downy growth which may be white, tan, gray, or purple.

Damage: Downy mildews vary in aggressiveness, and symptoms will vary among host species. As the disease progresses, lower leaves often wither and fall off. The fungus grows both locally and systemically and it can escape notice until conditions are right for sporulation. Systemic symptoms can include stunting, leaf distortion, epinasty, shortened internodes, and a decrease in the quantity and quality of flowers.

Lynn Ann Hyatt, Cornell University

Cultural management: Keep relative humidity below 85 percent to inhibit sporulation on infected plants and stop germination of spores on healthy plants. This can be done by venting and heating in a greenhouse. The use of fans improves horizontal air flow and prevents cold spots where condensation develops from occurring. The density of the plant canopy should be reduced by spacing plants to allow for maximum air circulation and sunlight availability.

Scout regularly for early symptoms of the disease. Diseased plants, infected debris, and soil should be promptly removed and destroyed. Asymptomatic plants immediately surrounding diseased plants should also be removed and destroyed. Control weeds as they may also serve as hosts for downy mildews.

Chemical management: Fungicides will be ineffective unless used in conjunction with good cultural management practices. Fungicides should be applied preventively for maximum efficacy. Many products which give excellent control preventively provide little to no control when used as curatives.

The use of fungicides to manage downy mildew diseases has become more difficult because of the development of fungicide resistant strains. Practices for the prevention of fungicide resistance development include:

Beginning a regular spray program with a protectant fungicide
Limiting the number of applications of particular fungicides applied per season
Rotating fungicide applications among FRAC groups (modes of action)
Applying systemic chemicals in combination with a protectant

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