Tree Diseases: Downy Mildew (Peronospora and Plasmopara)
Downy mildew is a plant disease that affects a bevy of economic and ornamental plants. Unlike powdery mildew, which is a true fungus, downy mildew infections are caused by the invasion of numerous microscopic oomycete parasites. There are many downy mildew parasites, but most belong to the genera Peronospora and Plasmopara. Some downy mildew organisms are obligate parasites, meaning they target specific plants. One notable example is Plasmopara halstedii, which infects members of the daisy family. Plants infected by downy mildew are often at least partially defoliated. Repeated downy mildew infections can ultimately cause affected plants to fail.
Distribution & Habitat
Downy mildew occurs globally, wherever susceptible plants are present.
Downy mildew infects a wide range of hosts. Common hosts include alyssum, aster, basil, butterfly bush, cantaloupe, coreopsis, crucifers, cucumber, crane’s bill geranium, hardy geranium, flowering plants belonging to the genus Impatiens, grapes, kale, members of the gourd family, mint, nettles, pansy, phlox, pumpkin, rose, sage, snapdragon, soybeans, squash, sunflowers, veronica, viburnum, violets, and vegetables that develop on vines.
Most downy mildew organisms overwinter on infected plant material, or in diseased soil in the form of mycelium or oospores. Downy mildews can reproduce sexually or asexually. In spring, white to gray, cotton-like fruiting bodies called sporangia appear on the underside of infected leaves. Once conditions are sufficiently moist, the sporangia either germinate by penetrating into the plant with a germ tube, or produce zoospores, which are released into the air, and disseminated to nearby plant foliage by air currents, splashes of rain, pruning equipment, or insect vectors.
The spores readily infect susceptible plant foliage. They linger on the surface of the leaves for a brief period, before becoming established in the plant tissue. Affected leaves will suffer from discoloration and become brittle to the touch. The affected leaves will then turn brown and are often prematurely cast. So long as conditions are favorable, vulnerable plants may be infected by downy mildew at any point during the growing season. The disease cycle for downy mildew is usually completed within 7 to 10 days. Rainy conditions can hasten the completion of the disease cycle. Downy mildew infections are typically most prevalent in spring or autumn, when rainfall tends to be more abundant.
Symptoms of Infection
When plants are initially infected, large, yellow patches, or lesions, will form on the upper surface of the diseased foliage. As the lesions mature, they enlarge and turn brown. The leaf veins of infected foliage will often manifest gray to brown spots. The spots will then develop into angular or block-like lesions. As infections progress, the lesions will grow larger. Multiple lesions may coalesce, damaging large portions of the leaf. The lower surfaces of infected leaves will appear water-soaked. The sporangia can be observed on infected plant foliage, especially during spring or fall. The zoospores released by the fruiting bodies are large enough to be discerned. Plants that are infected by downy mildew may be stunted, and decline. Repeated infection from downy mildew may eventually cause plants to be defoliated, and die.
- Use soaker hoses or irrigation to water plants. This will prevent the foliage from becoming wet during watering sessions.
- Ensure that plants are watered thoroughly, especially during extended periods of drought. Water plants in the morning to allow them ample time to dry.
- Apply a layer of organic mulch around the base of vulnerable plants to improve the soil quality, and insulate plant roots.
- Selectively prune plants to improve air circulation throughout the crown, and to promote a rapid drying of the foliage when wet.
- Systemic fungicides may provide some control against certain downy mildew organisms. Applications should begin in early spring, when conditions are favorable for infection. Repeat applications at a 2 to 3 week interval, until the growing season has ceased.
- Eliminate sources of moisture from around infected plants.
- Cull severely infected plants to prevent the downy mildew organisms from spreading.
- Prune or stake plants that grow on vines to elevate them, and provide them with support.