Downy mildew on Veronica
June 21, 2007 - Author: Jan Byrne, Michigan State University Diagnostic Services, Department of Plant Pathology
Editor’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.
Downy mildew problems have been showing up on perennials (view images). The name downy mildew refers to a group of related fungal pathogens that cause similar symptoms on a wide variety of plants. Downy mildew is not the same as powdery mildew, do not confuse the two. Each downy mildew fungus affects a limited group of plants, also know as a host range. For example, downy mildew on Veronica sp. is caused by Peronospora grisea; plants in the genus Veronica are the only known host of this pathogen. Downy mildew on Salvia sp. and Lamium sp. is caused by Peronospora lamii, a different fungus that is only known to infect plants in these two generas.
Symptoms caused by downy mildew fungi are fairly similar regardless of the host. There are chlorotic or necrotic lesions on the upper surface of infected leaves. Lesions may have angular edges, some lesions are bordered by veins. Also there is fuzzy gray to black mold on the underside of infected leaves. Infected foliage may be cupped and new growth may become distorted. Severely affected plants may also be stunted.
Warm days and cool nights with high humidity are favorable conditions for downy mildew spore production. Spores are released from the plants and dispersed by air currents. Hot dry weather will slow the development of downy mildew. Susceptible hosts such as Veronica should be scouted for disease, especially as new plant material arrives. Maintain good air circulation with good plant spacing to make the environment slightly less favorable to disease.
Once a downy mildew problem is present disease control is difficult. Consider removing and destroying plants that are heavily infected. This will reduce the number of spores in the growing areas. This can be especially helpful where disease is detected early and has not yet spread extensively. Fungicides for disease control are best used preventatively on especially susceptible crops. Downy mildews are capable of developing resistance to several effective systemic fungicides. Systemic fungicides should be used in rotation with protectant products to slow the development of resistance. Products labeled for downy mildew control on ornamentals include mancozeb, Stature DM, Aliette, Compass and Heritage. Refer to the product labels for specific application information.