Kevin Ong, Professor and Extension Plant Pathologist*
Downy mildew can affect roses in garden settings,but it is especially a problem for roses grown in greenhouses and rose fields. The disease destroys plants worldwide and is costly to growers. Although many species cause downy mildew, each one has a narrow host range. For example, the species that produces the disease on roses does not infect other plants.
Peronospora sparsa, an oomycete (a fungus-like organism more closely related to algae than to fungi), causes the disease. Other well-known pathogens in the oomycete family are Phytophthora infestans (potato late blight) and Phytophthora ramorum (sudden oak death). Cool weather with temperatures less than 80°F and relative humidity of 85 percent or more favor the development of the disease.
Initial symptoms include small, purplish red to dark brown, irregular spots on the upper leaf surface (Fig. 1). As the lesions enlarge, the leaf veins limit their growth, giving the spots an angular appearance. As the lesions grow larger, more leaf damage occurs and, eventually, the leaves drop off.
Because rose cultivars are so diverse, there is a wide range of disease symptoms. Purple to black areas are most common on the leaves of new growth, but can also appear on stems, peduncles (a stalk that supports a flower or fruit), and flower petals (Fig. 2). Dead tips can develop on new growth (Fig. 3). Because the infection is systemic (spreads throughout the plant), it can go undetected. But under favorable environmental conditions, symptoms can suddenly appear, along with rapid defoliation.
Check the lower leaf surfaces for signs of a whitish to grayish fungal growth (the mycelium) beneath the symptomatic areas of the upper leaves. This growth is not the same as the mycelium in powdery mildew infections that appears on the upper leaf surface. From this downy-looking growth, P. sparsa produces spores or “seeds” in enormous numbers to spread the disease (Fig. 4). The species gets its name “sparsa” because of its sparse growth and production of spores in less-than ideal conditions, compared to other downy mildews. The pathogen produces zoospores that have flagella they use to “swim” to ideal infection sites. That is why wet plant surfaces make the disease much more prevalent. The fungus overwinters in or on plant parts as a vegetative mycelium or in fallen plant debris as hardy Oospores.
Prevention is the most effective control measure for the disease. Other control measures are:
- Planting resistant cultivars such as roses in the Rugosa family.
- Choosing plants appropriate for the climate.
- Adequately spacing plants to promote air circulation and allow wet plants to dry rapidly.
Use cultural practices that help prevent the disease:
- Because spore release and movement commonly occurs in the morning, water in the late afternoon to reduce disease occurrence.
- Water plants at the base to minimize wetting the foliage.
Sanitation is crucial to disease management:
- Remove plant debris.
- Prune plants to keep them aerated.
- Remove infected plants as soon as you discover them since the pathogen can reproduce several times a season, potentially causing a devastating outbreak.
- Use fungicides as a preventive measure. Always use caution when applying chemical controls.
Beckerman, Janna. 2009. Downy Mildew. Purdue Extension, Diseases of Landscape Plants series,BP-68-W. https://www.extension.purdue.edu/extmedia/bp/bp-68-w.pdf
Fry, William E. and Niklaus J. Grünwald. 2010.“Introduction to Oomycetes.” The Plant Health Instructor. DOI:10.1094/PHI-I-2010-1207-01
Hammond, Gaye. The Low-Down on Downy Mildew.Houston Rose Society. http://www.houstonrose.org/mo041707.htm
Horst, Kenneth and R. Cloyd. 2007. Compendium of Rose Diseases and Pests, 2nd ed. The American Phytopathological Society, 16–18.
Wyckoff, Jeff. Downy Mildew. American Rose Society.
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