Downy mildew (Peronospora sparsa) can be a devastating disease of roses. Epidemics of downy mildew are sporadic, and many years may pass between outbreaks in some areas. The disease can destroy a greenhouse crop of roses or a landscape planting in a very short period of time if environmental conditions favor disease development. The disease can occur anywhere roses are grown, and all cultivated roses are susceptible.
Downy mildew diseases are caused by host-specific pathogens. For example, the pathogen that causes downy mildew of rose will not infect other cultivated plants known to have problems with downy mildew, such as impatiens, grapes, tobacco and pansy, but will infect some species of brambles (also members of the family Rosaceae).
Symptoms of downy mildew of rose are variable and can be confused with other diseases and abiotic stresses. This can make a correct diagnosis difficult, and a delayed diagnosis could lead to catastrophic destruction of roses. Often the first symptoms of downy mildew of rose are chlorotic (yellow) lesions (spots) on the upper leaf surface. These lesions quickly turn red, purple and/or black (Figure 1). Lesions are often bordered by leaf veins, which makes the lesions look angular or flattened on some sides. In some cases, especially if environmental conditions become dry, the lesions may develop necrotic centers (Figure 1 B). Unlike the disease black spot, where lesions start at the bottom of the plant, downy mildew lesions usually begin at the top of the bush. Another symptom is defoliation, which may occur before leaves become symptomatic with lesions (Figure 2). Although P. sparsa looks and behaves like a fungal pathogen, it is not a fungus. It is a member of the Oomycota; these organisms are more closely related to brown algae than they are to fungi and are therefore referred to as fungal-like organisms. All downy mildews are obligate parasites. This means they must have living tissue to complete their life cycle and they cannot be grown on media in a petri dish.
Courtesy of Mark Windham and Alan Windham
On infected roses, signs (the physical presence of the fungus) can be seen, with some difficulty and using a hand lens, on the underside of infected leaves. Look for whitish growth of the pathogen in blotches between leaf veins (Figure 3). Do not expect to see a lot of growth of P. sparsa; the signs of P. sparsa are sparse—this is why the species name is sparsa.
Outbreaks of downy mildew of rose are dependent on conducive environmental conditions. This disease is most likely to occur in cool weather (mid 50s F to mid 60s F) and when the humidity is high (> 85 percent). These conditions often occur in late winter/early spring in polyhouses, especially at night, and later in the spring in landscapes. Optimum temperature for spore germination and infection is 66 F. When the environment is conducive for disease development, symptoms can literally appear overnight, and complete defoliation of bushes can occur in as little as three days. Wet leaves and high humidity will trigger sporulation of the pathogen on lower leaf surfaces. Sporulation of the pathogen can occur on infected leaves before symptoms are evident. Cultural control measures are critical to prevent and reduce the impact of downy mildew of rose. Carefully inspect all new plants introduced to the landscape or nursery and monitor them vigilantly for disease outbreaks as the season progresses because the plants may be infected, but asymptomatic (latent infection). Space plants sufficiently apart to allow air movement between them. This type of spacing will allow wet foliage to dry more quickly. Avoid using overhead irrigation when possible, and water plants early enough in the day so foliage can dry before nightfall. In the evening, if outside temperatures are not too cold, use fans to exchange air in greenhouses/polyhouses to reduce humidity in the structures.
Sanitation is also important. If defoliation has begun, all dropped leaves should be removed as spores can survive on these leaves for several weeks, and the spores can therefore be splashed or blown to healthy foliage. In the landscape, if removing defoliated leaves is not practical, cover leaves with several inches of mulch. Symptomatic foliage should be stripped from plants to reduce inoculum for secondary disease outbreaks. Once weather warms into the mid 80s, disease pressure will drop significantly. As a preventive measure, fungicides should be applied when environmental conditions are conducive for disease development. Once an epidemic begins (plants are symptomatic), starting a chemical control regime is unlikely to yield satisfactory results. Homeowners should use Bonide mancozeb flowable with zinc (mancozeb) or Agri-Fos (salts of phosphorous acid) as preventive chemical sprays. In nurseries, Aliette (fosetyl aluminum), Heritage (azoxystrobin), Compass (trifloxystrobin) and Stature DM (dimethomorph) are available to control downy mildew. Fungicides should be rotated to prevent the pathogen from becoming less sensitive to fungicides, but avoid rotating fungicides with similar modes of action, such as Compass and Heritage. Fungicides commonly used for black spot control such as Daconil (chlorothalonil), Funginex (triforene), Bayer systemic fungicide (tebuconazole), Fertilome systemic fungicide (propriconazole, includes Banner Maxx), Immuninex (myclobutanil, also includes Eagle) and Clearys 3336 (thiophanate methyl) will be ineffective for control of downy mildew.
If you are not vigilant, downy mildew can destroy your roses. Cool temperatures and high humidity are environmental conditions needed for an epidemic to occur. Cultural control measures, such as spacing plants to promote good air movement, keeping foliage as dry as possible, quarantining new plants in the spring and using good sanitation, will lessen the impact of this disease. Use of preventive fungicides when environmental conditions encourage disease development is also recommended.
Mark Windham is a professor and distinguished chair in ornamental diseases, and Alan Windham is a professor and extension specialist in ornamental pathology. Both are members of the Department of Entomology and Plant Pathology at the University of Tennessee. Mark can be reached at email@example.com; Alan can be reached at firstname.lastname@example.org.