Downy mildew (Peronospora sparsa ) is not as common as other rose diseases like blackspot or powdery mildew, but it is many times more devastating and is often misdiagnosed. Unlike blackspot or powdery mildew, downy mildew is systemic in roses and can spread in as little as 12 hours in the right environmental conditions. Chemical control is very difficult, expensive and can take as many as 3 years for the disease to be eliminated in the garden. For this reason, immediate diagnosis and treatment, along with cultural controls, is imperative for the health of the infected plant and its garden setting.
Downy mildew is believed to have got its start in the United States in the 1880's. It is a very serious problem in California where environmental conditions provide the perfect host for development of the disease. California is reportedly one of only a few places in the county where downy mildew is found in great measure in outdoor rose fields. In contrast, greenhouse operations have always been plagued with this fungal problem and even a minor outbreak in a greenhouse setting can be financially devastating to the grower. Disease spores can be unknowingly shipped with plants to local nurseries and when the right conditions are present, symptoms will manifest themselves either at the nurseries or in our gardens.
Method of Infection. Downy mildew fungi are obligate parasites that can only grow on live plant tissue. Closely related to the Pythium and Phytophthora water molds, infection occurs inside the leaf during cool wet weather with temperatures between 50o and 75o F (optimal temperature for development is 64o) and high relative humidity (85%). Sporulation (the formation of fungal spores) can occur before symptoms are evident. Once weather conditions turn warm (above 85o F) and dry, the disease is kept in check until the next cool, humid period.
Dense canopy growth and tight plant spacing encourages downy mildew (and other fungal diseases) spore production. Pathogens are primarily spread by wind-borne and air-borne spores. It is a disease that is extremely contagious and can spread throughout a rose garden very quickly if left untreated.
A film of water is needed on the leaf surface for at least 4 hours for spore germination and infection. Prolonged periods of wetness promote the disease and fungal spores are easily spread by splashing water (the second most common method of disease transmission). Overhead irrigation and rainfall not only splash downy mildew spores from leaf to leaf and from plant to plant, these conditions also provide the wet surfaces required for spore germination. There are reports in the literature of disease transmission through cuttings and through seeds from infected plants.
Unlike blackspot that attacks lower foliage first, downy mildew attacks new growth at the top of the bush and works its way down to the lower foliage.
Downy mildew fungi overwinter in stems, crowns and root systems of roses. It can also overwinter in leaf debris and stems of nearby host plants and weeds. Many plants are susceptible to downy mildew. Fortunately, there is high host specificity between mildews and their individual hosts (rose downy mildew will not transfer to pansies, grape downy mildew will not transfer to roses, etc.).
Symptoms. Downy mildew is easily confused with chemical burn. Gardeners suspecting the fungal disease should look for faint downy fuzz on the underside of a symptomatic leaf very early in the morning. Another test is to put 20 suspect leaves in a zip-lock bag, blow some air into the bag and seal it. After a few days the sporangia (fuzz) should be evident if the disease is present. For those of us whose vision is not what it used to be, a 100-200 power hand lens may prove effective in identifying the sporangia. Left untreated, plants with downy mildew quickly decline and may appear to be dead, or almost dead. The most common symptoms are:
• Unexplained drop of green leaves;
• Faint light green or greenish yellows area of discoloration on leaves;
• Purplish red, dark brown and/or black irregular spots on leaves or canes. Major leaf veins often restrict fungal growth so that the lesions become angular as they enlarge.
• In areas where high humidity persists, a gray, brown or purple spore mass may appear on the underside of the leaf;
• Total defoliation may occur in extreme cases;
• Small spots or long purplish-black areas may form on canes and may kill twigs;
• Die back is commonly found on infected plants;
• In cases of severe infection the size, shape and appearance of rose blooms may be distorted;
• Decline of overall health and vigor of the plant.
The chronology of disease progression usually follows the pattern of faint yellowish green "islands" on the leaf surface followed by the appearance of dead/dying leaf tissue and then individual leaf drop, culminating in defoliation of the plant. If left unchecked, dieback of canes can occur. While disease progression can occur very rapidly - 48 hours from infection to symptoms, I have seen infected bushes appear to be symptom free for 5 weeks and display the first evidence of the disease in the 6th week after exposure. Even with treatment, an infected plant may continue to periodically display symptoms for as many as 3 years, with a reduction in the severity of the symptoms from year to year.
The disease does not always manifest itself identically from one cultivar to another. For example, a collection of Rio Samba at a local nursery was found to have dark brown rusty-looking leaf lesions, covering the majority of the leaf surface. Red Knock Out plants situated 20 feet way from the infected Rio Samba plants looked like someone sprayed Indian ink on the foliage. On further examination, both had heavy spore development.
Some people may confuse the symptoms of downy mildew with those of blackspot, botrytis, nutrient stress and/or chemical injury. It is even possible that infected plants have no outward immediate symptoms of the disease. For this reason gardeners should familiarize themselves with the symptoms of the disease and be on alert for its presence in spring and fall. When downy mildew is suspected, ask a Consulting Rosarian to confirm the presence of the disease before treatment is initiated.
New Purchases. Purchase roses from reputable nurseries that take good care of their plants, including doing regular preventative sprays beginning when the plants arrive in the nursery. Evaluate the bushes you are considering buying (especially modern grafted roses) for any evidence of the disease and ask yourself the question, "Do any of the bushes show evidence of infection?" If you see what you think is downy mildew, ask the nursery's horticulturist for confirmation of your suspicion. Ask the nurseryman if they have had any episodes of downy mildew in their inventory. A reputable nurseryman will give an honest response.
Cultural Controls. Regularly scout plants in the spring and fall when conditions are ripe for infection to develop. Experts believe that environmental management is much more effective than control with fungicides. Gardeners can minimize the incidences of disease and its impact with proper cultural controls:
1. Increase the air circulation between and around plants by maintaining a 1 to 2 foot open space between bushes. Proper spacing also reduces the humidity levels within each plant's canopy. Pruning to open up the center of the plant as well as the removal of blind shoots encourages more air circulation inside the plant canopy.
2. Avoid overhead irrigation whenever possible - but especially during periods of cool weather. Water early in the day so that the plant tissue has time to dry out before nightfall. Use drip irrigation whenever feasible.
3. Quarantine spring rose purchases (especially modern grafted roses) for several weeks and monitor these new acquisitions for evidence of the disease (especially when weather conditions are ripe for disease development) before introducing them into the garden.
4. Maintain good housekeeping in your rose beds by removing and disposing of leaf litter, spent blooms, fallen petals and rose clippings. Since the fungi are capable of overwintering in this material, eliminating the material from the bed eliminates one potential source of reinfection. Be aware that it is impossible to remove all sources of reinfection since the canes on the bushes will also contain spores just waiting for the right conditions to germinate and trigger a new attack. For this reason it is recommended that clippings from infected bushes not be shredded or added to a compost pile, and instead be bagged and sent to the area landfill or burned.
5. Maintain adequate plant nutrition and hydration. Plants in stress are more susceptible to disease pressure.
6. Strip infected leaves from the bush as soon as possible to prevent secondary infection.
7. Once the weather warms to 80o F, prune affected canes, cutting back damaged canes to solid green healthy tissue.
Chemical Controls. Fungicides are more effective when the disease is in its early stages. Chemical control tactics may help prevent infection when conditions are ripe for spore development, but may not be very helpful once the disease progresses. As a preventative only a very few fungicides are truly effective on downy mildew. Even fewer have the ability to eradicate the disease and those products are very expensive.
Downy mildew pathogens enter the plant through the stoma (minute openings) on the underside of the leaf. For this reason it is imperative that chemical spray applications provide good coverage on the underside of the leaf as well as on the leaf surface.
1. Protect plants with regular broad-spectrum preventative fungicide applications during cool wet weather when conditions are ripe for disease development. Repeated applications will be needed.
2. Rotate chemical treatment among fungicide classes to delay the development of resistance by the fungi to the chemical treatment.
3. After spring pruning, spray the mulch and ground surrounding the bushes thoroughly with a fungicide.
4. Do not use metalaxyl or mefenoxam-based products alone as they are not registered and may not offer much control and may actually encourage resistance to develop.
5. Chemical controls with a rotation of Heritage (azoxystrobin), Stature DM (dimethomorph) and Aliette (aluminum phosphonate) have proven to be the most effective in our area. Compass (trifloxystrobin), a non-systemic fungicide, is also effective in the treatment of downy mildew, but be aware that it has the same mode of action as Heritage. For this reason it should not be alternated with the use of Heritage alone in a treatment plan. Heritage and Compass are broad spectrum fungicides and are effective on blackspot as well as downy mildew. Stature is selective to downy mildew oomycetes and primarily rated for this fungal disease. Repeated applications of Heritage or Compass and Stature, along with Aliette have proven effective in eradicating the disease.
Dr. Karl Steddom, Associate Professor and Extension Plant Pathologist at the Texas Agricultural Experiment Station in Overton, recommends the inclusion of Aliette in the spray rotation because this is the only fungicide capable of traveling throughout the system of the plant - from leaves to roots on a fungal seek-and-destroy mission.
As a preventative measure only, some rosarians report success with Mancozeb (ethylene bisdithiocarbamate) - however this product is usually not an effective curative. Carefully follow the label directions when using any fungicide.
According to Dr. Steddom, "Downy mildew in Houston is a sporadic problem. Even without treatment an infected plant will usually recover once the temperature rises above 80o F. If proper horticultural practices are employed downy mildew should not be a big issue. The disease is rarely found in landscape plantings in any great concentrations in the South."
The important thing is not to panic. All rosarians, at one time or another, have most likely had downy mildew (whether they knew it or not) in their gardens. Prompt identification, good cultural practices and immediate action during cool wet weather should keep this garden gorilla under control.
Resources:
Notes from interview with Dr. Karl Steddom, Associate Professor and Extension Plant Pathologist, Texas A&M University System, Overton, Texas, April 2007
Barnes, Larry, Ph.D., "Mildews", Texas Cooperative Extension, Texas A&M University, online publication, hortipm.tamu.edu/publication/pathup1.html
Windham, Alan, Ph.D., "Observations of Downy Mildew of Rose at Retail Nurseries", SNA Research Conference, Volume 38, Section 5, Pathology & Nematology, 1993
Horst, R. Kenneth, "Compendium of Rose Diseases", American Phytopathological Society, APS Press, St. Paul, Minnesota, 1996
"Integrated Pest Management: Plant Diseases - Downy Mildew", University of Connecticut online fact sheet, http://www.hort.uconn.edu/Ipm
"Rose - Downy Mildew", Oregon State University Extension Service online fact sheet, http://plant-disease.ippc.orst.edu
Pearce, Mila, "Rose Diseases in the Landscape", University of Georgia College of Agricultural and Environmental Sciences, Cooperative Extension Services, Bulletin 1280, June, 2005.
Wycoff, Jeff, "Diseases of Roses - Downy Mildew", American Rose Society online publication, http://www.ars.org/About_Roses/disease_downey2.html