Downy Mildew is a serious and widespread disease that affects both outdoor and protected crops of roses. It is caused by the obligate parasite, Peronospora sparsa, and can lead to unsightliness, poor growth, and reduced marketability of rose plants. Severe attacks of the disease can result in rapid defoliation of plants. The disease can be difficult to identify and control, making it a significant challenge for rose growers.
Downy Mildew Management in Rose
The Disease cycle of Downy Mildew
- The disease cycle of downy mildew in roses begins with the presence of the pathogen’s spores (sporangia) on the underside of the leaves. These sporangia are easily dispersed by air currents and water splashing from infected plants onto healthy ones.
- Once the sporangia come into contact with a suitable host, they germinate and infect the plant, penetrating the surface and growing into the plant tissue. This initial infection can take a few days to a few weeks to become visible, depending on environmental conditions.
- As the pathogen grows, it produces more sporangia on the surface of the infected plant tissue, which can then be dispersed to nearby plants. The sporangia can also survive the winter in infected plant material or the soil, allowing the pathogen to persist from one growing season to the next.
Factors Favoring Population Increase in Downy Mildew
- Cool and humid conditions: The incidence of Downy Mildew disease increases during cool and humid conditions, with prolonged leaf wetness periods. Disease development is favored by 90-100% humidity and relatively low temperatures.
- Temperature: Germination of P. sparsa sporangia has been observed to be high from 2-18°C and declines under and over these temperatures. No germination has been observed at 26°C.
- Spore production: Spores are produced in great numbers on the undersurface of infected leaves in short periods, about three days, and for as long as one month under favorable conditions. In drier conditions, spores are often sparse and not easily noticed.
Symptoms of Downy Mildew
- Leaf symptoms: Infected leaves develop purplish-red to dark-brown irregularly-shaped spots and blotches. The lesions usually have clearly defined edges and may be angular in appearance. Affected leaves may turn yellow and fall prematurely, resulting in rapid and severe plant defoliation. Sporulation of downy mildew occurs on the undersurface of leaves but can be difficult to see; it appears as a sparse, white to light grey downy growth. In the later stages of infection, leaves may become necrotic.
- Flower symptoms: Infection of the flower bud or flower results in retarded and distorted growth. Black or brown dead areas form on the petals, and the calyx can also be affected. Infected flower stalks or stems show reddish-brown elongate lesions.
- Stalk symptoms: Occasionally, young shoots become heavily infected, causing dieback.
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Downy Mildew Management in Rose by Biological Method
- Biological control is a sustainable and environmentally friendly approach to managing plant diseases, including Downy Mildew in roses. One example of a biological control agent is the bacteria Bacillus subtilis. When used as part integrated pest management program, Bacillus subtilis can help control the spread of P. sparsa, the causal agent of Downy Mildew in roses.
- Bacillus subtilis is a soil-dwelling bacteria that have been shown to have antifungal properties. It works by colonizing the plant’s root system and producing antimicrobial compounds that inhibit the growth of P. sparse. Doing so reduces the incidence and severity of Downy Mildew in roses.
Downy Mildew Management in Rose by Chemical Method
Chemical management is also an important tool for downy mildew in roses. The following steps can be taken for effective chemical control of downy mildew in roses:
- Early detection: Early disease detection is important for effective chemical control. Regular crop monitoring is necessary to detect the disease at an early stage. Infected plants should be removed and destroyed.
- Fungicide rotation: A well-managed chemical spray program starting early with a rotation of chemicals is important for preventing and controlling downy mildew. Fungicides against downy mildew can be categorized as preventive, early, or late curative products. It is recommended to rotate the use of fungicides from different chemical classes to prevent the development of resistance.
- Preventive fungicides: Preventive fungicides such as Antracol must be applied before an infection period begins. They work by protecting new growth from infection. It is important to note that new growth following applications will not be protected.
- Early curative fungicides: Early curative fungicides such as Infinito work against spore germination, sporangia elongation, and penetration. They should be applied as soon as symptoms are detected or as a preventive measure before an infection.
- Late curative fungicides: Late curative fungicides such as Verita and Melody Duo deal with intracellular infection levels and are applied when symptoms are visible. They work by stopping the development of the disease and preventing its spread.
- Drenching with previcur energy: As downy mildew can overwinter in the crop that was infected in the previous season, early drenching with Previcur Energy can control the spread of the disease.
Preventive Measures for Control of Downy Mildew
Preventing downy mildew in plants is key to avoiding the need for control measures. The following preventive measures can help reduce the risk of downy mildew infection:
- Proper spacing: Proper plant spacing is important for reducing the risk of downy mildew infection. Adequate space between plants ensures good air circulation, which can help prevent the spread of the disease.
- Pruning: Keeping plants well pruned can help prevent the spread of downy mildew by removing infected plant material. This also helps to improve air circulation and reduce humidity levels, which can create conditions for disease development.
- Watering: Watering at the soil level rather than on the foliage can help reduce the risk of downy mildew infection. If you do water on the foliage, try to do it in the morning so the water can evaporate quickly. This reduces the amount of time that the foliage is wet, which can help prevent the development of the disease.
- Clean garden debris: Cleaning up any debris in the garden in the fall can help prevent the overwintering of the pathogen responsible for downy mildew. This reduces the risk of the disease developing in the following growing season.
- Clean tools: Cleaning your gardening tools well in between plants can help prevent the spread of the disease from one plant to another. Wiping your blades with an alcohol wipe or a rag soaked in soapy water can help prevent the spread of downy mildew and other plant diseases.
- Prune well: Proper pruning practices can help reduce the risk of downy mildew infection. This involves removing infected plant material and creating good air circulation, which can help prevent the spread of the disease.
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Conclusion
Downy mildew is a common and destructive disease in roses that can cause significant damage to plants if left unchecked. Fortunately, many measures can be taken to prevent and manage the disease. It’s important to be aware of the disease cycle of downy mildew, influenced by environmental conditions such as temperature and humidity, and to take steps to break this cycle by removing and destroying infected plant material and using good sanitation practices.
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