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.
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: