Downy mildew in roses

Ornamentals Advisory Blog

Roses are the leading cut flowers in global floriculture trade. Cut roses, either planted outdoors or in plastic greenhouses are susceptible to many phytopathogens. Downy mildew is among the most serious foliar diseases which affects the quality and yield of cut roses during production. Any damage on leaves, flower buds or stems affects quality standards and impact revenues. All rose cultivars grown in East Africa are susceptible to downy mildew, although they can differ greatly in their sensitivity.

Downy mildew is caused by the oomycete Peronospora sparsa. It may cause severe losses under plastic greenhouses, but in heated greenhouses the disease may not be severe. Peronospora sparsa infects young leaves and stems at the shoot apex, peduncles, calyxes, sepals and petals. The first signs of the disease are angularly-edged, purple-red spots that evolve into dark brown lesions followed by defoliation of affected plants. In infected flowers and stems, purple brown lesions are common, accompanied by malformed floral buds.

Rose leaves infected by downy mildew develop purplish red to dark brown irregular spots

Severe defoliation of infected plants reduces plant vigor while rootstock cuttings may fail to root, and the canes cannot reach the diameter required for the market. In addition, there is the possibility of spreading the pathogen through propagation materials. The incidence of downy mildew disease increases during cool humid conditions with prolonged leaf wetness periods. Germination of P. sparsa sporangia has been observed to be high from 2-18°C and to decline under and over these temperatures. No germination has been observed at 26°C. Spores are produced in great numbers on the undersurface of infected leaves in short periods of time, about three days, and for as long as one month under favorable conditions. In drier conditions, spores are often sparse and not easily noticed. Sporulation may take place in infected rose leaves before symptoms appear. 

Disease development is favored by 90-100% humidity and relatively low temperatures. Therefore, rose downy mildew occurs mainly in greenhouses, rather than outdoors. In greenhouse production of cut roses, epidemics usually begin in localized areas of the crop. The frequent appearance of the disease in “hot spots” and the tendency for severe symptoms to appear suddenly under favorable conditions supports the hypothesis that the pathogen overwinters in the plant probably as dormant mycelium.

Under optimal conditions, P. sparsa can quickly spread over the whole field. Different studies have shown that roses grown under less than 85% relative humidity were not infected. These factors indicate the importance of maintaining low relative humidity levels by using heat if necessary, and of avoiding sudden temperature drops during the night, as they greatly increase relative humidity. Good ventilation and air circulation are also important. Indeed, the feasible environmental conditions that may limit the development of rose downy mildew in greenhouses seem to be those provided by hot air heating systems and motile ventilation in the roof. The reduction or elimination of water leaks in the greenhouses is strongly recommended.

Most rose growers regularly and systematically monitor and inspect plants or areas of the crop to detect the disease and determine the adequate control method and corresponded timing when necessary. Integrated pest management (IPM) implies a multifaceted approach and stresses the importance of early pest detection. 

Control measures should include cultural practices such as the removal and destruction of cuttings and symptomatic leaves along with infected fallen leaves and stems. Downy mildew crop protection programs in rose production currently involves the use of foliar fungicides that are applied preventively. Protectant sprays with fungicides are recommended when environmental conditions are ideal for the infection. Chemicals like Daconil (chlorothalonil), Revus (mandipropamid) and Ortiva (Azoxystrobin) provide good protection. Phenylamide containing fungicides e.g. Ridomil Gold and Folio Gold are used to control and reduce the severity of the disease. Disease management is more effective when full coverage of the underside of the leaves is achieved during spray application with contact fungicide treatments or when systemic products such as those in FRAC groups 4 (mefenoxam) are employed in rotation. To slow the development of resistance, it is highly recommended that label directions are followed closely and that rotations occur among different modes of action as established by the Fungicide Resistance Action Committee (FRAC).