Downy Mildew in Vegetable Crops

Not all water mold pathogens just infect roots and crowns of vegetables. Some species of Peronospora and a few Phytophthora cause downy mildew on leaves of certain crops. Many vegetable crops including cole crops, radish, lettuce, spinach and onions to name a few, can be infected by a downy mildew, however, each different crop is infected with a different species of the downy mildew pathogens which tend to be limited to a few hosts and some related weeds. With the cool to moderate temperatures, frequent rains together with prolonged leaf wetness periods experienced recently, downy mildew has become a problem in some crops and may become a problem in others if this weather trend continues.

Downy mildew usually infects leaves including cotyledons of susceptible crops but as the leaves become older they also become tend to become less susceptible. If infection occurs during the plant's seedling stage, it may become systemic which frequently results in severe stunting and premature plant death. Often radish, rutabaga and turnip become systemically infected with downy mildew resulting in internal brown discolouration as well as russeting and cracking of the roots.

Although cool temperatures with frequent rains and high relative humidity favour infection, symptoms often don't appear until 5 to 7 days later. Downy mildew symptoms first appear as greasy yellow angular patches on the top of infected leaves (Figure 1). Flipping the leaves over will reveal a thick mat of downy fungal growth on the underside immediately below the chlorotic patches observed on the top of the leaf. Sometimes the growth will appear white, purplish or grey depending on the downy mildew species, age of infection and host. The downy fungal growth on the underside of infected leaves contains sporangium and spores, which are blown or splashed to other host plants. Diseased leaves eventually turn pale, yellow and sometimes become white and papery before they collapse. Severely infected plants often appear stunted. The disease significantly reduces both the quality and quantity of yield. Downy mildew infections are often invaded by secondary pathogens such as soft rot bacteria, which further reduces yield.

The cool temperatures together with the on and off rain periods experienced recently are conducive for many downy mildews to infect crops such as onions, lettuce, spinach and cole crops. In fact the incidence of downy mildew on spinach caused by Peronospora farinosa f.sp. spinaciae has increased in recent weeks resulting in significant losses to growers. Several cases of downy mildew on peas caused by P. viciae have been reported in recent weeks, which could weaken and stunt plants, resulting in some yield loss. Downy mildew on onions caused by Peronospora destructor caused losses in the past, particularly during the wet spring of 2000. Although downy mildew has not been reported in onions yet this spring in Ontario, it is a disease worth watching for, should the weather return to cool and wet conditions. Downey mildew overwinters as persistent oospores in the debris of systemically infected hosts from the previous year. Some downy mildew pathogens can also survive in infested seed depending on the host and pathogen. During the spring when long periods of wet and cool conditions prevail, the oospores germinate and infect host plants.

Managing downy mildew involves the use of cultural practices integrated with fungicide applications where registered. Crop rotation for 3 years will reduce the over wintering oospore populations in soil of most downy mildew pathogens. Obtaining seed from regions with dry conditions where the pathogen is unlikely to survive will reduce the probability of introducing the disease into a field. Controlling alternative weed hosts in and around the field is also important to eliminate overwintering inoculum. Using registered seed treatments for damping-off control will also help control seed borne and early downy mildew infection. Unfortunately, there are no fungicides registered to control downy in some crops like spinach, however, other crops have registered fungicides for downy mildew control, so consult publication 363. Several applications may be required to control this disease. For best results, applications of a registered product should always be made prior to infection. Some regions also have a forecasting system that alerts growers to infection periods in certain crops. Growers who follow the predictions and infection period warnings can effectively apply fungicides before rainy weather occurs.

Figure 1: Downy mildew symptoms (a) on top of cauliflower leaf,
and underside of (b) spinach, (c) Chinsese cabbage leaves, and (d) onion leaf