Greenhouse Disease Management
Downy Mildew
Downy mildew diseases pose an increasing problem in the horticultural industry causing serious losses in many greenhouse crops. Downy mildews present a challenge to growers both because the disease can be present but not obvious; and because they are difficult to control with fungicides once established. The pathogens are very different from powdery mildew—they attack different plants under very different environmental conditions, and are controlled by different classes of fungicides. Downy mildew diseases are caused by a group of fungus-like organisms: they are not true fungi, and are similar to Pythium and Phytophthora species. Most of the downy mildew fungi are host specific and infect only one plant family. Pathogens include species of Peronospora, Bremia, Plasmopara, and Basidiophora. Often confused with powdery mildew, downy mildew almost always is found patches on the underside of foliage as soft and fluffy gray, purplish, or light brown sporulation.
Disease Cycle
The fungus overwinters in or on plant parts as mycelium or oospores (thick-walled, gumball-like structures that form the resting stage of the pathogen). Temperature and humidity play key roles in the pathogen’s development.
Symptoms
Symptoms of downy mildew include blotchy yellow or brown lesions on leaves, a general yellowing, leaf distortion and stunting (See Figure 28.3). These fungi are primarily foliage blights but will attack and spread rapidly into young green tissues of growing tips and flower buds, causing stunting and distortion. These symptoms can be mistaken for other infectious or noninfectious problems.
Cultural Management Strategies
Infection is favored by cool greenhouse temperatures and high humidity. Free moisture is required for spore germination. Increasing night temperatures using both crop and overhead heating lines and raising minimum night temperatures to reduce the radiant energy loss from leaf surfaces can be effective. Many epidemics begin during extended periods of cool, damp weather, when growers attempt to economize on fuel consumption by adopting low-temperature set points or by not operating the heating system during the late spring or early fall.
Chemical Management Strategies
Implement appropriate control procedures immediately, because the spores are readily dispersed by air currents, and further disease development may be very rapid under favorable environmental conditions. Ensure good coverage, particularly with protectant fungicides. Make fungicide applications every seven to ten days, being sure to rotate or tank mix fungicides with different chemical classes to prevent fungicide resistance from developing.
Click on the following topics for more information on greenhouse disease management.
