Greenhouse Disease Management
Powdery Mildew
Powdery mildew is probably one of the most common and widely distributed plant diseases of plants in greenhouse production. Powdery mildews are a group of fungi including the genera Erysiphe, Leveillula, Microsphaera, and Sphaerotheca that produce gray or white powdery growth. This disease is responsible for significant economic losses in many greenhouses floricultural (e.g., roses, violas, African daisy, zinnias), and vegetable (e.g., tomatoes, cucumbers) crops. Although infections usually do not result in plant death, they reduce crop aesthetics and value.
Disease Cycle
In the greenhouse, powdery mildew fungi have a simple life cycle. Single-celled conidia (spores) form in a long chain on short, erect fungal stalks. This creates the “fluffiness” usually associated with powdery mildew. Conidia need a relative humidity of 95 percent or a near-zero vapor pressure deficit for more than 3 to 4 hours to germinate and penetrate the host’s leaf or stem epidermal cells. Conidia are “powdery” and are readily disseminated by air currents in the greenhouse.
Symptoms
Powdery mildews are easily recognized by the white, powdery growth of the fungus on infected portions of the plant host (See Figure 28.7). The powdery appearance results from the superficial growth of the fungus as threadlike strands (hyphae) over the plant surface and the production of chains of spores (conidia). Colonies vary in appearance from fluffy and white to sparse and gray. Powdery mildew fungi usually attack young developing shoots, foliage, stems, and flowers but can also colonize mature tissues.
Cultural Management Strategies
Monitor crops on a regular basis for powdery mildew diseases. Epidemics that seem to develop overnight are often the result of undetected low-level infections that have spread spores throughout the greenhouse. Rogue infected plants or prune out diseased tissue. Perform this operation when plants are wet or immediately place diseased material into a plastic bag to prevent spores from spreading.
Chemical Management Strategies
Fungicides may be necessary when conditions are favorable for disease. Both eradicant; protectant fungicides are registered for powdery mildew controls. Protectants prevent powdery mildew from developing, whereas eradicants stop pathogen development from any further damage. Therefore, apply protectants when the conditions are conducive to powdery mildew outbreaks; yet apply eradicants only when the pathogen is present. Because the genera and species of fungi causing powdery mildew are diverse, there may be some variation in fungicide effectiveness across crops.
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