Powdery mildew is a common fungal disease encountered in greenhouse production of lettuce. Many growers struggle with controlling the disease on lettuce crops grown in protected environments, especially during the fall and winter months. This fact sheet details the causal pathogen, favorable environmental factors and recommendations for control for greenhouse producers.
|Figure 1. Powdery mildew produces patches of white powdery growth on the upper and lower leaves and stems. Photo by Beth Scheckelhoff, Ohio State University Extension.|
Powdery Mildew Symptoms
Symptoms of powdery mildew include patches of white powdery growth on the upper and lower leaves and stems, as shown in Figure 1. Disease severity depends upon lettuce type and variety, age and overall condition of the plant, as well as microclimate conditions. Older leaves and mature plants are usually affected first and become chlorotic and deformed, limiting plant growth and marketable yield. Severely affected tissue may turn necrotic and die.
Powdery mildews are typically host-specific, although plants in the same family can be susceptible to the same mildew species. The fungus Golovinomyces cichoracearum (formerly Erysiphe cichoracearum) causes powdery mildew on lettuce. Strains of G. cichoracearum can infect over 150 species, including other greenhouse crops in the Asteraceae family such as Achillea, Dahlia, Cosmos, Dendranthema, Leucanthemum, Zinnia and Gerbera, among others.
The Perfect Storm
The greenhouse environment provides ideal conditions for development of powdery mildew on lettuce. Mildew spores are spread by air currents, and if conditions are favorable, the spores will germinate and infect a plant, initiating the disease. Conditions favoring development of powdery mildew are:
- Dense foliage canopy
- Restricted air flow
- Ideal temperature for spore germination
- Optimal temperature is 64 to 77 degrees Fahrenheit (18 to 25 degrees Celsius).
- Able to occur at a broad temperature range of 41-86 degrees Fahrenheit (5 to 30 degrees Celsius).
- Ideal relative humidity (RH) levels for spore germination
- Optimal RH is 95 to 98 percent.
- Could occur when the RH is at 50 percent, enabling the disease to develop in both humid and dry environments.
Interestingly, free water on plant surfaces prevents spore germination and infection for powdery mildew.
Controlling Powdery Mildew
Powdery mildew can be controlled with a multi-pronged approach involving plant selection, environmental control and the use of fungicides. The first step in alleviating powdery mildew concerns in lettuce production is to select varieties less susceptible to the pathogen. Studies have shown leaf and butterhead lettuce types show more resistance to powdery mildew than romaine and crisphead lettuces. Check seed company and university trial results for varieties and cultivars that show resistance, though performance may vary depending on your specific greenhouse conditions and cultural practices.
Figure 2. Nutrient film technique troughs have a fixed plant spacing, and scouting efforts should increase when the crop canopy begins to close (leaves from one plant touch those of a neighboring plant). Photo by Beth Scheckelhoff, Ohio State University Extension.
Monitor and scout for disease presence regularly, especially in sections of the greenhouse with reduced air flow, dense plant canopies, or standing water. Examine both the upper and lower side of leaves. Powdery mildew forming on the underside of leaves can go undetected and move quickly through the greenhouse before it is noticed.
Growers can manage powdery mildew with:
- Proper greenhouse ventilation to keep humidity levels low.
- Increased air movement within the plant canopy.
- Recommended fungicides.
When detected on plants, treat with a fungicide labeled for powdery mildew management on lettuce. Carefully read the label and follow all guidelines. Note that products labeled for control of powdery mildew on ornamentals may not be labeled for use on greenhouse vegetables (e.g., Insignia®). Likewise, products labeled for application to lettuce may have a label restriction for use in the greenhouse (e.g., some products with chlorothalonil). Remember to rotate fungicide products with different modes of action (designated as group or FRAC codes) to manage resistance issues.
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- Simko, I, Rauscher, G., Sideman, R.G., McCreight, J.D., & Hayes, R.J. (2014). Evaluation and QTL mapping of resistance to powdery mildew in lettuce. Plant Pathology, 63, 344-353.
- Lebeda, A. & Mieslerova, B. (2011). Taxonomy, distribution and biology of lettuce powdery mildew (Golovinomyces cichoracearum sensu stricto). Plant Pathology, 60, 400-415.