Problems with powdery mildew

Growers easily recognize the characteristic white colonies of powdery mildew. Begonia, calibrachoa, dahlia, gerbera daisy, hydrangea, rose, phlox, petunia, rosemary, torenia and verbena are all prone to this disease. Although powdery mildews rarely kill a plant, epidemics can occur that can cause complete crop loss on sensitive crops or cultivars.

What to look for
Symptoms may appear first on the upper leaf surface, but they can also develop on the underside, with pale spots on the upper leaf surface. Sometimes symptoms resemble those of a nutrient deficiency, with the oldest leaves turning yellow or a reddish discoloration developing.

When infected with powdery mildew, sedum develops brown scabby spots that can easily be confused with a leaf spot disease or spray injury. Flowers, petals and bracts can also become infected. When symptoms develop on the more mature leaves, powdery mildew is harder to detect and seems to occur almost overnight, catching many growers by surprise.

All powdery mildew fungi need a living plant host in order to grow and develop. Because they tend to look alike, growers often assume that powdery mildew infections are caused by the same fungus. Some of the powdery mildew fungi have wide host ranges, however, and others are relatively host-specific. For example, Erysiphe cichoracearum infects many different annuals, perennials and vegetables, whereas Sphaerotheca pannosa var. rosae only infects roses.

The powdery mildew that infects verbena also infects cucurbits, so it’s important not to grow squash and cucumber transplants in the same house with susceptible verbena varieties. Gary Moorman of Penn State has developed a powdery mildew by host plant cross-list that is available online (bit.ly/powdery-mildew-host). If powdery mildew is affecting one type of plant in your greenhouse, this can help you decide if other plants in your greenhouse need to be monitored and treated.

Unlike many foliar diseases, powdery mildew does not need free moisture on the leaf to develop. Favorable environmental conditions include high relative humidity, moderate temperatures and relatively low light levels. Infections may be more common in the spring and fall when changes between day and night temperatures encourage high relative humidity levels, especially at night.
 

Prevention tips
To prevent powdery mildew, maintain proper plant spacing to reduce relative humidity levels within the plant canopy. This will also help you gain better spray coverage. Grow susceptible crops on wire benches to help increase airflow.

Heat and ventilate in the late afternoon and early morning to reduce relative humidity levels at night, and avoid watering late in the day. Clean your greenhouse thoroughly between crops, removing all weeds that could be reservoir hosts. Select less susceptible cultivars whenever possible. For example, verbena cultivars vary greatly in their susceptibility to powdery mildew.

You can also try to group mildew-susceptible crops or varieties together. This makes scouting and spraying easier. Begin scouting early and often, at least once a week, even every two to three days, if possible. Check areas with poor air circulation and known hosts, and look closely on lower and mature leaves where powdery mildew is often overlooked.Turn the leaves over, especially if you see pale spots on the upper surface. Take a closer look with your hand lens to look for fungal strands if you suspect nutrient deficiency symptoms.

Fungicide management
Alternate between systemic materials, contact materials and biologicals. Some powdery mildews are resistant to certain systemic fungicides, so rotate among FRAC codes of the systemic fungicides and alternate with contact fungicides to help preserve their effectiveness. Be sure to consult and follow pesticide labels for registered uses.
 

Pundt is an extension educator at the University of Connecticut and a frequent contributor to Greenhouse Management. Have a question? Reach her at leanne.pundt@uconn.edu.