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Greenhouse Updates: Apr 24, 2015

Powdery Mildew
April 24, 2015

Powdery mildew was reported on rosemary this week.

In addition to rosemary, the characteristic white spots of powdery mildew usually first appear first on the upper surfaces of the lowermost leaves of many types of plants such as non-stop begonias, calibrachoas, dahlias, asters, phlox, monarda and other susceptible crops. Stems and flowers may also become infected.

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. Powdery mildew diseases are caused by different species of fungi such as Erisyphe, Leveillula, Microsphaera, Podosphaera, Odium, and Sphaerotheca. 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 Podosphaera xanthii that infects verbena and calibrachoa also infects cucurbits, so it’s important not to grow squash and cucumber transplants in the same house with susceptible verbena varieties and calibrachoas. For more examples of cross listing see the link at the end.

The whitish powder on leaves is composed of fine threads of fungal vegetative tissue (mycelium) and light colored mats of asexual spores (conidia). Some powdery mildews produce conidia on short, erect branches that resemble tiny chains, while others form threads so sparse that the mildew cannot be seen without the aid of a microscope. These spores (conidia) are easily moved by air movement and water splash. Because powdery mildews are obligate parasites, they require living hosts.  So, unlike Botrytis blight they do not colonize dying or weakened plant tissue. When spores land upon a susceptible host, they germinate and send a specialized feeding structure into the epidermis to obtain their nutrients from the plants. This infection process may take as little as 3 days or as long as 7 days. The pathogen survives in the greenhouse in weed hosts or on crops. Outside, the pathogen can overwinter as mycelium in infected plant parts or in resting structures and are visible as small, dark specks on dying leaves.

Powdery mildews, unlike most other fungal diseases, do not need free water on the leaf surface to germinate and infect. They are favored by high relative humidity (greater than 95%), moderate temperatures (68°-86° F), and low light levels.
To prevent powdery mildew, maintain proper plant spacing to reduce relative humidity levels within the plant canopy. This will also help with 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. Select less susceptible cultivars whenever possible. For example, monarda and phlox cultivars vary greatly in their susceptibility to powdery mildew.

Early detection is needed to prevent the plants from becoming covered with the white fungal mycelium. Powdery mildews only colonize the upper layer of cells, so chemical eradication is possible. Fungicides should be applied as soon as the disease is first detected. Additional treatments may be necessary to protect new growth. Rotate among fungicide classes to discourage development of resistance.

There are many fungicide labeled for powdery mildew on ornamentals, however fewer fungicides are labeled for herbs. Fungicides labeled for herbs include Bacillus subtillus (Cease), hydrogen dioxide (OxiDate), copper sulfate pentahydrate (Phyton 35), potassium bicarbonate (Milstop) and Streptomyces (Actinovate). Most fungicides work as protectants and repeat applications may be needed to protect the new growth from infection. Read label directions carefully for rates and registered crops.

More information:

Powdery Mildew Diseases of Ornamental Plants, UMass Extension

Powdery Mildew Cross Listing, Gary Moorman, Penn State Extension

Leanne Pundt, UConn Extension and Tina Smith, UMass Extension