Look for powdery mildew in the greenhouse

Stop powdery mildew before it explodes into an epidemic

Powdery mildew epidemics can occur without warning. Depending on how the disease is managed, there may be minimal impact or complete crop loss. The white talcum-like growths, or colonies, are initially small but can rapidly blight the leaves, stems and flowers of susceptible crops. Growing crops susceptible to powdery mildew can be a challenge, and fungicides have typically played a key role. Powdery mildews are tricky and have been known to genetically adapt to overcome some of the most effective fungicides. Since the powdery mildews are a problem each year, a long-term strategy is needed. This strategy may include fungicides, but could also take advantage of the plant’s natural resistance to the powdery mildew fungus. Some powdery mildews are caused by Erysiphe cichoracearum, which can infect many different annual and perennial flowers.

Powdery mildew can occur on all above-ground plant parts and results in white growth on the plant’s surface. When the fungus reproduces, the abundant conidia, or spores, give a white, powdery or fluffy appearance. Severe infection can cause yellowing and withering of leaves and restrict plant growth. Powdery mildew can infect plants even when the relative humidity is low, but epidemics are prompted when relative humidity is high. Sometimes the powdery mildew progresses unnoticed until many plants are infected.

Some plant species such as gerbera daisy, verbena and begonia are very susceptible and should be sprayed more frequently with the most effective fungicides. Other plant species may not need frequent applications, but should be scouted regularly for signs of the disease.

Powdery Mildew “A” Team
Eagle myclobutanil
Terraguard triflumizole
Powdery Mildew “A-/B” Team
Compass O trifloxystrobin
Heritage WDG azoxystrobin
Insignia WG pyraclostrobin
Zyban thiophanate-methyl + mancozeb
Strike triadimefon
Pageant pyraclostrobin + boscalid

For more information, visit MSU’s Department of Plant Pathology’s Ornamental Research web site.

Powdery mildew on gerbera flower.
Photo A. Powdery mildew on gerbera flower.

Powdery mildew on gerbera leaf.
Photo B. Powdery mildew on gerbera leaf.

Powdery mildew infection on poinsettia bracts.
Photo C. Powdery mildew infection on poinsettia bracts.

Powdery mildew infection on a rose leaf.
Photo D. Powdery mildew infection on a rose leaf.

Powdery mildew infection on verbena foliage
Photo E. Powdery mildew infection on verbena foliage.

Powdery mildew conidium that has been germinated and infected a poinsettia bract
Photo F. Powdery mildew conidium (arrow) that has germinated and infected a poinsettia bract and is producing a new conidium (C) on a stalk (S).

This research was funded in part by Floriculture Nursery and Research Initiative of the Agricultural Research Service under Cooperative Agreement #59-1907-5-553 and by the American Floral Endowment.

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