Management of Powdery Mildew Review

 Powdery mildew is a true fungus, with several genera (now grouped into tribes) based on how the conidia (spores) are produced. Conidia are windblown and/or splashed, land on leaf surfaces and then begin to germinate. As spores develop into hyphae, a pentration peg is inserted in the plant cells to obtain nutrients for growth. High humidity (70%+) favors development but infections can also occur at lower humidities. Leaf moisture and wetness on upper leaf surfaces limits disease development, opposite of requirements for most other foliar plant pathogens. Powdery mildew is often confused with downy mildew, which is a true water mold. The difference in the two is that powdery mildew produces spores on the top of the leaf (although can sometimes can be on bottom) and downy mildew almost always produces spores on the underside of leaves.

Powdery mildews develop as thin strands of hyphae, form a mycelium mat that colonizes leaf surfaces. Typically white in color, the mycelium does not kill the plant but will interfere with photosynthesis and can slow plant development. Heavy coatings can limit salabilitiy of plants and be a active source of infective conidia. It takes as little as 3 day up to as much as 7 days for infection to occur and new conidia to form. The are many tribes of powdery mildew that affect bedding plants such as zinnia, rex begonia, sedum, gerbera, dahlia, verbena, poinsettia and more. Many herbaceous perennials such as phlox, aster, solidago to roses, hydrangea, dogwood trees and others are prone to powdery  mildew.  In the fall, on ornamentals grown outside, black specks within the powdery mildew colony may be noticed. These structures are known as chasmothecia and are formed to be the overwintering structure of the pathogen. 

Be aware of a newer powdery mildew showing up on petunia and calibrachoa.  Symptoms start with the yellowing of lower leaves, then faint, fuzzy, white spores begin to appear. As the powdery mildew colonies form, the typical white mycelium strands are sutble and my not be observed easily. As the disease progresses, lower leaves turn brown and dry. This strain or tribe of powdery mildew can be transferred to and from verbena and squash plants. A good rule is to grow ornamentals separate from vegetable crops. Powdery mildew can infect herbs (rosemary) and vegetables like lettuce, legumes and curcurbits.

Cultural management options include keeping humdity under 85%, providing good air movement by using HAF fans, utilizing plant spacing and venting to reduce excessive humidity. Choose cultivars with disease resistance when possible. When scouting, examine the underside of leaves and when detected, remove leaves prompty by bagging and discarding.

Chemical fungicides should be used preventatively on highly susceptible crops and used in rotation, always alternating modes of action (MOA) to reduce the possibility of pesticide resistance . 

All applications for powdery mildew will be by foliar sprays. Always select a fungicide based on crop and site locations. There are several biological and soft biopesticides available that can be useful to rotate into spray programs.

Green -preventative and curative

Pink -preventative only