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Plant Insect & Pest Profiles: Powdery Mildew

Plant Insect & Pest Profiles: Powdery Mildew

Control of Powdery Mildew

Powdery Mildew is a white fungus found growing on the leaves of various trees, shrubs and annuals. It afflicts many commonly found trees and shrubs: Red-tipped Photinia (Photinia x fraseri), Sycamore Trees (Platanus acerifolia), Crape Myrtles (Lagerstroemia indica varieties), some Cultivars of Ninebark (Physocarpus species), some varieties of Euonymus shrubs, Rose bushes and some Stone Fruit Tree species (Rosa & Prunus ), Apple Trees (Malus), on both the leaves and fruit, and some species of squash and cucumber plants. Powdery Mildew can also be a big problem when growing grape vines.
It can grow on the tops or undersides of leaves, stems and fruits.

How to Identify

Powdery Mildew is white or pale gray in color, and tends to grow in circular patterns on leaves. The spores are spread by the wind. They do not need water to germinate.
Powdery Mildew can distort and stunt new growth on plants, and cause premature leaf drop, as well as being unsightly. Powdery mildew is ugly, but will not kill plants.

Cultural Control of Powdery Mildew

Despite being called Mildew, Powdery Mildew, does prefer and thrive in dry conditions, which is why it takes off in our drier seasons. The spores will survive on plant tissue only, on leaves and buds, but will not live in soil. So it is important to dispose of all diseased plant material, don’t put it in your compost pile, as spores can over-winter and infect plants in the Spring.

Favorable conditions for spores growing, are areas with warm days and cool nights. Ideal temperatures for the fungus spreading are 70-85 Deg. F. Direct Sunlight and rain will prevent powdery mildew from spreading.

Improve Cultural Conditions of Plants

A good first step of cultural control, is to pay attention to the cultural conditions of afflicted plants. If plants are stressed, they will be more susceptible to disease:

  • Pruning to increase air circulation while dormant, or earlier in the season, will help, particularly with branches or plants that are in the shade.
  • Adequate Water Make sure that the plants are healthy, pay attention to their water requirements
  • Drainage make sure that drainage is good, dig in some soil amendment
  • Fertilize plants to make sure that they’re as healthy as they can be.
  • Remove Dead Leaves, or buds that drop to prevent mildew from spreading

Treatment

  • Wash off leaves regularly with a jet of water
  • Spray with Neem Oil or Horticultural Oils
  • Spray with Organic Fungicides, such as sulfur

Choose Mildew Resistant Varieties:

  • Platanus acerifolia ‘Columbia’, ‘Liberty’ and ‘Yarwood’ have some resistance.
  • Popular Roses such as ‘Just Joey’ and ‘Olympiad’ are resistant. Here’s a list of Roses resistant to different diseases
  • Crape Myrtles with excellent resistance are: Tonto, Muskogee, Acoma, Souix, Tuskegee

As always, it is worth seeking out the advice of Tree and Landscape professionals to ensure the very best care for your plants.

Good references for more information on Powdery Mildew: UC Davis and University of Washington

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