Powdery Mildew on a rose leafAt this time of year, it is common to see a whitish-gray powdery substance on the leaves of some plants. Powdery mildew can affect any plant, but lilacs, roses, fruit trees, vegetables, begonias, and lawns are particularly susceptible.

The term “powdery mildew” actually refers to an entire group of fungi, each one attacking different types of plants. Since the fungi are species specific, powdery mildew on your fruit trees won’t spread to camellias or other plants. It usually starts with a few round white or grayish spots that you can rub off with your finger. They spread and join until the entire top leaf surface is covered, then it moves on to the underneath, stems, flowers, and fruit.

Powdery mildew on a lilac leafAs powdery mildew takes over, photosynthesis becomes very difficult for the plant. This can cause growth of the plant to slow and distort, leaves to fall, and flowers and fruits to fail to form properly.

Powdery mildew thrives in dry weather with high relative humidity. Unlike other fungi, it doesn’t like rain or extreme heat, and it tends to slow when temperatures soar over 90° F. Powdery mildew needs a dry leaf surface, moderate temperature, and high humidity to form. These conditions are often caused by:

  • Seasonal weather: A summer that combines drought with high humidity is an invitation to powdery mildew.
  • Crowded plants: Densely packed plants, overgrown shrubs, or plants under trees, are more susceptible to powdery mildew due to poor air circulation and cool, humid conditions.
  • Overfeeding: Lush, succulent growth caused by over fertilizing is particularly susceptible to powdery mildew.

How to Treat Powdery Mildew

Powdery mildew may seem to spring up faster than you can control it, but the good news is that it’s easy to treat and not immediately fatal. Take these steps to treat powdery mildew on your plants:

  • Prune: Cut off affected growth, prune the plant to open it up to more air circulation, and trim back tree limbs that might be shading too densely.
  • Disinfect: Clean pruning tools in a bleach/water solution to kill any remaining spores.
  • Clean Up: Pick up all fallen leaves and pruning debris and put either in the trash or in a hot compost pile.
  • Stop Fertilizing: Reduce nitrogen fertilizer in order to slow down succulent growth.
  • Spray with Water: If plants are in the sun, try washing the patches off the leaves with a spray of water. Avoid extra water in shady or damp areas.
  • Spray with Fungicide: If all else fails, spray plants with an eradicating fungicide. Check the label to make sure it’s rated both for powdery mildew and for your plant type.[ NOTE: Meadows Farms carries Daconil, Fung-Onil, Copper fungicide, and Dragon Dust, to name a few)

Natural treatments include neem oil, copper, and potassium bicarbonate.