Preventing and Controlling Powdery Mildew

Powdery mildew on rose foliage
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Powdery mildew is a fungal disease that results in a powdery gray or white coating on the leaves and stems of infected plants.  Several different species of fungi in the order Erysiphales can cause the disease, though the symptoms are always similar. A powdery mildew infection generally starts out as a few spores on the leaves but quickly spreads. The white powdery surface is a thick coating of the fungi spores.

The fungi easily overwinters in garden debris, and thus is very hard to eradicate entirely.  It can eventually cause yellowing the leaves and premature leaf drop.

As virtually all gardeners know, powdery mildew thrives in humid conditions with moderate temperatures. In any region with humid summers, powdery mildew virtually always makes its appearance. While it can affect any plants, there are certain species much more susceptible to heavy infection: 

Some Plants Susceptible to Powdery Mildew:

  • Apples
  • Begonia
  • California poppy
  • Dahlia
  • Delphinium
  • Hollyhock
  • Hydrangea
  • Lilacs
  • Monarda
  • Oak
  • Strawberries
  • Phlox
  • Roses
  • Strawberries
  • Zinnia

Damage to Plants

In many cases, powdery mildew does little damage to plants but is merely unattractive. And some plants seem so susceptible to powdery mildew that it is virtually unavoidable--an expected cost if you choose to grow these species. Many gardeners simply resign themselves to the fact that powdery mildew will make an appearance nearly every year and don't  bother to fight it aggressively.

 

But besides being unattractive, powdery mildew can in extreme cases result in leaf yellowing and dropping, stunted plant growth, distortion of buds, blooms, and fruit, and eventual overall weakening of the plant.

Disease Life Cycle

Spores overwinter on diseased plant parts and begin asexual production of new spores once the weather warms.

New spores are carried on the wind to other parts of the plant or to other nearby plants. Spores never stop producing more spores, so if infected leaves are not destroyed, the problem can never be eradicated and will only get worse.

Treatment and Prevention

Powdery mildew thrives in temperatures between 60° and 80°F.  Dry, shady conditions are ideal, as are areas with poor air circulation. Few chemical fungicides offer much cure for powdery mildew; the best strategies are more mechanical in nature, such as simply removing and destroying diseased plants and plant parts.

 Planting disease-resistant cultivars and making sure you allow for good air flow are two ways to guard against powdery mildew. Beyond this, possible control methods to keep powdery mildew in check include the following: 

  • Plant species that are most susceptible in a location where they can receive early morning sun. This will allow condensation to dry out quickly and reduce the humid conditions that foster the fungus.
  • Enhance air circulation by spacing plants well apart. Better ventilation will reduce the disease. Dense plants can be thinned out to improve air flow.  
  • Inspect plants regularly during warm, dry conditions, and remove any leaves that show signs of infection. Destroy (do not compost!) infected plant parts.
  • A spray made with baking soda, if applied weekly at the first signs of infection, can protect plants against further damage.
  • Plants that are badly infected should be ripped out and destroyed to prevent the disease from spreading further.
  • A commercially available organic option is Neem oil, which both treats existing powdery mildew and protects the plant against further infection.

Interestingly enough, an effective measure in preventing and treating powdery mildew is to spray the foliage of your plants daily with plain water from the hose. Powdery mildew hates water! The only caveat with this method is to be sure you do it early in the day so that the foliage completely dries before cooler evening temperatures arrive, otherwise you may invite other fungal diseases such as black ​spotinto your garden.