Powdery mildew on a verbena.

Welcome to the Powdery Mildew edition of Stump the Gardening Stars.

See how much you know about one of our least favorite fungi.

True or false?

1.  Overhead watering increases powdery mildew.

2.  Powdery mildew is worst in humid climates.

3.  Shade is the happiest home for powdery mildew.

4.  The powdery mildew on your cucumbers is the same as the one on your tomatoes.

5.  Powdery mildew always shows up as white powder on plant leaves or fruit.

Answers:

1.  False
2.  False
3.  True
4.  False
5.  False

Are you surprised? Here are the explanations:

1.  A good dose of overhead watering can actually rinse powdery mildew spores off your plants. Don’t do this all season long or you’ll encourage other plant problems, but feel free to give the plants a robust bath periodically to get the spores off.

2.  Mildew sounds like it would thrive in a damp climate, but the powdery mildew fungi grow most quickly in dry, warm conditions.

3.  Sunlight and good air circulation help keep powdery mildew at bay.

4.  There are eight species of powdery mildew fungi that attack vegetables, so don’t worry that the disease will jump from one row to another. For instance, Erysiphe cichoracearum preys on cucumbers, lettuce, melons, pumpkins, and squash. Leveillula taurica feeds on your tomatoes, eggplants, peppers, and artichokes.

5.  Is this the biggest surprise? Powdery mildew is not always white and powdery. Leveillula taurica, the tomato/artichoke fungi, creates yellow patches on leaves, not a spread of white dust.

Treatments

As Tricia showed in our video about powdery mildew, there are several organic weapons to combat powdery mildew. Prevention is the best path and, as Tricia says, if you know powdery mildew is a problem in your garden, choose resistant varieties of your favorite vegetables and fruits.

Serenade Biofungicide is a good preventive spray with a broad spectrum biofungicide that controls or suppresses powdery mildew and many other plant diseases.

Sulfur sprays, like Safer Garden Fungicide, are a classic organic gardening measure to use at the first sign of powdery mildew.

If you’re reading this after powdery mildew has already taken a firm hold, go straight for the hose. See if a good water bath will wash off all traces of the fungus. If it does not, turn to the horticultural oils. All the oils, whether a paraffinic oil like Saf-T-Side, a sesame and fish oil such as Organocide, or a neem oil like Green Light Concentrate suffocate the fungi and prevent more spores from adhering to the plants. The oils can be used for prevention as well.

Do not use horticultural oils if you have applied sulfur within the last 14 days.

For further information:

The first step to solving a garden problem is proper identification. Try the handy pack of Tree Fruit Pest ID and Monitoring Cards—easy to carry around your garden, and they will guide you to University of California resources for solutions.

The University of California Integrated Pest Management site has pages discussing powdery mildew on vegetables, fruits and berries, and ornamental plants.