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Warm almost-spring days bring out powdery mildew in the garden

Fungal disease, which infects a wide range of plants, is most active right now

Photinia with powdery mildew
This photinia is plastered with powdery mildew. Trim off infected foliage to try and curb its spread. (Photo: Debbie Arrington)

Spring seemed to arrive early this year – and so did the powdery mildew.

This fungal disease looks like fairies dusted leaves and emerging buds with powdered sugar. But there’s nothing sweet about this fungus. It clogs the leaves’ pores and eventually kills the foliage. Infected foliage drops off, leaving the plant susceptible to sunburn.

Like other fungal diseases, it’s triggered by temperature. Once we hit pleasant afternoon temperatures around 70 degrees, powdery mildew literally explodes. The spore factories, located on the underside of leaves, pop open and blast their white dust on nearby foliage and plants.

Those spores are easily carried by the breeze to other susceptible plants, spreading the infection throughout your garden – and the neighborhood. It particularly likes roses but also can infect crape myrtle and sycamore trees, plus azaleas, rhododendrons, begonias, mums, dahlias, sunflowers, coral bells, calendulas and zinnias as well as many other ornamentals such as photinia. It can be problematic on apple and peach trees as well as raspberry vines. In the vegetable garden, it attacks the foliage on melons, pumpkins, squash, cucumbers, beans and peas.

Although it’s a mildew, powdery mildew doesn’t need wet conditions to multiply. In fact, rain (or overhead sprinklers) can stop an outbreak.

Powdery mildew particularly likes plants growing in partial shade or next to a wall or fence. Those factors keep air temperature cool and just the way this fungus likes it. Walls and fences also cut down on air circulation, allowing powdery mildew to thrive undisturbed.

Once you see the telltale white fungus, it’s too late; the spores have already launched their attack. But you can minimize their spread. Clip off infected plant material, bag it up and dispose of it in the trash.

Sulfur sprays or powders are only effective if applied BEFORE the disease appears. Neem oil may help contain an outbreak, but only if applied in the very earliest stages.

Two other tricks are more effective, easier and cheaper:

-- Plain water works wonders against powdery mildew. Keep the plant (and the soil beneath it) well watered. Water on leaves inhibits the fungi’s germination and kills spores.

To knock off spores before they “sugar coat” your roses, give your plants a shower. Sprinkle water on the foliage from above, preferably in the morning. That gives the plant plenty of time to dry so the leaves don’t stay wet (and cause other fungal problems).

-- Inspect your susceptible plants regularly (daily if possible) and be on the look out for puckered or deformed leaves. Before releasing its white powdery spores, this fungus creates a spore-generating fruiting body, usually on the underside of a leaf. As it sucks energy from the leaf, the spore factory deforms its host. When it’s ready, it releases the telltale white spores all over that leaf and neighboring foliage.

Don’t wait; clip that puckered leaf off the plant and dispose of it before it can spread spores everywhere.

The good news: Powdery mildew doesn’t stick around in high heat. It can’t stand temperatures above 90 degrees; 95 degrees pretty much wipes it out. Damaged foliage will fall off to be replaced by healthy leaves.

For more tips, check out the UC Integrated Pest Management pest notes on powdery mildew:


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Garden Checklist for week of April 14

It's still not warm enough to transplant tomatoes directly in the ground, but we’re getting there.

* April is the last chance to plant citrus trees such as dwarf orange, lemon and kumquat. These trees also look good in landscaping and provide fresh fruit in winter.

* Smell orange blossoms? Feed citrus trees with a low dose of balanced fertilizer (such as 10-10-10) during bloom to help set fruit. Keep an eye out for ants.

* Apply slow-release fertilizer to the lawn.

* Thoroughly clean debris from the bottom of outdoor ponds or fountains.

* Spring brings a flush of rapid growth, and that means your garden needs nutrients. Fertilize shrubs and trees with a slow-release fertilizer. Or mulch with a 1-inch layer of compost.

* Azaleas and camellias looking a little yellow? If leaves are turning yellow between the veins, give them a boost with chelated iron.

* Trim dead flowers but not leaves from spring-flowering bulbs such as daffodils and tulips. Those leaves gather energy to create next year's flowers. Also, give the bulbs a fertilizer boost after bloom.

* Pinch chrysanthemums back to 12 inches for fall flowers. Cut old stems to the ground.

* Mulch around plants to conserve moisture and control weeds.

* From seed, plant beans, beets, cantaloupes, carrots, corn, cucumbers, melons, radishes and squash.

* Plant onion sets.

* In the flower garden, plant seeds for asters, cosmos, celosia, marigolds, salvia, sunflowers and zinnias.

* Transplant petunias, zinnias, geraniums and other summer bloomers.

* Plant perennials and dahlia tubers for summer bloom.

* Mid to late April is about the last chance to plant summer bulbs, such as gladiolus and tuberous begonias.

* Transplant lettuce seedlings. Choose varieties that mature quickly such as loose leaf.

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