No sugar-coating powdery mildew

Powdery mildew overwhelms the buds and new leaves on

this Dorothy Perkins rose. (Photos: Debbie Arrington)

This fungal disease loves mild spring weather, hates water

By Debbie Arrington

Does it look like someone dumped powdered sugar all over your roses and other plants? That’s an outbreak of powdery mildew, a very common condition this spring.

This fungal disease thrives in mild spring weather with afternoon temperatures in the 70s. 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. 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.

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 effective only before the disease appears. Neem oil may help contain an outbreak, but only if applied in the very early stages (and coverage must be thorough).

Two other tricks are more effective, easier and cheaper:

* Plain water works wonders against powdery mildew. Although it’s a fungus, it hates wet conditions. Keep the plant (and the soil beneath it) well watered. Water on plants 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).

Before the white powder shows up, infected leaves will pucker

 and become deformed. 

* 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 explodes with 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 will all be gone soon. 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: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7493.html