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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

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:


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Dig In: Garden Checklist

For week of March 26:

Sacramento can expect another inch of rain from this latest storm. Leave the sprinklers off at least another week. Temps will dip down into the low 30s early in the week, so avoid planting tender seedlings (such as tomatoes). Concentrate on these tasks before or after this week’s rain:

* Fertilize roses, annual flowers and berries as spring growth begins to appear.

* Knock off aphids with a strong blast of water or some bug soap as soon as they appear.

* Pull weeds now! Don’t let them get started. Take a hoe and whack them as soon as they sprout.

* Prepare summer vegetable beds. Spade in compost and other amendments.

* Prune and fertilize spring-flowering shrubs after bloom.

* Feed camellias at the end of their bloom cycle. Pick up browned and fallen flowers to help corral blossom blight.

* Feed citrus trees, which are now in bloom and setting fruit.

To prevent sunburn and borer problems on young trees, paint the exposed portion of the trunk with diluted white latex (water-based) interior paint. Dilute the paint with an equal amount of cold water before application.

* Cut back and fertilize perennial herbs to encourage new growth.

* Seed and renovate the lawn (if you still have one). Feed cool-season grasses such as bent, blue, rye and fescue with a slow-release fertilizer. Check the irrigation system and perform maintenance. Make sure sprinkler heads are turned toward the lawn, not the sidewalk.

* In the vegetable garden, transplant lettuce and kale.

* Seed chard and beets directly into the ground.

* Plant summer bulbs, including gladiolus, tuberous begonias and callas. Also plant dahlia tubers.

* Shop for perennials. Many varieties are available in local nurseries and at plant events. They can be transplanted now while the weather remains relatively cool.

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