Homegrown: Mildew issues

I have noticed that a few of my shrubs are getting a little bit of mildew during the growing season. I don’t want it to spread. How should I treat these bushes (spirea and weigela)?


— Barb

The first thing to keep in mind when dealing with mildew is that powdery mildew isn’t the end of the world.

Unlike some other diseases, this parasite won’t kill its host. It may make its host look pretty sad and pitiful to the point where you wish it would kill it, but it almost never will.

Another thing to understand is that although powdery mildew is our most common foliar disease around here, it isn’t all the same. That is, the powdery mildew that affects roses is a different strain than the mildew that gets on lilac, and that one is different from the one that gets on euonymus.

There generally is a very specific strain or race of mildew for each type of plant out there. That means you don’t have to worry about rose mildew spreading to adjacent plants unless they’re roses.

The presence of mildew in a shrub bed, especially if it’s affecting several different types of plants, indicates that the environmental conditions in that area are conducive to the development of mildew.

Mildew usually thrives when days are warm, nights are cool and more humid and in areas where air movement is restricted.

I tend to see more mildew where plants are shaded and crowded. There are several things you can do to reduce the chances of powdery mildew affecting your plants.

The first is some judicious pruning. If your shrubs are overshadowed by trees, some minor thinning will improve sunlight and even air circulation. Some thinning of the shrubs themselves also can help sometimes.

When and how the plants in the area are watered also can make a big difference.

If you’re using spray type sprinklers to water, then minimize or eliminate any water on the foliage.

I know this is often not possible. After all, the water spray must reach the plant to water it, which requires getting the foliage wet.

If this is the case, then when you water makes a big difference.

You don’t want to water in the late afternoon or early evening hours. It’s best to water between midnight and sunrise or between 9 a.m. and noon.

We have a period of time every night when our humidity is naturally higher and you don’t want your watering to extend that period.

Running the sprinklers in the evening will wet the foliage and wet the surrounding area, raising the humidity that persists as night comes on, dovetailing with that nightly period of high humidity.

Watering during our natural high humidity period between midnight and sunrise may increase the relative humidity during that time but doesn’t lengthen it significantly. The same is true if you wait to water later in the morning.

If doing these things doesn’t help, start planning on doing some preventative and/or corrective spraying.

As a preventative spray, I like to apply a dormant spray mixture of spray oil and lime sulfur. This spray needs to be applied in early to mid-March while the plant is still dormant. Never use this combination when leaves are present. It will fry them.

It may not completely eliminate the mildew but it should reduce it by a significant degree.

Oh, just one word of caution. The sulfur is very safe to use, but it smells like rotten eggs, so wear old clothes and maybe warn the neighbors.

If, during the growing season, you run into a bigger mildew problem, you’ll need to apply a corrective spray.

There are a number of fungicides that will cure an existing mildew problem. Plan on using a systemic fungicide such as Fertilome Systemic Fungicide. You’ll have to spray it on the plant two or three times at 10-day intervals.

Dennis Hill is the nursery manager at Bookcliff Gardens, online at http://www.bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or e-mail .(JavaScript must be enabled to view this email address).


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