Not all mildew is created equal

My neighbor behind our house has many mature shrubs and bushes which have mildew in the late summer and fall. Now I notice that a few of my shrubs that border her yard are getting a little bit of mildew during the growing season. I don’t want it to spread anymore in my yard. How should I treat these bushes (spirea and weigela)? Perhaps I should discuss a cooperative action with my neighbor?

— Molly

The question I always ask when talking with folks about powdery mildew is “are you absolutely sure it’s mildew?” I really don’t mean to be insulting, but so often people mistake the residue left on the leaves from ditch water for powdery mildew. If in fact it’s mildew we’re dealing with, then we get into a different realm.

The first thing to keep in mind when dealing with mildew is that powdery mildew isn’t the end of the world. Actually, powdery mildew is a pretty smart little parasite. Unlike some other diseases, it won’t kill its host. Now, it may make it 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 is generally a very specific strain or race of mildew for each individual type of plant out there. That means that you don’t have to worry about rose mildew spreading to adjacent plants unless they’re roses.

So, the question is, do you have the same type of plants in your yard as your neighbor has in his that have mildew? If they’re different, then you don’t have to worry about the mildew spreading from his plants. However, 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 and 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 or limbing up of the trees will improve sunlight and even air circulation. Some thinning of the shrubs themselves can also help sometimes.

When and how the plants in the area are watered can also make a big difference. If you’re using spray-type sprinklers to water, minimize or eliminate any water on the foliage. Now, I know this is often not possible. After all, the water spray has to reach the plant to water it, which requires getting the foliage wet. If this is the case, when you water makes a big difference. Never water in the late afternoon or early evening hours. You only want to water between midnight and sunrise or between 9 a.m. and noon.

You see, we have a naturally higher period of humidity every night between midnight and sunrise. You don’t want your watering to extend beyond that period. Running the sprinklers in the evening will wet the foliage and wet the surrounding area, raising the humidity which persists as night comes on, dovetailing with that nightly period of high humidity. So instead of a six-hour period of humidity, it’s now 10 or 12 hours, which makes a huge different in disease development.

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. Actually, you could water in the afternoon as well, but because you lose so much water to direct evaporation when it’s so hot and dry in the summer, we don’t recommend it as a rule.

If doing these things doesn’t help, then we start planning on doing some preventative and/or corrective spraying. As a preventative spray, I like to apply a dormant spray of a 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. 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 spray systemic fungicide like Ferti-Lome 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, Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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