#2 Powdery Mildew

Wild Nights, Hot and Crazy Days 

Doctor: I’ve got good news and bad news.

Patient: What’s the bad news?

Doctor: You’ve got powdery mildew.

Patient: What’s the good news?

Doctor: It’s not fatal, just horribly disfiguring.

The invitation has been sent and powdery mildew has arrived. Hot humid days and cool dry nights are a delight for this hideous fungus. Rain, which usually stops the spread of the spores, has been absent allowing it to grow and propagate. It can manifest itself in various ways. Sometimes it appears like a sugary glaze on leaves and flower buds, and sometimes it looks like someone spilt a powdery donut at your plant. If you don’t pay close attention to your plants, you may not have noticed it yet, but once it is seen it can’t be unseen. Like a giant zit on your nose, once you’re aware of it that’s all that you can think about.

What is it? Powdery mildew is a fungus. It’s spores are spread by the wind then land on your plant attaching itself to leaves, stems, and buds. The fungi feed on the surface of the plant absorbing it’s water and nutrients. It is rarely fatal, but it can reduce the vigor of your plant, stunting flower production, causing leaf drop, and reducing fruit production. Powdery mildew can occur on a wide variety of plants most commonly seen on cucumbers, squash, garden phlox, peonies, roses, and bee balm (monarda). However, under the right conditions it can infect almost any plant including turf grass. This year I have even found it on my black eyed Susans and crepe myrtles. The good news is that powdery mildew is host specific. Meaning the powdery mildew on your cucumbers will not infect your peonies. Although it is not lethal, it can be disgusting to look at.

Powdery Mildew

Oh no, what do I do? You can do nothing and your plant will probably survive. Flowers can be stunted or suppressed completely. Leaves may dry up and drop. In perennial plants, this will prevent photosynthesis decreasing vigor and weakening the plant for following year. There is also a good chance that the spores will over winter on the plant or in the soil making it a reoccurring problem next year.

There is not cure. Like most fungal infections there is no cure, but you can stop the spread. Stopping the spread is key. We still have plenty of sun left, so if you stop the spread there is still plenty of time left for your plant to grow and thrive. Cucumbers, a common host of powdery mildew, are a perfect example. Left unchecked it can completely devastate the plant ruining your harvest. But if treated in time, you will still have plenty of cucumbers to enjoy.

Five Step Plan:

  1. Remove and destroy all infected tissue. Infected plant parts must be put in the trash or burned. Do not leave infected tissue in your yard. If you have allowed the infection to get to an advanced stage, this can be a difficult decision. Removing a majority of the plant can be as damaging as the fungus. However, certain plants, like cucumbers and roses, grow like weeds. I have personally given them a severe pruning to see them reinvigorated in a couple of weeks. Naturally, the sooner you notice the infection and amputate the infected tissue the better.
  2. Give them some air. Just like us plants like to breath. Prune away crowded areas, remove crisscrossing branches and leaves laying on top of leaves. This will improve the circulation inside your plant reducing the relative humidity inside of it. Remember, humidity makes the fungus thrive.
  3. There are numerous remedies that you can use to treat powdery mildew. 
    A. The best overall remedy that I have found is copper spray. It is completely organic elemental copper that can be purchased affordably in any garden center. It is completely harmless to people, pets and children, and will not harm beneficial insects. I have had great success using copper spray to treat powdery mildew this season. Just avoid apply it in direct sunlight like most fungicides.
    Sulfur is another completely organic element, but it is harder to apply.
    C. If you are not into the whole organic thing then Chlorothalonil and Propiconazole are two ready to use fungicides that can be purchased at any garden center. They are highly effective broad-spectrum fungicides just avoid skin contact, inhalation, eye exposure and definitely don’t drink them! Caution must be observed when applying them, but they are completely harmless once they are dried.
    D. Finally, there is my all time favorite, still reigning heavyweight pesticide champion and one compound that every good gardener should always have on hand - Neem oil! Neem oil is always my go to favorite for garden problems. It is completely organic, derived from the oil of the neem tree, and harmless, you can spray it on your fruit and eat it the same day. The great thing about neem oil is that it doesn’t require a clinical diagnosis. If you are unsure what symptom is ailing your plant, neem oil covers all bases. It is a broad spectrum fungicide, insecticide and miticide. So if you are unsure what is affecting your plant, neem oil probably has you covered. Just don’t apply it in direct sunlight and probably don’t drink it either. As with all pesticides, the label is the law. Always read the label, even if you don’t follow it.
  4. Never water from overhead! I know, but rain comes from overhead? This is true. Nature provides rain to us from overhead. That is the circle of life Simba, but so is fungus. When we create an ornamental garden we are providing an optimum environment for plants to thrive and provide beauty. Wet leaves, especially overnight, provide optimum circumstances for all fungi to thrive, not just powdery mildew. I know this may be hard to wrap your head around but it is the way. If you must use a sprinkler to water from overhead, be sure to do it early on a sunny day so that any water on leaf tissue will quickly evaporate.
  5. Avoid fertilization. New growth is especially susceptible to fungal infections. Soft new leaves and infant flower buds are much more vulnerable to infection. Besides, at this point in the season most plants should be focusing on hardening up for autumn, not producing lush new growth. If you prepared your plants properly, there should already be sufficient nutrients in the soil for them to thrive. You may have to sacrifice some of your blooms this year for a healthier plant next year.

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