Sunday, May 29, 2011

Take a Powder

One of my good friends called me the other day with a problem. It looked like somebody had taken baby powder and sprinkled it on one of his Citrus trees. After a little discussion, I concluded it was Powdery Mildew. This is somewhat unusual, not completely out of the question, but not one of your ordinary Citrus problems either.
Powdery Mildew is a fungal disease that does affect a wide range of plants. It is actually caused by many different species of fungi in the order Erysiphales. It is one of the most widespread and luckily, easily recognized plant diseases.
As many of you know, I am NOT a fan of Crepe Myrtles. There are many other reason, but one of them is their susceptibility to Powdery Mildew. Here we are at the end of May and I already have a wicked case of it:


Even though there are several types of Powdery Mildew fungi, they all produce similar symptoms on plant parts. Powdery Mildews are characterized by spots or patches of white to grayish, baby powder like growth, however leaf curling and twisting may be seen before the fungus is noticed. While it is not usually considered fatal, plant damage can occur when the infestation is severe, and if left untreated, it can eventually kill the plant. On vegetable plants, Powdery Mildew will weaken the taste and quality of the produce. It can also stunt plant growth and distort the shape of buds, blooms and fruit. It can cause leaves to turn yellow and drop prematurely.

Powdery Mildew on Squash

The fungus survives the Winter attached to plant parts and plant debris such as fallen leaves. This is one reason why I stress sanitation in the yard and garden. Clean up any leaves, dropped fruit, or dead plant matter as soon as the crop is done. When the weather warms in Spring, the process begins.
As the daytime temperatures rise above 60 degrees the fungi responsible for Powdery Mildew begin to produce spores which are dispersed into the air. Infections occur when they contact a suitable host and environmental conditions are favorable. The severity of the disease depends on many factors, such as variety of the host plant, age and condition of the plant, and weather conditions during the growing season.
Powdery Mildews are severe in warm, dry climates. This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. While high relative humidity favors spore formation, low relative humidity favors spore dispersal, which explains why Powdery Mildew tends to be a problem when the days are cooler and the nights are humid. Temperature can also be a factor. Although Powdery Mildew can occur all season long, it is less common during the heat of the Summer.

Early Stages of growth

If there is one good thing about the Powdery Mildews, it is this, they are host specific. This means they cannot survive without the proper host plant. For example, the species Uncinula necator, which causes Powdery Mildew on grapes does not attack lilac. Similarly, Microsphaea alni which affects elm, lilac and oak, does not affect turfgrass.
How does one go about to control this pest? As with all diseases, optimum plant health is the first line of defense. Make sure your plant is well watered, fed, and growing in the correct place. Avoid planting sun loving plants in shady areas, especially very susceptible plants.
Purchase only top quality, disease-free plants of resistant cultivars and species from a reputable nursery, greenhouse or garden center.
If the disease has already become an issue in your yard:
Avoid late Summer applications of nitrogen fertilizer. When you apply high levels of nitrogen late in the season it produces new tender growth which is more susceptible to infection.
Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus in the average homeowners compost bin. Burn it or dispose of it in the trash.
Selectively prune overcrowded plant material, or space the plants for good air circulation. This helps reduce relative humidity.
Remember to sanitize garden tools to prevent them from carrying the spores elsewhere.
If you have attempted the above cultural controls and are still having a problem, there are many fungicides out there to help. Please follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest. This is the law!
For best results with fungicides, spray programs must begin as soon as mildews are detected. When ranges are given, use the shorter interval during cool, damp weather. Be sure to cover both the upper and lower surfaces of the leaves.
Something else to consider, is a late season spray of fungicide worthwhile if the plant is deciduous? The leaves have already produced food for the plant and are going to fall off soon anyway. Just be sure to rake and dispose of them as they fall. Sanitation, Sanitation, Sanitation!
There are some homemade remedies out there. I don't or won't recommend them, they can be useless or dangerous, but thought you should at least know about some of them. I have heard of people using one part milk with 2 parts of water and spraying every 3 to 4 days at the first sign of mildew.
How about the use of mixing 3 tablespoons of apple cider vinegar with one gallon of water and spraying every few days.
Then there is this, researchers at the University of Rhode Island have confirmed that a combination of 1 tablespoon baking soda plus 2.5 tablespoons light horticultural oil in 1 gallon of water is effective against Powdery Mildew on roses.
I would like to say again, the use of homemade remedies can be useless or in some extreme cases, dangerous to you or the plant. Please use caution if you decide to go that route.
One last thing, for all you plant geeks out there, and I count myself as one of you.....the Powdery Mildew that is attacking my Crepe Myrtle is Erysiphe lagerstroemiae.....luckily, that is it's only host.
Happy Growing!


  1. Hi I am fairly new to growing veg, and some of my plants have already got powdery mildew on. I done a bit of research on the net, and have now cut away all the infected leaves, and sprayed the plants with a baking soda spray. This was the mix I used.

    1 gallon of water

    1 tbsp of Baking Soda

    1 tbsp of Vegetable Oil

    1 tbsp of Washing Up Liquid

    Hope that helps anyone else who is having this problem.

  2. Please be very careful when using a homemade remedy such as the one above.
    I am assuming the "washing up liquid" is dish detergent. Many of the dish washing products out there today have a grease cutting agent added to them.....this can rip the protective layer off of the plant leaf.
    Vegetable oil needs to be used carefully when the plant is in full sun. It can easily burn the plant.

  3. Thanks for the advice Darren, I would never of given it that much thought. I wouldn't of realised about the washing up liquid being a grease cutting agent.