Powdery mildew

Asked July 13, 2016, 5:31 PM EDT

My grapes look like they are covered in a very lite coat of white powder. I think it is powdery mildew. The internet seems to have a lot of ideas for the best way to treat my plants. One site says sulfur another says soap-oil-water and baking powder. Do you know the best treatment?

Minnesota

2 Responses

http://www.practicalwinery.com/marapr03/marapr03p16.htm
Sulfur actually kills the powdery mildew however it is most effective if applied just after flowering. Baking soda in water stops the powdery mildew but does not prevent a
new infection. Water also inhibits powdery mildew but is not preventive but why it can appear to go away and then come back. I think the most useful information can be found from the wine grape growers.

Your question has also been answered before for another grape grower.

This sounds like powdery mildew which is a fungus and is one of the most common and widespread plant diseases. Here is a good fact sheet on powdery mildews:

http://www.ext.colostate.edu/pubs/garden/02902.html

The fungus likes hot, dry weather and does not need water to spread, however, high humidity is usually present when the fungus is seen. Our almost daily afternoon showers help to keep the humidity up even though the days are hot. One good factor regarding this fungus is that it is host specific. You may see powdery mildew on other plants but this would not be the same species of fungus that is presently on your hollies.

The fact sheet will give you ways to control powdery mildew:

"Control

Cultural

Several practices will reduce or prevent powdery mildews. Many plants, such as roses, vegetables and Kentucky bluegrass, have cultivars, which have been developed to be resistant or tolerant to powdery mildew. Inquire about resistant varieties before a purchase. If resistant varieties are unavailable, do not plant in low, shady locations.

Once the disease becomes a problem:

  • Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection.
  • Avoid overhead watering to help reduce the relative humidity.
  • Remove and destroy all infected plant parts (leaves, etc.). For infected vegetables and other annuals, remove as much of the plant and its debris in the fall as possible. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus.
  • Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.

Chemical

If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary. These include:

These include:

  • sulfur
  • neem oil (Rose Defense, Shield-All, Triact)
  • triforine (Ortho Funginex), ornamental use only
  • potassium bicarbonate (Kaligreen, First Step)

Chemicals are most effective when combined with cultural controls. Apply fungicides at seven to 14-day intervals to provide continuous protection throughout the growing season. 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.

An alternative nontoxic control for mildew is baking soda (similar to the potassium bicarbonate listed above) combined with a lightweight horticultural oil (Sunspray). Researchers at Cornell University have discovered the fungicidal properties of this combination against powdery mildew on roses. Applications of one tablespoon baking soda plus 2.5 tablespoons of Sunspray oil in 1 gallon of water are still experimental. Use it at your own risk."

Be sure to watch for powdery mildew on other plants in your landscape and follow the advise in the fact sheet.


-Debbie, Colorado Master Gardener, Arapahoe County