Control of Powdery Mildew, Now Prevalent in Greenhouses

The word mildew means many things to many people.

To the housewife, it may be any discoloration or growth caused by fungi on the vegetable matter or any other substance, including old shoes. 

Powdery MildewPin

To the greenhouse hobbyist and commercial grower, it may be any one of three distinctly different fungus diseases: 

  • Black mildew, which appears as a soot-like coating on leaves of slow-growing tropical plants; 
  • Downy mildew, which forms a delicate, white, frosty coating on leaf undersides; and 
  • Powdery mildew makes leaves and stems look like they’ve been sprinkled with flour.

Powdery Mildew: Most Common And Destructive

The latter, powdery mildew, is perhaps the most common and destructive mildew in greenhouses during the winter and spring. 

Therefore, a detailed discussion of this mildew is appropriate for this season.

Powdery mildews are obligate parasites; they grow only on living tissues and mainly on the surface of those tissues. 

Although they are primarily leaf parasites, they may grow upon stems, flower parts, or fruits. 

Some cause relatively little apparent injury; others are highly destructive.

A few, such as crape-myrtle mildew, are highly specific regarding food preferences and hence attack a single kind of plant. 

Phlox Mildew Attacks Ornamentals

Others, such as phlox mildew, can attack more titan 280 different kinds of ornamentals, including:

  • Aster
  • Begonia 
  • Calendula 
  • Chrysanthemum 
  • Delphinium
  • Zinnia

Powdery Mildew Fungi

Powdery mildew fungi differ in one important respect from most other fungi. Their spores do not require free water to germinate. 

High humidity on the leaf surface is sufficient. Such a situation frequently exists when plants are grown without good air circulation or when cold nights are followed by warm clays.

Powdery Mildew Of Rose

The powdery mildew of rose has been the most widely studied.

Most of our information on the behavior of mildews and their control is based on this particular one, which goes under the botanical name Sphaerotheca pannosa, variety rosae. 

This mildew has become increasingly prevalent in greenhouses where miticides instead of old-fashioned syringing are used to control spider mites.

Controlling Mildew Severity 

It is now well established that syringing with water reduces mildew severity.

But this practice cannot be recommended for mildew control because it will bring on other more destructive diseases such as black spots.

Many statements appear in garden literature to the effect that careful management of the greenhouse to avoid excessive humidities, drafts, and sudden temperature changes will help to prevent outbreaks of mildew.

However, Karla Longree conducted the most exhaustive studies at Cornell on the effect of temperature and humidity on the powdery mildew of roses. 

She concluded that this disease could not be effectively controlled under greenhouse conditions through temperature and humidity regulation.

Low Temperature And Humidity

It may be held somewhat in check by keeping temperature and relative humidity as low as possible for cultivating roses and avoiding drafts and extreme temperature changes.

Since these conditions are difficult to meet, except possibly in winter, the application of fungicides seems indispensable in controlling the disease.

Fungicides For Mildew Control

Over the years, many great fungicides have been tested for mildew control.

Some have proved effective eradication even after the mildew is quite abundant. 

One reason is that the powdery mildew does not penetrate deeply into leaf tissue, like some diseases such as black spots.

Vaporizing Sulfur: Combating Mildew

Vaporizing sulfur has long been a favorite way of combating mildew.

During the winter, the heating pipes in the greenhouse may be painted with a smooth paste composed of equal parts of dusting sulfur and lime mixed with water. 

Or the sulfur may be put in a vessel with a small opening and the sulfur vaporized by a steady beat until a heavy vapor fills the house.

Care must be taken to keep the sulfur from igniting, for fumes of burning sulfur seriously injure foliage.

Malachite Green

Although malachite green is sometimes used to eradicate existing infections of mildew on roses, it has no lasting protective effect. 

A stock solution is made by dissolving 1 ounce of malachite green crystals in 5 pints of water. 

Spraying Of Stock Solution

At the time of application, this stock solution is then a diluted-1 tablespoon solution to 1 gallon of water. 

The spray mixture works better if a good synthetic spreader like Vatsol, Dreft, or Dupont Spreader-Sticker is added.

Correct Timing Of Spray Application

As with disease control on outdoor plants, the correct timing of spray applications is essential. 

Mildew is more easily controlled if the sprays are applied when the disease first appears. It is far more difficult to control plants already heavily infected.

Heavy Outbreak Of Mildew On Plants

Occasionally a commercial grower is confronted with a heavy outbreak of mildew on roses, snapdragons, or begonias while plants bloom and the peak demand is still several weeks away.

Sulfur cannot be used because it will bleach the blooms. 

To prevent serious loss in such a case, Dr. A. W. Dimock of Cornell suggests spraying with the powerful and quick-acting mildew eradicant Mildex, 4 tablespoonfuls per 100 gallons of water, plus 8 ounces of Dreft, completely wetting the mildew growth. 

Then, to avoid any chance of injury to the blooms, the plants should be syringed with clear water a half hour later.

Dr. Dimock, however, does not suggest this procedure for routine mildew control.

Chlordane: Effective Insecticide

Users of chlordane, a very efficient and effective insecticide, will be glad to hear that this chemical is far less dangerous to handle than previously supposed. 

Dr. Lester Ingle of the Department of Zoology at the University of Illinois says that earlier reports that chlordane vapor is toxic to experimental animals were due to the presence of an “unreacted intermediate” in the chlordane as it was originally prepared. 

This harmful intermediate, Dr. Engle reports, has been reduced to a point where it is no longer present in sufficient quantity to produce significant vapor toxicity to mice.

Maintaining Thrifty Plants In The Garden

House plant enthusiasts who use softened water may have difficulty maintaining thrifty plants. 

Softened water contains sodium, which is toxic to plants. According to R. C. Andreason of Cornell University, the plants take up the water but leave the sodium in the soil.

Soon the sodium accumulates in sufficient quantities to actually kill the plants.

The solution to the problem, according to Mr. Andreason, is to use non-softened water.

Rainwater, melted snow, and a neighbor’s water supply are all possible sources.

44659 by P. P. Pirone