Tips On Controlling Mildew And Black Spot

In the Pacific Northwest, where the rose has won international acclaim for its size, beauty, and case of culture, there are two major fungus diseases and four persistent pests (of the insect type) which demand the attention of rose growers.

Fungus DiseasesPin

Two Major Fungus Diseases

Let us take the fungus diseases first.

Mildew and black spot constitute the major fungus problems in the Pacific Northwest.

Of the two, mildew is undoubtedly the most common. Starting in early summer, when warm days are followed by cool, moist nights, the spores of this disease spread with unbelievable rapidity.

The unsightly, whitish fungus always attacks new foliage first, deforming the leaves and detracting greatly from the beauty of the rose.

Mildew may be successfully controlled when observed, but prevention is the real key to conquering it.

A thorough spraying with a calcium polysulfide solution, or the newer Mildex or Karathane, every week beginning early in April will usually be adequate to ward off the initial attack.

Spraying Versus Dusting

The fungus, being on the leaf’s surface, may be effectively destroyed, once established, with these same sprays. However, the deformed leaves and scars remain.

Spraying is really better than dusting to combat the mildew once it is present since only a liquid can penetrate the oily mycelium of the fungus to destroy it.

Many gardeners prefer dusting because of its ease of application. Nevertheless, dusting for mildew is effective only as a preventive measure, frequently applied.

Black Spot Epidemic

The year 1954 saw, perhaps, one of the worst epidemics of black spots observed in the Pacific Northwest.

This fungus thrives in moisture at temperatures above 70° degrees Fahrenheit.

It is more generally destructive than mildew since it causes defoliation, thereby robbing the plant of a source of food supply.

The fungus spores reach beneath the surface of the leaf tissue, so they cannot be eliminated without destroying the leaf.

For this reason, black spot control calls for preventive measures.

A spray of Captan fungicide may ward off the disease effectively.

Once the obvious, fuzzy-edged black spot is present, hand picking of each leaf affected is still recommended treatment.

With black spots, unlike mildew, the old foliage is attacked first.

All foliage, therefore, must be carefully covered with an adequate amount of fungicide in periods when moisture is in evidence for longer than six hours.

Black spot spores germinate only in the presence of water. Consequently, fungicides must be applied before rains to be most effective.

Four Different Persistent Pests

No Japanese Beetles

In reviewing the insect pests of roses in the Pacific Northwest, the picture is somewhat brighter.

There are no Japanese beetles and no rose midges in this area.

Rose Aphids

Of the four main rose pests, the most common is the rose aphid.

Not as harmful as other insects, this small, soft-bodied, sucking creature robs the leaf of its color, sometimes even causing bud-drop.

It is easily controlled with the application of a spray containing any one of the following:

  • Malathion
  • Pyrethrum
  • Neem Oil

The application should be made on three successive days to eliminate the complete cycle of insects, larvae, and eggs.


In recent years, thrips have been in evidence in more and more gardens in the Pacific Northwest.

These tiny (1/24-inch) narrow-bodied insects feed on the unopened buds, causing malformed blooms and balling the blossoms.

They are difficult to control because they can secrete themselves within the petals where the spray cannot reach.

Sprays of Talstar, horticultural oil, natural pyrethrins, insecticidal soaps or neem oil on the freshly-raked surfaces of rose beds about the 10th of March will significantly eliminate the first onslaught of this very detrimental pest.

During the growing season, a weekly insecticide spray will help in control. For this, a malathion spray appears as effective as any.

Leaf Rollers

Leaf rollers are particularly harmful to roses which are prospective show winners.

The damage to the leaf is so obvious that the rose is deemed worthless for exhibition.

This tiny worm weaves a web around one or more leaves, rolling himself within them and feeding.

Leaf-rollers appear in early May and again later in the season. A spray of malathion or dust of Diatomaceous earth is effective.

Red Spider Mites

In seasons of dry, warm weather, the tiny red spider mites attack rose foliage, sometimes leaving the whole garden with a dull, lack-luster appearance tinged with a colorless, gray look.

They form a webbing on the underside of the leaf, which is difficult to penetrate.

Their size is so tiny that the naked eye may barely see them, but the damage they cause is quite evident.

Malathion or Neem is an effective control used in a spray with pressure enough to penetrate the web.

Wherever protection is necessary, it should be provided to maintain the perfection of the world’s most beautiful flower.