Controlling Black Spot in the Rose Garden

Somewhere in rose growing country the season will prove to be one of the worst for blackspot in recent years. Often this can be attributed to a very early and warm spring which produces early growth of roses and of blackspot. It greatly emphasizes the importance of spraying early.

Gardeners who began their spray programs with the first growth have blackspot free plants; those waiting until the usual starting time do not. This lesson can well be remembered by all.

When a season comes along like this it again shows that dusts cannot be expected to give thorough blackspot control under severe conditions. Information continues to mount indicating that home gardeners can obtain superior blackspot control with a proper spray program.

Best Control Method

The necessary elements for best blackspot control are:

• The use of maneb fungicide as a finely ground wettable powder… Neem Oil has also been used as a natural alternative
• A good spreader-sticker agent
• A satisfactory garden sprayer
• An early start
• A continuous weekly program, during spring-time particularly

These ingredients have produced blackspot free gardens in very severe seasons.

If you have missed an early start, all is not lost. It has been shown that rose fields infested with blackspot can be cleared of the effects of the disease with a weekly spray program. The infected leaves will be lost but new growth can be protected.

Some Important Points.

There are several important points to recognize and remember in the fight against blackspot. One, once the fungus infects a leaf we have no way to stop it, the methods are entirely preventive. The fungicide must be on the leaf surfaces at all times to prevent infection and spread of the disease. Thus thorough coverage, the use of spreader-sticker and frequent spraying and treatment of the underside of leaves are of extreme importance. Fungicide spraying probably is more effective than dusting because of penetration and killing of spores in the spore masses prior to distribution of the spores.

Another important fact, water is necessary for the spread of the spores. The only source of infection being from the “black spots” of the mature spores, and these spore clusters can be liberated only by water contacting the “black spot.” These liberated spores, when disseminated to other leaf surfaces, can germinate and grow only if maintained in a moist condition continuously for six hours. Germination and growth is most rapid at temperatures close to 70° Fahrenheit. These facts account for the rapid spread of blackspot in rainy, humid and temperate seasons and for the necessity of frequent spraying during these seasons. They account for the practice of discontinuation of control measures during hot, dry summer months and for the virtual absence of blackspot in large areas of the Southwest where rainfall is sparse and humidity is low.

Black spots become visible in 3 to 10 days after spore infection on new leaves. Mature reproductive spores are present in the spots within a day or two after appearance of the spots. These spores are released only by water contacting the spot and then are disseminated throughout the garden by air currents wind-blown rain and rain.

Spreading Spores

Respattering can carry the spores even farther. This time lag between actual infection and observable symptoms, and the fact that we have no control after infection starts, results in frequent widespread infection in a rose garden without the gardener realizing the presence of blackspot. It also accounts for the often futility of picking off the black spotted leaves, since many infected leaves may be missed due to infection not showing yet, furthermore is the fact that the condition cannot be cleared up quickly and the only means of protection is preventive spraying against the unobserved infection.

The popular idea that blackspot passes the winter in the soil is not substantiated by fact. The living fungus has never been isolated from the soil and plants have never been given blackspot from “infested soils” alone. Plants are infected only from spores from other infected leaves or stems. This indicates the extreme importance in removing and destroying all foliage from plants during the winter and of pruning all tender growth.

Experiments have demonstrated that heavy pruning plus thorough clean-up of all foliage can materially delay the onset of blackspot infection in the spring. Reducing the over-wintering infection in this way can materially aid in the success of the season’s control program. Also for this reason blackspot control is more difficult where climbing roses are grown. Large climbers cannot be cleaned of foliage and pruned as thoroughly as bush roses and hence are a source of heavy and early infestation of the entire garden.

A better understanding of the habits of rose diseases can be a great aid to their control.

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