Most varieties of roses are seriously affected by two fungus diseases which, unless controlled, can take all of the fun out of trying to grow them: black spot and powdery mildew. Both can be controlled, but it may involve regular spraying throughout the growing season.
Black spot can be recognized by the nearly circular black spots that appear on the upper leaf surface. The spots can be distinguished from other leaf spots by the uniform black color and fringed margin.
The spots enlarge from pinhead size to the diameter of a dime, and as they enlarge they coalesce to form large irregular blotches on the leaves. The entire leaf may become yellow or there may be a yellow area around individual spots. Premature defoliation of the plant follows.
Although the leaf spotting is unslightly, the most serious effect on the plant is from the premature defoliation. The early loss of leaves reduces the plant's vigor by reducing the food manufacturing area, and thereby predisposes it to other diseases, insects and drought. If it occurs near the end of the growing season, the plant goes into winter in a weakened condition which makes if far more susceptible to winter injury.
For blackspot control, specialists recommend spraying during dry weather about once every 10 days with Benlate or about once every seven days with Daconil 2787 or Phaltan or Fore. Shorten the intervals with all materials during rainy or very humid weather. Use a sticker with Benlate, Phaltan and Fore. In all cases, read and follow label directions for mix and application.
These materials will not cure a leaf that has become infected. Properly applied, they can protect a leaf from infection.
Powdery mildew develops rapidly during warm, humid weather. Mildew disease of other plants, such as those of crape myrtle and lilac, do not spread to roses. Certain strains of mildrew affect roses only. The disease usually worsens during spring and fall when there is a pronounced difference between dry and night temperatures.
The disease is more severe on roses than most other species of plants because the young leaves and buds are affected. Rambler roses may become infected soon after new leaves develop. Hybrid tea and floribunda varieties may be infected by May or June but the disease is more serious on these roses in late summer as nights become cooler and there is more water condensation on the plant from heavy dew and ground fog.
The first symptous of powdery mildew are slight curling of the leaves followed by a white powdery coating on leaves, buds and stems.
Flower buds may not open or if they do open the flowers are malformed. The leaves become blistered and have a reddish cast. The disease is more severe on succulent growth such as that brought about by rather heavy applications of nitrogen fertilizers.
For powdery mildew control, specialists recommend spraying at seven day intervals with Actidion PM or at 14-day intervals with Benlate.
It is important to spray more often during rainy weather and to thoroughly spray both sides of the leaves.
Most of the fungicide dust formulations are only slightly soluble in water and they should be mixed slowly and carefully to make sure they go inot solution. It is also important to keep them constantly agitated (by shaking them) while in the sprayer in order to maintain a uniform concentration.