Diseases
Black Spot and Its Control Black spot is a fungus disease which attacks the foliage of roses in late spring and during the summer. It not only makes the leaves unsightly, but, if neglected, seriously injures the growth and vitality of the plant. Bordeaux mixture has been found to be the most efficient means of control. After careful investigations for the American Rose Society, Professor Massey has made the following report on the disease:- "The treatment that has been most successful in the control of black spot is that of protecting the rose by covering the leaf with some fungicide that will prevent the spores of the fungus from penetrating the cuticle. Spores need moisture for germination, and under favorable conditions will send forth a germ-tube and penetrate the leaf within a very few hours. Once within the tissue, no treatment noninjurious to the leaf can be given which will kill the parasite. The problem, then, is to have the fungicide present on the leaf when the necessary moisture for germination and infection is present. This means that the fungicide must be applied before rains; that it must adhere to the leaf; that it must be so highly insoluble that it is not readily washed away, and that it must be fatal to the fungus while noninjurious to the rose." Bordeaux mixture seems to answer all the above requirements and if applied systematically throughout the season will keep black spot, this most troublesome of all rose diseases, practically under control. Another measure of great importance is the removal and burning of all diseased foliage, as the spores of the fungus are harbored during the winter in dead rose leaves. Mildew This often causes serious trouble in a rose garden and is most prevalent during a long damp season or where roses are planted in low, damp spots. Fortunately, however, it is one of the few diseases that are comparatively easy to control. The young tender foliage of the new shoots is usually the first part of the plant to become infected with the powdery, grayish spots which are indicative of the disease. Treatment should be given immediately upon the first sign of trouble.
The most effective and most easily applied remedy is finely ground sulphur dust (flowers of sulphur). This can be applied with a small dust gun or can be shaken through a little homemade cheesecloth bag.