Most rose varieties are susceptible to two widespread diseases, blackspot and powdery mildew, either of which can spoil a lot of their natural charm. Both can be prevented by spraying. Some gardeners are able to get nice flowers without spraying, but the odds are against it. Now is the time to start if it's necessary.
Blackspot shows up as a small round black spot, generally on the upper side of the leaves. Leaf tissue near the spot turns yellow. Infected leaves usually turn yellow and drop. Early loss of leaves prevents the plant from producing adequate food.
Growth may stop gradually or quickly, resulting in dieback and dead canes. The disease often leads to winter-killing due to lack of freeze resistance.
Besides affecting the size of the plant, the blackspot often is responsible for small flowers. It may decrease the intensity of flower color and fragrance.
The spores of the blackspot fungus are spread by wind and splashing water, and infection occurs only when water remains on the leaves for several hours. Therefore, the disease is most serious in rainy and moist regions. Overhead watering or syringing the plants during dry weather should be avoided. During rainy weather it may be necessary to spray the plants twice a week. During dry weather, spraying every seven to ten days is usually enough. For control (prevention) of blackspot, specialists recommend Benlate, Daconil, or Phaltan; follow the label's mixing and application directly closely. Benlate, a systemic, need not be applied as oftenas the other two, but be sure it is mixed thoroughly.
Powdery mildew spores germinate best when the temperature is between 65 and 75 degrees and relative humidity between95 and 99 per cent. Germinaton is poor between 75 and 95 per cent, at 100 per cent, and in standing water, and doesn't take place below 75 per cent.
The first symptoms of powdery mildew are a slight curling of leaves followed by a white powdery coating on leaves, buds and stems. Flower buds may not open, if they do the flowers are deformed. The leaves become blistered and have a reddish cast. Powdery mildew is more severe on succulent growth brought about by heavy use of nitrogen fertilizers.
For control (prevention) of powdery mildew, specialists recommend spraying with Benlate or Actidione PM, again following the directions closely.
Correct timing of applications is important If you had powdery mildew or blackspot on your roses last, summer chances are you'll have them again this year. Start keeping a watch on your roses. If you see signs of diseases, start spraying immediately and try to get rid of it before it can build up. Most of the chemicals used for disease control on roses are only slightly soluble in water and must be constantly agitated to keep the concentration uniform.