Black-spot, scientifically known as “Diplocarpon Rosae” is a nasty fungus found on rose bushes as a black spot surrounded by yellow rings on both sides of the leaves. Usually young leaves, less than two weeks old are the ones first infected. If left untreated, the whole leaf will change to yellow and then fall off the plant. Eventually, the entire plant will become defoliated. Rainy, warm and humid weather is the cause of black-spot fungus especially when the temperatures run between 75 and 85 degrees and the foliage remains wet for a period of only 7 hours.
To prevent black-spot fungus from starting it is important to only water in the morning and only down near the soil. Roses as will as most plants do better if they go into the nighttime with dry foliage. When two or three days of rain occur, you must use a fungicide as a preventative so that black-spot will not get started. This information may seem to be too late at this time since black-spot has shown up almost everywhere but it is never too late to stop the spread of black-spot. The spores of black-spot are produced over and over again every three weeks and without cleanup, can survive over winter.
Systemic fungicides work best however wettable sulphur sprays, lime sulphur and dormant oil sprays have always been the most popular treatments. The problems with the sulphur is that you must re-apply after every hard rain and the dormant oil and lime sulphur can only be applied during the winter. There is a “summer” dormant oil that can be used during the summer months, but it is hard to find at the garden centers. Cleanup is very important as the spores do survive on the leaves that fall off the rose plants, so removal is most important. Spores do not live in the soil. One word of caution…do not place infected leaves into the compost pile or you will be asking for trouble.
Here is a homemade fungicide for fighting black-spot…dissolve 1 teaspoon baking soda in a quart of water and add a couple of drops of liquid soap to the mix to help it cling to the foliage and stems of you roses. Other products include Maneb and Zaneb, copper products, as well as Bordeaux Mix fungicide and many other fungicides for roses and tomatoes labeled for flowers and vegetables.