Q. Black spots my rose leaves can you help? Evelyn Moore – Parkland, WA
A. It’s that time of year again. One of the problems with many of gardener’s favorite roses is that they are susceptible to a common fungus called black spot. The breeding of roses over the centuries rivals the plots of afternoon soap operas. Hybrids were made with roses from around the globe. The Mediterranean roses contributed to many modern roses being susceptible to black spot and other fungal diseases. This is because fungal diseases were at a major disadvantage in these dry climates so the roses didn’t evolve strategies to combat fungal diseases. I have heard ancestors of the Persian yellow roses tend to have the least disease resistance. But there are many good heirloom roses and Rugosa roses that have very good disease resistance. But since you are probably not interested in yanking out your prize roses and replacing them you will need to have a strategy to control the disease. First locate roses in an area that gets good ventilation or cross wind. This allows the surface of the leaves to dry more quickly after rainfall. Moisture on the leaves is needed for the development of fungal spores. So it follows that overhead watering should be discontinued or should take place in the morning keeping the amount of time the leaves stay wet to a minimum. In addition water is the vehicle that allows black spot spores to spread so splashing water only exacerbates the problem. Now that you have sings of the disease it is best to remove and destroy infected plant parts. Also rake up leaves that drop from the disease. This reduces the chances for future infections. Finally you may wish to maintain a spray regimen to further combat black spot. One popular home remedy was developed at Cornell University. Cornell’s baking soda spray consists of 1 tablespoon of baking soda (sodium bicarbonate), 2 ½ teaspoons of summer oil, a natural oil based insecticide used on plant, mixed with 1 gallon of water and sprayed on the leaves. The increased pH of the leaf surface seems to interfere with black spots ability to infect the leaves. Also Neem Oil has been now labeled for use beyond insect control. It now can be labeled as a miticide and a fungicide. You may see it marketed for the Rosarian as Rose 3-in-1. This is a good option if you also have the insect bane of the Rosarian the aphid.