Black Spot on Roses
Perhaps the worst problem associated with roses in the rose garden is black spot. First small black spots appear usually surrounded by a yellow ring. Before you know it the whole leaf looks yellow and is dropping from your rose bush.
Black spot is a fungus which can overwinter in fallen debris from previously infected plants. It has been shown that the spores can also overwinter at the leaf nodes of infected plants. Black spot spores are easily spread by touch or splashed water drops. In cleaning up, if you accidentally brush an infected leaf against an unaffected leaf, you can pass this disease along. If you don’t not wash your hands after touching infected leaves or parts of a rose, or use the same gloves, then you can pass the disease around.
It is a disease that is more a concern in humid locations with more rainfall. Apparently the spores must be wet for at least seven hours for an infection to occur. That is why the spores begin spreading in the spring as temperatures warm up and the spring rains fall. This is also why it is a very good idea not to crowd your roses together but plant them so there is plenty of air flow around each shrub. This idea also applies to pruning your rose bushes. Do not allow the canes to become crowded within the bush.
Make sure your roses are in full sunlight to help dry them quickly after a rain. Don’t allow surrounding trees and other plants or structures to impede the sunlight and air flow. Do not use sprinklers or any overhead type watering system that will keep the roses wet. And water early in the day to allow plenty of time for the water that may have splashed to dry up. Even the splashing of drops from infected places can carry the spores to uninfected plants and leaves.
In the fall, or winter depending on your location and climate, prune out any cankered canes as these may be, and probably are, harboring black spot spores. If you miss in the fall, prune them out in the early spring before it starts warming up into the 50s at night. (Studies say that night time temperatures of 60 to 65 along with the spring rains are the ideal conditions for black spot, so I figure make sure you get the cankered canes before then.)
Some say at the first sign of balck spot begin a spraying regimin but if you live in a humid area and know black spot is prevalent in your area, start as soon as the rose leaves begin to appear. One would probably think a fungicide would be best to use as a spray but there are few legal fungicides left and they may be very dangerous to you and the environment.
Perhaps the best first course of action is to use baking soda mixed with horticultural oil as a spray. Mix 2 -1/2 teaspoons horticultual oil and 3 teaspoons baking soda to one gallon of water. Use weekly. You may need to cut down on the horticultural oil in hot weather.
If this treatment does not keep black spot at bay in your rose garden, then try flowable sulfur. Follow the directions on the container carefully. Apply twice montly. In hot weather, do not use the sulfur as it may cause leaf burn.
Taking care to not plant rose cultivars that are more suseptible to black spot in your area and following the tips above should help you creat a lovely and beautiful rose garden with little incident of black spot.