Answer: Black spot is a fungal disease that requires certain environmental conditions to infect plants. If you keep a close eye on your plants, you can catch the disease early and pinch off the affected leaves and dispose of them in the trash. To help prevent further infection, spray with a solution of compost tea, made by mixing one part mature compost that contains some manure with five parts water. Let the mixture sit for 2 weeks, then filter and spray at 2-week intervals. An alternate spray recipe is 3 teaspoons baking soda, and 2 tablespoons of summer horticultural oil mixed in 1 gallon of water, sprayed every 10 days to 2 weeks. Compost tea contains organisms that fight the fungi, and baking soda changes the pH of the leaf surface, which discourages fungal growth. Other routine maintenance can help prevent black spot: Avoid getting the leaves wet when you water your plants, and water early in the day so that moisture on leaves can evaporate during the heat of the day; keep the mulch around the bushes free of fallen rose leaves and replace it every spring; trim your roses to remove old blooms; in late winter, prune the bushes hard to remove all leaves left from the previous season and remove canes to open the center of the bushes. Or, you can use a fungicide formulated especially for black spot on roses.
There are some rose varieties that are more tolerant of black spot, so you may want to look for them when buying new plants. Incidentally, yellow roses are the most susceptible to the disease.
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