Black Spot On Roses - Knowledgebase Question

Joppa, MD
May 7, 2000
How do I control black spot on roses?

Answer from NGA
May 7, 2000
Black spot is a fungal disease spread by moisture on the foliage. It can be held in check if you have only a few bushes, and like to spend time in your garden. 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 far from your roses. To help prevent further infection, spray with a solution of compost tea, made by mixing one part mature compost that contains some manure with 5 parts water. Let the mixture sit for 2 weeks, then filterand spray at 2-week intervals. An alternate spray recipie is 3 teaspoons baking soda, 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; 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. There are some rose varietiesthat 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.

There are several low-toxicity fungicides now available, including those made from neem, a plant extract, and one from Safers, made with copper combined with fatty acids. Follow all label directions carefully.

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