Black Spot disease appears on the leaves of trees and shrubs as small black circular spots that range in size from about ¼ inch to 1 inch in diameter. They normally appear on the upper surface of the leaf, starting in the early to mid-spring. As the disease progresses, the leaf tissue around the black spot often yellows and premature leaf drop often occurs. In cases of severe infection, a plant or tree can lose many of its leaves by mid-season. Flowering trees and roses that are infected have fewer or no blossoms as a result of the stress caused by the Black Spot.
Black spot is caused by a fungus called Diplocarpon rosae which overwinters on infected leaves. In the spring, especially in areas of high humidity and rain, the fungal spores are splashed on to new leaves and begin their infection cycle. The fungal spores enter the leaf tissue and as they reproduce they cause the black spots. Normally infections are limited to the leaves, but they can also infect new twig growth. Repeated exposure to black spot can cause death or increase susceptibility to other infections.
Recommend Steps to Control Black Spot
To control Black Spot, spray the tree with Monterey Complete Disease Control. Repeat the spraying every 7 to 14 days as necessary, as long as the weather remains humid and damp. Avoid watering plants and trees from above, always water at the ground level. In the fall remove all leaf debris to avoid re-infecting trees and plants the following spring. If infected trees are pruned during the dormant fall or early spring period, spray the tree with Liquid Copper Fungicide Spray after pruning and be sure to sanitize the pruning tools after each branch cut.