Roses are among the most popular flowering shrubs in landscaping and gardens, and black spot disease is the most common threat to them. The fungus attacks all species and cultivars; tea roses are especially prone to it. While there are some cultivars that display mild resistance, the fungus should be taken seriously. Its presence weakens the plant and can lead to other problems that may ultimately prove fatal.
Initially, the upper surface of the leaves display black round spots about ½ inch in diameter. These spots can grow larger and become irregularly-shaped. The area around the spot becomes yellow and spreads throughout the leaf. Next, the affected leaves drop. Lesions that appear fist purple, then black, can also develop along the canes. As more and more leaves fall, the plant becomes weakened and susceptible to other disease and injury.
The black spots contain spore-producing bodies. The spores must have access to water, and need temperatures between 65 and 75 degrees F in order to germinate. Temperatures above or below this range will slow or halt the development and spread of the spores. Once a leaf is infected, the spore-producing bodies grow in about two weeks. The spores are transferred by wind movement, splashing water or physical transfer through touch. The spores can live for about one month and will overwinter in fallen leaf debris.
Site selection is an important part of preventing black spot. A well-drained, well-circulated area is ideal. Air flow will dry morning dew or water from irrigation before nightfall. Black spot needs seven hours of moisture to germinate, so keeping rose bushes dry is essential. Water plants only in the morning to help ensure the moisture has been absorbed or evaporated by nightfall.
During the dormant period, remove any stems that have died or have lesions, as they may harbor the fungus. As the disease becomes apparent, remove and destroy any infected leaves or canes. Wash pruning tools after each cut to avoid spreading the fungus. Remove fallen leaves from the base of the bush where black spot is present to eliminate an environment where the fungus can overwinter.
Fungicides for black spot are quite effective but labor-intensive. They should be applied at the first appearance of spots and reapplied weekly until weather conditions return to dry and warm. After that, fungicide should be applied after rain to prevent the onset of the disease again. Check that the fungicide you buy is labeled for black spot. Effective fungicides are chlorothalonil and triforine.