Informational table showing disease name, symptoms, pathogen/cause, and management of Mountain Ash diseases. Disease Symptoms Pathogen/Cause Management Cytospora canker Trunks and larger branches have brown sunken cankers, circular or irregular in shape, that gradually enlarge and girdle the wood. Black, pimple-like fruiting structures form on infected bark. Yellow threads of spores ooze out of the fruiting structures during wet weather. Cytospora Prune infected limbs well below the canker. Disinfest the pruning tools thoroughly between cuts. Remove severely infected trees. Fire blight Flower clusters are killed and turn dark brown to black. Dead leaves and aborted flower parts remain on the tree. Long, slightly sunken cankers are seen where the dead wood meets the live wood. In the spring, slime may ooze from the canker if the weather is warm and wet. No fungal fruiting structures are found in the canker. Flower clusters are killed and turn dark brown to black. Erwinia amylovora During dormancy when the weather is dry, prune infected branches, cutting at least 4 inches below the base of the canker. Disinfest pruning tools frequently. Use fertilization practices that do not promote excessive succulent growth. Remove root suckers and water sprouts while they are small. Remove unwanted plants that are susceptible to the disease from near cultivated plants. Apply a bactericide to protect against flower infection. Sorbus aucuparia and S. intermedia are resistant. Leaf spot Round to irregular, brown spots form on leaflets. Tiny, black fruiting structures form within older spots. Phyllosticta No control is recommended.