Twig blighting in successive seasons may cause the formation of witches- brooms. The spots may be surrounded by a white to light yellow halo. Repeated infections in early summer result in multiple spotting and premature defoliation. The fungus overwinters in dead leaves on the ground and in dormant buds. These spores infect newly developing leaves and stems.
Leaves are most susceptible to infection in spring and early summer.
Certain cultivars of Chinese elm appear to be highly susceptible. Remove leaf debris and dead shoots from trees during winter to reduce overwintering inoculum of the fungus. Avoid close spacing and overhead irrigation of cuttings and young trees. Preventive fungicide applications may be required for susceptible elm cultivars. Make the first application at bud break in spring and continue at regular intervals until leaves are fully developed.