Environmental stresses are often responsible for causing fungus diseases on shrubs. Stresses such as drought, excess rainfall or planting the shrub in poorly-drained soil make it susceptible to fungal spores of different diseases. Diagnose and treat the disease immediately to prevent it from spreading to other parts of the same shrub or to nearby vegetation.
Although most fungal diseases vary from one shrub species to another, the more common ones include Verticillium Wilt, Anthracnose, Powdery mildew, Phytophthora canker and Phomopsis Gall. Each disease infects and damages a different area of the shrub and control methods vary accordingly.
Verticillium wilt infects American cranberry, border privet, Korean barberry and bottlebrush buckeye. Anthracnose infects Japanese maple and Amur maple, while powdery mildew commonly infects Amur honeysuckle, late lilac, oakleaf hydrangea and winter honeysuckle, among others. Phytophthora canker infects Japanese yew, Flame azalea and Korean rhododendron while Phomopsis gall infects border forsythia, early forsythia, Carolina rhododendron and Flame azalea.
Symptoms of Verticillium wilt, the disease that starts in the roots and progresses upward, streaking of xylum vascular tissues, curling or wilting of leaves or defoliation and stem or branch dieback. Anthracnose causes patches or spots on infected leaves in early summer, and new growth becomes distorted or curled. Symptoms of powdery mildew include white or gray powdery blotches or patches on buds, leaves, stems and fruit that enlarge in time to cover the entire area, accompanied by leaf drop and bud deformation. According to the University of Illinois Extension, this disease causes more cosmetic damage than harm. Phytophthora canker affects roots of shrubs, stunting growth and causing wilting, dieback and eventually death of the shrub. The inner bark at the base of the infected shrub is discolored when scraped away, and cankers emit a deep-brown exudation. Galls on parts of the infected shrub resemble round balls that vary in size from as small as a pea to an inch large, and appear singly or in clusters.
No chemical control exists to prevent Verticillium wilt. Spray fungicide specifically formulated to treat Anthracnose disease over infected areas. Cultural control practices prevent the spread of Powdery mildew, such as planting in well-drained soil, appropriate spacing for air circulation and avoiding excessive watering and fertilizing. Spray fungicide over infections that occur early in the growing season. Phytophthora fungus lives in the soil; therefore control method includes improving plant drainage and watering practices, and treating the soil with a recommended fungicide. The best control practice to manage Phomopsis gall is to prune and destroy infected areas.
Grow resistant shrubs adapted to the environment conditions to prevent Verticillium wilt. Immediately water and fertilize shrubs showing symptoms of this disease, and remove infected shrubs to prevent spread. Prevent Anthracnose disease spread by removing fallen stems and leaves from infected shrubs immediately. Remove fallen infected parts of shrubs infected with powdery mildew.