Horticultural Forum: July 2001, ORNAMENTAL MAINTENANCE: Battling Tree & Shrub Diseases

VERTICILLIUM WILT DISEASES. Verticillium is a fungus that causes wilting of a large number of woody ornamentals and some herbaceous plants such as maples, redbud, Russian olive and chrysanthemum. The fungus invades injured roots, grows into the stem and plugs the plant’s vascular system. Wilting, yellowing and browning of foliage are followed by premature defoliation. Yellow-brown, brown, black or greenish-black streaks are found in the outer wood rings of infected branches or in the stems of herbaceous plants, differentiating this disease from other types of plant wilting and dieback.

Integrated control programs, such as controlling the insect vector of the fungus with proper insecticide treatments, promptly and completely pruning out infections when identified, removing and destroying dead trees in the area, isolating the root grafts of healthy trees, using resistant elm hybrids or varieties, and injecting fungicides can slow the disease’s progress.

JUNIPER TIP BLIGHTS. Correctly identifying the causal agent is important because while symptoms of these various diseases are identical, their control methods differ. Visible symptoms include browning and dieback of young needles and shoot tips. Gray lesions usually girdle the shoot at the base of the dead tissue and tiny, black or grayish fungal fruiting bodies may be visible in the gray lesions. On highly susceptible hosts, the fungus may invade and girdle larger stems, browning and killing major branches; however, this degree of disease severity is rare. Both Kabatina tip blight and Phomopsis tip blight are most damaging to younger plants.

To eliminate the sources of infection, remove and burn or bury all blighted twig tips. Prune or shear on a dry day to reduce fungus spread to other plants on wet tools, and avoid overhead irrigation, which spreads fungal spores.

Because fungi have different infection periods, fungicide application timing can differ. For instance, treatment for certain types of Juniper tip blight, such as Phomopsis tip blight, should begin in early spring and continue at 10- to 14-day intervals. But spring treatments are ineffective for Kabatina tip blight, so fungicide applications should begin in the fall.

POWDERY MILDEW DISEASES. These diseases can cause leaf reddening and distortion in addition to whitish fungal growth, and often are caused by fungi specific to the host they infect. Generally, each plant type is unique in that the powdery mildew fungus that infects it will not infect any other. Powdery mildew is common on dogwoods, roses, lilacs, English oak, sycamore, some deciduous azaleas and zinnias.

Powdery mildew fungus grows on the plant tissue surface in white blotches. Much of this growth consists of spores, which can be blown to plants nearby. Small structures, called haustoria, grow within host cells, injuring them as they obtain food. Powdery mildew will not usually kill a plant, but may weaken it and reduce winter hardiness, and the unsightly fungal growth reduces the plant’s aesthetic quality.

Some mildew fungi affect older leaves first, such as on lilacs. Others affect newer shoots, such as on roses or crabapples. When new shoots are affected, leaf curling and shoot stunting and twisting is severe.

Powdery mildew can be treated with fungicides, but white spots may remain after the fungus dies.