Powdery mildew diseases are caused by fungi that are often 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 roses, lilacs, English oak, sycamore, some deciduous azaleas and zinnias, and many other plants (Chapter 7).
The white growth on leaves and stems is the powdery mildew fungus growing on the surface of the plant tissue. Much of this growth consists of spores, which can be blown about to susceptible 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. The unsightly fungal growth greatly reduces the aesthetic quality of the plant. If the infection is widespread, the entire leaf blade may be covered with a thin, white fungal growth. 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 may be severe.
Powdery mildew can be treated effectively with chemicals. The white appearance may remain, however, even though the fungus may be dead. For highly susceptible plants such as roses, a regular preventive spray program should be planned and carried out. For other plants, watering only early in the day and pruning out surrounding vegetation to improve sunlight and drying of plant foliage may be all that is needed for control.
Rust diseases are also caused by fungi that are highly specific to the host plant they infect. Rust fungi usually sporulate on plant tissues forming masses of orange to dark red spores. Orange to reddish-brown spots or lesions will form on leaves, twigs, or fruit. Sometimes galling and disfigurement will be associated with the infections. In summer, infections on some plants will produce cup-like structures or tubular growths that will bear abundant orange spores. Commonly affected trees are hawthorns, crabapples, or junipers. Some kinds of rust fungi must have two different species (hosts) of plants in the area before they become a problem. An example is Cedar-Apple rust, where both a rosaceous host (crabapple, hawthorn) and junipers are required for disease development. Some other common rust diseases include rust on aster, hollyhock, and snapdragon.
Most of these diseases are also caused by fungi; however, bacteria can also be the cause. Common bacterial diseases include bacterial leaf spot of English ivy and zinnia; fireblight on crabapple, mountain ash, hawthorn, cotoneaster, and pyracantha; crown gall on rose and euonymus; and bacterial leaf scorch of shade trees.
The fungi that cause leaf spotting and blight are common and are usually spread by microscopic spores either long distances by air or shorter distances by splashing water. Most notable among this group are Entomosporium leaf spot on hawthorn, scab on crabapple and pyracantha, Botrytis blights, black spot of rose, and anthracnose on shade trees.
Anthracnose diseases are common and often result in irregular tan or brown areas on the leaves, especially along major veins of the leaf blade. The condition usually is noticed in late spring on lower parts of the plant. Deformation and twisting of leaves often results from infections that occurred during bud break and leaf expansion. Anthracnose blight is commonly found on ash, dogwood, white oak, maple, sycamore, and London plane tree and others, particularly when such plants are growing in damp, cool places. Frost injury, herbicide damage, and leaf scorch are often mistaken for anthracnose infection. Examine affected leaves closely for fungal fruiting bodies within or near the edge of browned areas and on the veins.
Anthracnose is also common on walnut, causing early leaf fall later in the summer. On river birch, small circular leaf spots are the symptoms. On some hosts, especially sycamore and flowering dogwood, twig blight and cankers may occur.
Pythium and Phytophthora fungi are often called water molds because they have a spore stage that is adapted to spread by swimming in water. These organisms attack a wide variety of plants, causing root rots, stem rots, and cutting rots. Many times they do not kill a plant. Instead, they may "prune" the root system, resulting in poor growth, yellowing, or stunting of the top portion of the plant. However, under conditions of poor soil drainage (wet soil) these fungi can girdle the crown or kill major roots resulting in plant death.
These fungi are found in almost all soils. Environmental control can be achieved by improving the drainage of the soil. The fungi do not survive well in well-drained planting beds.
Aside from water molds, many other fungi cause root and crown rots. The fungus Rhizoctonia lives in the soil and attacks a wide variety of crops. Some species of Fusarium, Sclerotium, Cylindrocladium, Sclerotinia and Thielaviopsis behave similarly. All of these fungi can persist in soil for many months through specially adapted resting structures.
Many fungi such as Botryosphaeria, Phomopsis, Sphaeropsis (formerly Diplodia), and Fusicoccum cause stem or twig cankers. A canker is an infection of the bark and outer vascular tissue of a tree or shrub. The infections vary in color from the surrounding healthy bark. Infected tissue may be sunken or swollen. When cankers girdle the twig or branch, the end of the branch dies. Small black or brown bumps, spore-forming bodies of the fungus, are often seen in the canker. These may produce an ooze of orange, white, salmon, or tan spores during rainy weather. Phytophthora, mentioned under root rots caused by water molds, sometimes causes cankers as well as twig and stem blight.
Most notable examples include Phomopsis twig blight on juniper, Dothiorella branch blight on Shumard oak, Sphaeropsis tip blight on red or Austrian pine, and Volutella blight and canker on pachysandra. These pathogens sporulate primarily in the spring. The spores are spread by splashing water, insects, and wind.
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 vascular system of the plant. Wilting, yellowing, and browning of foliage may be followed by premature defoliation. One limb of the plant may be affected first. Gradual dieback may be apparent. Yellow-brown, brown, black, or greenish-black streaks may be found in the outer rings of wood of infected branches, or in the stems of herbaceous plants. These streaks differentiate Verticillium wilt from most other types of plant wilting and dieback. The fungus that causes Verticillium wilt has specialized "resting" structures that allow it to persist in soil for many years.
Ophiostoma ulmi (formerly Ceratocystis ulmi) is the plant-wilting fungus that causes Dutch elm disease in a variety of elms. It is spread from plant to plant by root grafts or by elm bark beetles feeding on small twigs or by root grafts.