Sometimes, adverse growing conditions or environmental factors produce symptoms similar to those of plant diseases. These problems need to be distinguished from plant diseases. Problems such as frost injury, dog urine burn, nutrient deficiencies, drought, girdling roots, changes in grade, chemical injury, air pollution injury, and mechanical damage are not corrected by fungicide applications.
By knowing the disease-causing organisms, the proper chemical or cultural practice can be chosen to control the problem. A root problem identified as a root rot when it is actually a nematode infestation will not be cured by applying a fungicide; a nematicide is required.
Accurate identification and diagnosis is an art as well as a science, and experience is essential. This section is not intended to make anyone an expert in identifying diseases. It is meant to acquaint the pesticide applicator with the general symptoms of diseases. For more accurate disease diagnosis, consult your county Extension agent.
Leaf Spots—Fungal leaf spots (also known as anthracnose, scab, leaf blotch, or shot hole) are usually definite spots of varying sizes, shapes and colors. (ex. Anthracnose of maple) The spots have a distinct margin and are sometimes surrounded by a yellow halo. Spots or dead areas may enlarge to cover an entire leaf. As the spots become more abundant, leaves may yellow, die, and drop. Usually, leaf spots first occur on the lower leaves and progress up the plant. Fungus growth in the spot may consist of tiny pimple-like structures or a moldy growth of spores. A hand lens or microscope may be needed to see these symptoms. (ex. Black Spot on Rose)
Leaf spots are more common in the early spring and fall when an abundance of moisture, necessary for infection, is present. When infected leaves fall and become part of the refuse around a plant, fungus spores may be produced. If carried to healthy plants, these spores can begin a new infection under appropriate environmental conditions. Leaf spots occur on virtually all ornamental plants.
Leaf blights may have the same effect on plants as leaf spots, but are generally larger diseased areas and less regular in shape. Dogwood anthracnose disease may begin as a leaf spot, become a leaf blight, and even progress to twigs and branches, causing dieback.
Rust—Rusts often produce spots called pustules that are similar to leaf spots. Pustules may be on the upper and/or lower leaf surface and are brown, reddish brown, orange, or yellow. Rust pustules are usually raised above the leaf surface, and rubbing the affected leaf surface will leave a dusty rust color (caused by the spores) on fingers. Rust fungi may also attack twigs, branches, and fruit. (ex. Cedar-Apple Rust on Crabapple)
Rust spores are often carried by wind and can be blown from infected plants to healthy plants, spreading the infection.
Rust diseases have very complicated life cycles and, in many cases, require two separate hosts to complete their life cycle. In such cases, removing either one of the hosts can break the cycle and stop rust. Cedar-apple rust and related rusts are common ornamental disease problems. (ex. Cedar-Apple Rust on Cedar)
Powdery Mildew—The typical symptom of powdery mildew disease is the white or gray layer of fungus growth produced on the surface of the plant leaves and stems (Figure 6). Plants affected by powdery mildew may also show crooked stems or bubbled and curled leaves. Spores produced by the fungus can be blown by the wind or carried by rainsplash to new plants, initiating new infections. The fungus produces small, black, overwintering structures that can withstand the cold winter. Roses, oaks, tulip poplars, lilacs, zinnias, and euonymous are commonly affected by powdery mildew. (ex. Powdery Mildew—Rose)
Leaf Galls—Leaf gall diseases are caused by fungi and are favored by cool, moist weather. However, some galls are caused by insects or mites. Leaf galls can usually be seen shortly after new growth begins in the spring. Part of the leaf becomes distorted with a pale green to whitish bladder-like thickening. The young, thickened, fleshy leaf is covered with a white growth. As the galls age, they turn brown, dry up, and fall to the ground. If the disease is severe, plant vigor can be affected due to leaf loss. The dead, dry leaves which have fallen to the ground will be a source of spores for infection the following season. Leaf galls occur on azalea, camellia, and plum. (ex. Azalea Leaf Gall)
Root Rot—The first symptoms of root rot generally appear on the above-ground parts as a gradual loss of vigor, yellowing of leaves, or wilting. Attempts to correct the problem with fertilizers and water generally yield little or no response. Diseased roots appear decayed, generally brown to black, and may be mushy or spongy ). The fungi Pythium, Phytophthora, Fusarium, Rhizoctonia, and Thielaviopsis are common root rotting organisms. Excess soil moisture favors root rot disease on ornamental plants. (ex. Pythium Root Rot—Geranium)
Stem Rot—The pathogens commonly associated with stem rot of ornamentals include Pythium, Phytophthora, Rhizoctonia, Sclerotium, and Botrytis—all common soil-inhabiting fungi. These fungi can be spread in infected debris, on cuttings, or when soil is moved. (ex. Seedling Stem Rot and Damping-Off)
Affected plants wilt slightly, become more severely wilted, and eventually die. The stems may be brown and shrunken at the soil line. Under extremely moist conditions, the white, cottony fungus mycelium may be visible on the surface of the stem. Chrysanthemums, geraniums, petunias, and other herbaceous ornamental plants are quite susceptible to stem rot. Damping-off kills ornamental seedlings when seedling roots and stems decay.
Cankers—Cankers are localized diseased areas on trunks, stems, or branches of woody plants such as shrubs and trees. Canker diseases cause bark tissues to shrink and die. The dead tissues then crack open and expose the wood underneath. (ex. Nectria canker—Black Cherry) Cankers begin as small, discolored yellow, brown, or red spots that sometimes appear watersoaked. These spots enlarge and their centers may become tan or gray. Small, black, pimple-like structures (fungus fruiting bodies) may form in the canker. Cankers can enlarge and girdle the stem, causing death to parts of the plant above the canker. The fungi causing cankers usually begin infection in a wound or injury to the bark or wood of the plant. Rose canker is a common example of a disease showing this symptom.
Vascular Wilt—Fungus parasites such as Fusarium, Verticillium, and Ophiostoma can cause wilting of many ornamental species by restricting the water flow to leaves and stems. (ex. Verticillium wilt—Jerusalem Cherry) The wilting caused by such parasites is sometimes due to the toxins they produce; later, the water-conducting vessels may become plugged by fungus growth. Vascular wilt diseases often affect one side of the plant first, causing individual limbs or branches to wilt and die back . Fusarium and Verticillium infections usually begin in the roots and gradually spread internally throughout the infected plant. Verticillium wilt of maple is an example of this kind of vascular wilt. The fungus Ophiostoma, the cause of Dutch Elm Disease, usually initiates infections in the top of the tree, in the crotches of small branches where wounds have been created by the feeding of elm bark beetles. The wilt-causing fungus slowly spreads internally throughout the tree, gradually killing it. Symptoms of vascular wilt disease often include discolored streaks in the wood of infected branches (ex. Discoloration of Maple Wood caused by Verticillium wilt)
Bacteria are single-celled microbes. Some bacteria attack living plants and cause plant disease. Bacteria can be carried from plant to plant by wind, rain splash, insects, and tools. Few bacterial diseases can be controlled using chemical bactericides.
Crown Gall—Crown gall first appears as a soft swelling of the roots or stems. As the disease develops, the swelling enlarges, becoming firmer and darker. The outer surface of the gall may become rough. (ex. Crown Gall - Euonymous)
Crown gall bacteria may survive in the soil for years in slowly decomposing galls from a previous crop. The bacteria enter the plant only through wounds and natural microscopic openings. Once inside, normal cell development of the plant is altered. After galls have fully developed, they begin to decompose and release additional bacteria that can reinfect the plant or infect new plants. Euonymous and rose are frequently affected by crown gall.
Bacterial Leaf Spot—Bacterial leaf spot may begin as a light green, water-soaked area. Later, the spots may turn brown to black. (ex. Bacterial Leaf Spot - Ivy)These spots generally are more irregular in shape than fungal leaf spots. Under warm, moist conditions, the leaf spot will appear soft and mushy. However, under dry conditions, the same spot may be brittle.
The bacteria generally overwinter in plant debris and are introduced to the plant by splashing rain or through watering. Bacteria require either a wound or a natural opening for entrance into the plant. Once inside, the bacteria multiply enough to cause symptoms, and leaves may fall prematurely. Infected leaves become a source of bacteria the following year. This disease occurs frequently on ivy.
Bacterial Blight—Fire blight is perhaps the most common bacterial blight, affecting a large number of woody plants. It is commonly found on flowering pears and crabapples, cotoneaster, and pyracantha.
Infection generally occurs in the spring and may be more damaging during long periods of cool, wet weather. The bacteria, which overwinter in stems and in bud scales, can be transported by bees during pollination so that symptoms may occur on blossoms and fruit spurs.
Symptoms first appear on new shoots as a dark discoloration as if the leaves and twigs have been scorched by fire. Later, discolored, sunken cankers may appear on infected limbs and branches. Entire plants may be killed. (ex. Fire Blight)
Bacterial Leaf Scorch—This disease of shade trees causes premature leaf browning and loss in late summer. Individual leaves may have brown margins due to the bacterial pathogen living in the tree xylem. Infected trees gradually decline. Bacteria are thought to be transported by certain leafhoppers.