Diseases occur on landscape plants when environmental conditions are suitable for pathogens to develop on them. Some pathogens attack a wide variety of plants, whereas others are host-specific. Pathogens can attack all plant parts, although many pathogens only attack selected tissues. Prevention is the key to the management of diseases in the landscape. Although foliar diseases, such as leaf spots and mildews, are generally manageable once they are observed, root diseases generally are not. Many fungicides are available to aid in the management of diseases in the landscape, but they should be used in conjunction with cultural practices intended to modify the environment to make it less conducive to disease development.

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Powdery Mildew

Powdery mildew is a type of fungal disease that commonly occurs on a variety of landscape plants, including roses, crape myrtles and dogwoods. It is visible as a grayish or whitish powdery growth on the surface of infected plant tissues. The disease develops most frequently on plants grown in shade or partial shade when temperatures are moderate and humidity is high.

Controls. Choose resistant plants and varieties or plant susceptible plants in sunny locations with good air movement. Protect the foliage with sprays of potassium bicarbonate, horticultural oils, sulfur or other fungicides.

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Leaf Spots and Blights

Leaf spots and blights caused by various fungi and bacteria are common in the landscape, particularly when the plants are subject to overhead watering. Many of these pathogens, particularly bacteria, are spread in splashing water and need extended periods of leaf wetness to gain entry into the plant.

Controls. Choose resistant plants and varieties. Prevent the foliage from getting wet from overhead irrigation. Fixed-copper or other fungicide sprays applied prior to the onset of rainy periods may provide some protection.

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Anthracnose

Anthracnose is a group of fungal diseases that occur on leaves, shoots and twigs on various shrubs and trees. These diseases generally develop in the spring during prolonged rainy periods as the spores of the fungi that cause these diseases are dispersed, primarily in splashing water. These fungi survive from year to year in cankers on infected branches.

Controls. Choose resistant plants and varieties. Fungicide sprays applied prior to the onset of rainy periods in the spring may provide some protection.

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Fire Blight

Fire blight is a bacterial disease of flowering pears, pyracantha, loquats and certain other rosaceous (rose family) plants. The bacterium that causes the disease is spread in splashing water or by honeybees and enters the plants primarily through the flowers. Once in the plant, the bacterium moves into the new shoots, which it kills. Characteristic symptoms of this disease are dead branch tips to which the leaves are still attached.

Controls. Choose resistant plants and varieties. Prune out infected branches (6 to 8 inches below the dead portion of the branch) and protect flowers by spraying with streptomycin.

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Root and Crown Rots

Root and crown rots commonly affect both annual and perennial plants in the landscape, particularly in sites with poor drainage. The first noticeable symptoms may include stunting of the plants or wilting of the leaves. Additional symptoms, such as defoliation and branch dieback over the entire plant, are often observed in the later stages of disease development.

Controls. Choose resistant plants and varieties. Visibly check the condition of the roots prior to purchasing plants. Choose a well-drained planting site. Water regularly, but avoid overwatering.

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Viruses

Viruses can affect many types of plants. Typical virus symptoms include mosaic, mottle, ring spot or line patterns on the foliage or flowers. They are spread primarily by various insects of which aphids, whiteflies and thrips are the most common, but many also can be transmitted mechanically in sap on pruning shears or even hands. Once infected with a virus, a plant cannot be cured.

Controls. Choose resistant plants or varieties when possible. Do not introduce virus-infected plants into the landscape. Prevent their spread by controlling insects and regularly cleaning pruning shears.

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Turfgrass Patch Diseases

Many diseases of turfgrasses occur as circular or irregular patches of affected plants. These patches may range in size from a few inches to several feet, depending upon the disease. Many of these diseases occur on turf that is stressed from environmental factors such as drought or on turf that has been overfertilized.

Control. Follow the best management practices for growing and maintaining each type of turfgrass to prevent stresses that predispose turf to infection. Several fungicides are available to homeowners to aid in the control of these diseases, but they should be used in conjunction with cultural practices to modify the environment and make it less conducive to disease development.

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Fairy Rings

Fairy rings are generally caused by the group of fungi that produce mushrooms or “toadstools.” These fungi live on organic matter in the soil, such as construction materials, wood debris, tree roots, etc. As these fungi grow through the soil, they may kill the grass, cause it to be a darker shade of green or produce no symptoms at all until the mushrooms are produced. Mats of fungal tissue produced in the soil can interfere with water penetration into the soil resulting in localized dry spots.

Control. Fairy rings can be quite difficult to control. If the source of the nutrition (i.e., buried wood or tree roots) can be removed, their growth can be halted. Otherwise, steps must be taken to break up the hydrophobic layer and rewet the soil. Fungicide applications may help, but the fungicide must penetrate deep enough into the soil so that it comes in contact with the fungus.

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Environmental Factors

Many environmental factors can cause disorders of plants. These include drought, excess water, soil compaction, nutrient deficiencies or toxicities, heat or cold damage, herbicides and improper soil pH. They can generally be recognized because they do not spread from plant to plant.

Control. Remedial action will depend upon the cause of the disorder.