Common Plant Diseases and Disorders

Learn to identify common plant diseases

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Rots
Rots are diseases that decay roots, stems, wood, flowers, and fruit. Some diseases cause leaves to rot, but those symptoms tend to be described as leaf spots and blights. Rots can be soft and squishy or hard and dry. They are caused by various bacteria and fungi. Many are very active in stored fruits, roots, bulbs, or tubers.

Fruit rots: Grapes infected with black rot turn brown, then harden into small, black, mummified berries. Brown rot of stone fruits causes whole fruit to turn brown and soft. Control fruit rots by planting resistant cultivars, removing and destroying infected fruit, and pruning to increase air movement. Applying compost tea or Bacillus subtilis may help prevent the disease from developing. Sulfur sprays throughout the season can be effective, too, as a last resort.

Root and stem rots: Control these troublesome rots by providing good drainage and good air circulation. Try drenching the soil with beneficial fungi or bacteria. Start cuttings in sterilized mix, and plant only healthy plants. Dispose of all infected plant material. Winter injury may invite problems on woody plants.

Mushroom and wood rots: These rots can damage or kill trees. Some of them form obvious mushrooms or other fungal growths. Cutting out infected areas can provide control. Keep soil well drained, and plant resistant species and cultivars where problems are severe.

Rusts
Rusts are a specific type of fungal disease. Many of them require two different plant species as hosts to complete their life cycle. Typical rust symptoms include a powdery tan to rust-colored coating. Applying neem oil can help prevent rust by killing spores on the leaves.

Asparagus rust: This disease appears as a browning or reddening of the small twigs and needles, and a release of rusty, powdery spores. It overwinters on stalks and infects new shoots as they emerge the following spring. Rust is also carried to other plants by wind. To control, space plants to allow air circulation. Plant resistant cultivars. Remove infected plants and burn them in fall.

Other rusts: Wheat rust, cedar-apple rust, and white pine blister rust require alternate hosts. Wheat rust needs barberry to survive, cedar-apple rust needs both juniper and an apple relative, and white pine blister rust needs a susceptible member of the currant family. Removing the alternate hosts in the area can control outbreaks.

Wilts
Plants wilt when they don't get enough water. When fungi or bacteria attack or clog a plant's water-conducting system, they can cause permanent wilting, often followed by the death of all or part of the plant. Wilt symptoms may resemble those of blights.

Stewart's wilt: This bacterial disease is widespread on sweet corn in eastern North America. It overwinters in flea beetles and infects corn when they begin feeding on its leaves. Infected leaves wilt and may have long streaks with wavy margins. Bacterial slime will ooze out if the stalks or leaves are cut. Plants eventually die or are sufficiently stunted that no ears are produced. To control, plant resistant cultivars and eliminate flea beetles. Destroy infected plants.

Fusarium and Verticillium wilt: These fungal wilts attack a wide range of flowers, vegetables, fruits, and ornamentals. Plants wilt and may turn yellow. To control, plant resistant cultivars. Rotate crops, or do not replant in areas where problems have occurred. If wilt only affects a branch, it may help to cut it out well below the wilt symptoms. Destroy infected branches or plants.

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