If you can identify the symptoms as a blight or wilt, you may be able to take steps to limit the disease, even if you don’t know the specific pathogen causing the infection.
The common names of plant diseases often reflect the type of symptom they cause. If you can identify the symptoms as a blight or wilt, for example, you may be able to successfully take steps to limit the disease, even if you don't know the specific pathogen causing the infection. The most common garden plant diseases and disorders are described below.
Remember: If you're considering applying a spray or dust, take time to identify the specific disease problem first so that you apply the appropriate product at the correct time to be effective.
When plants suffer from blight, leaves or branches suddenly wither, stop growing, and die. Later, plant parts may rot.
Fire blight: This bacterial disease affects apples, pears, fruit trees, roses, and small fruits. Infected shoots wilt and look blackened.
Alternaria blight (early blight): This fungal blight infects ornamental plants, vegetables, fruit trees, and shade trees worldwide. On tomatoes, potatoes, and peppers, it is called early blight. On leaves, brown to black spots form and enlarge, developing concentric rings. Heavily blighted leaves dry up and die as spots grow together. Lower leaves usually show symptoms first. Targetlike, sunken spots will develop on tomato branches and stems. Fruits and potato tubers also develop dark, sunken spots. Alternaria spores are carried by air currents and are common in dust and air everywhere. They are a common cause of hay fever allergies. Alternaria fungi overwinter on infected plant parts and debris, or in or on seeds. Control this disease by planting resistant cultivars and growing your own transplants from disease-free seed. Apply Trichoderma harzianum to the soil just before planting. Promote good air circulation. For early blight, apply potassium bicarbonate (baking soda) sprays starting 2 weeks before the time of year when symptoms would normally first appear. Dispose of infected plants and when possible, use a 3-year rotation.
Phytophthora blight (late blight): Lilacs, rhododendrons, azaleas, and holly infected by Phytophthora fungi suffer dieback of shoots and develop stem cankers. Prune to remove infected branches and to increase air movement.
On peppers, potatoes, and tomatoes, Phytophthora infection is known as late blight. The first symptom is water-soaked spots on the lower leaves. The spots enlarge and are mirrored on the undersurface of the leaf with a white downy growth. Dark-colored blotches penetrate the flesh of tubers. These spots may dry and appear as sunken lesions. During a wet season, plants will rot and die. The pathogen overwinters on infected tubers and in plant debris. Avoid problems by planting only in well-drained soil, and use resistant varieties if possible. For late blight, keep foliage dry as much as possible, and check frequently for symptoms whenever the weather is wet. Preventive sprays of compost tea or Bacillus subtilis may help prevent the disease. Immediately remove and destroy plants infected with late blight; prune off cankered shoots of shrubs. After harvest, remove and destroy all plant debris that may be infected.
Bacterial blight: This bacterial disease is particularly severe on legumes in eastern and southern North America. Foliage and pods display water-soaked spots that dry and drop out. On stems, lesions are long and dark colored. Some spots may ooze a bacterial slime. To control, plant resistant cultivars, remove infected plants, and dispose of plant debris. Use a 3-year rotation and don't touch plants while they are wet, as you may spread the disease.
Cankers usually form on woody stems and may be cracks, sunken areas, or raised areas of dead or abnormal tissue. Sometimes cankers ooze conspicuously. Cankers can girdle shoots or trunks, causing everything above the canker to wilt and die. Blights and diebacks due to cankers look quite similar. Cold-injury symptoms may look like, or lead to the development of, cankers and diebacks.
Cytospora canker: This fungal disease attacks poplars, spruces, and stone fruits. The cankers are circular, discolored areas on the bark. To control, plant resistant trees and cut out branches or trees with cankers.
Nectria canker: This fungus attacks most hardwoods and some vines and shrubs. It is most damaging on maples. Small sunken areas appear on the bark near wounds, and small pink spore-producing structures are formed. It kills twigs and branches and may girdle young trees. Control by limiting pruning cuts and removing diseased branches.