Common Plant Diseases and Disorders

Learn to identify common plant diseases


Other Diseases
Anthracnose: Anthracnose, or bird's-eye spot, is a fungal disease. It causes small dead spots that often have a raised border and a sunken center, and that may have concentric rings of pink and brown.

Bean anthracnose infects beans and other legumes. The symptoms are most obvious on the pods as circular, black, sunken spots that may ooze pink slime and develop red borders as they age. To control, buy disease-free seed, rotate crops, turn under or hot-compost infected plants, and avoid touching plants when they are wet so you won't spread the disease.

Club root: Club root affects vegetables and flowers in the cabbage family. Plants infected by the fungus wilt during the heat of the day, and older leaves yellow and drop. Roots are distorted and swollen. Avoid club root by choosing resistant cultivars and raising your own seedlings. The fungus has spores that can persist in soil for many years. If you've had past club root problems, adjust the soil pH to at least 6.8 before planting susceptible crops.

Damping-off: Damping-off is caused by a variety of soilborne fungi. Seeds rot before they germinate, or seedlings rot at the soil line and fall over. It can be a problem with indoor seedlings and also in garden beds. Prevent damping-off by keeping soil moist, but not waterlogged. Provide good air movement in seed-starting areas. Wait until soil is warm enough for the specific plant before seeding. Sterile seed-starting mix or a mix that includes compost can help prevent problems, too. If you've had past problems with this disease, add compost to your soil, and use a product containing Trichoderma harzianum to drench the soil before planting.

Downy mildew: Downy mildews are fungal diseases that attack many fruits, vegetables, flowers, and grasses. The primary symptom is a white to purple, downy growth, usually on the undersides of leaves and along stems, which turns black with age. Upper leaf surfaces have a pale color. Lima bean pods may be covered completely, while leaves are distorted. The disease overwinters on infected plant parts and remains viable in the soil for several years. It is spread by wind, by rain, and in seeds. To control it, buy disease-free seeds and plants, follow a 3-year rotation, and remove and dispose of infected plants. Preventive sprays of bicarbonate may be effective.

Galls: Galls are swollen masses of abnormal tissue. They can be caused by fungi and bacteria as well as certain insects. If you cut open a gall and there is no sign of an insect, suspect disease.

Crown gall is a serious bacterial disease that infects and kills grapes, roses, fruit trees, brambles, shade trees, flowers, and vegetables. Galls are rounded with rough surfaces and are made up of corky tissue. They often occur on the stem near the soil line or graft union but can also form on roots or branches. To control it, buy healthy plants, and reject any suspicious ones. Don't replant in an area where you have had crown gall. Avoid wounding stems, and disinfect tools between plants when pruning. Remove and destroy infected plants, or cut out galls.

Leaf blisters and curls: Leaf blister and leaf curl are fungal diseases that cause distorted, curled leaves on many trees. Oak leaf blister can defoliate and even kill oak trees. Blisters are yellow bumps on the upper surface of the leaves, with gray depressions on the lower surface. Peach leaf curl attacks peaches and almonds. New leaves are pale or reddish and the midrib doesn't grow along with the leaves, so the leaves become puckered and curled as they expand. Fruit is damaged, and bad cases can kill the tree. Both diseases are controlled with a single dormant oil spray just before buds begin to swell.

Leaf spots: A vast number of fungi can cause spots on the leaves of plants. Most of them are of little consequence. A typical spot has a definite edge and often has a darker border. When lots of spots are present, they can grow together and become a blight or a blotch.

Blackspot is a common disease on roses. The spots appear on the leaves and are up to 1/2 inch across with yellow margins. Severe cases cause leaves to drop. To control blackspot, plant resistant cultivars, and destroy all dropped leaves and prunings. Mulch to prevent dirt and spores from being splashed up onto plants. Bicarbonate sprays can be very helpful in preventing leaf spot diseases.

Molds: Molds are characterized by a powdery or woolly appearance on the surface of the infected part.

Gray mold, or botrytis, is a common problem on many fruits and flowers. It thrives in moist conditions and is often seen on dropped flower petals or overripe fruit. It appears as a thick, gray mold or as water-soaked, blighted regions of petals, leaves, or stems. In most cases it first infects dead or dying tissue, so removing faded flowers and blighted buds or shoots will control the problem. Peonies, tulips, and lilies can be severely damaged in wet seasons. Destroy infected material, and space, prune, and support plants to encourage good air movement.

Nematodes: Nematodes themselves are described earlier in this entry. Symptoms of nematode invasion include reduced growth, wilting, and lack of vigor.

Some nematodes cause excessive branching of roots, rotted roots, and enlarged lumps on roots. Other nematodes attack leaves, causing triangular wedges of dead tissue.

Root knot nematodes attack a variety of plant root systems, including most vegetable and ornamental crops. Carrot plants will be stunted, with yellowed leaves, and roots may be distorted. Roots of other plants will have swollen areas. Remember that legumes are supposed to have swellings on their roots that are caused by nitrogen-fixing bacteria.

Prevent nematodes from invading your plants by maintaining your soil organic matter. Plant resistant varieties when possible. Take care not to spread soil from nematode-infested areas to other parts of your garden or yard. Reduce nematode populations by solarizing soil. Use a marigold (Tagetes patula or T. erecta) cover crop to reduce nematodes. Rotate susceptible crops. Adding products containing chitin to the soil can help reduce problems.

Hot-water dips can eradicate nematodes from within roots, bulbs, and the soil on them.

Powdery mildew: Mildews are one of the most widespread and easily recognized fungi. They are common on phlox, lilac, melons, cucumbers, and many other plants. Mildew forms a white to grayish powdery growth, usually on the upper surfaces of leaves. Small black dots appear and produce spores that are blown by wind to infect new plants. Leaves will become brown and shrivel when mildew is extensive. Fruits ripen prematurely and have poor texture and flavor. To control mildews, prune or stake plants to improve air circulation and dispose of infected plants before spores form. Apply bicarbonate sprays to prevent the spread of infection.

Scabs: Scabs are fungal diseases that cause fruits, leaves, and tubers to develop areas of hardened, overgrown, and sometimes cracked tissue. Fruit scab can be a major problem on apples and peaches. Control by disposing of fallen leaves and pruning to increase air movement. If you've had past serious problems with scab, ask your local extension service about the best spray schedule for sulfur to control the disease.

Smuts: Smuts are fungal diseases. They are most commonly seen on grasses, grains, and corn. Enlarged galls are soft and spongy when young but change to a dark, powdery mass as they age.

Corn smut can form on kernels, tassels, stalks, and leaves. Smut galls ripen and rupture, releasing spores that travel through the air to infect new plants and overwinter in the soil, awaiting future crops. To control corn smut, select resistant cultivars. Remove and burn galls before they break open, and follow a 4-year rotation.

Viruses: Infected plants often grow slowly and yield poorly. Leaves may cup or twist, and develop mottling, streaking, or ring-shaped spots. Identification is often the elimination of all other possible causes. Professional growers use heat treatments and tissue culture to control viral disease. Purchase certified plants to avoid problems. Control insects that spread viruses. Remove and burn all plants with viral disease to prevent the disease from spreading.

Common Cultural Disorders
Cold injury: Freezing injury can cause death or dieback. Symptoms of cold stress are stunting, yellowing, bud or leaf drop, and stem cracking. Fruit may form a layer of corky tissue or be russeted if exposed to cold when young.

Heat injury: Temperatures that are too high cause sunscald of fruits, leaves, or trunks on the sunny side of the plant. Discoloration, blistering, or a water-soaked and sunken appearance are other symptoms of heat stress.

Moisture imbalance: Plants need a relatively constant supply of water. If they don’t have enough, they will wilt. Long periods of wilting, or repeated wilting, can cause stunting, pale color, and reduced flowering and fruit production. Plant roots also need oxygen. Too much water in the soil damages roots and will cause symptoms like frequent wilting, pale color, root decay, leaf dropping, and lack of vigor.

Wind damage: High winds also take their toll on plant appearance. Silvery discoloration and tattered leaves are symptoms of wind damage.

Salt damage: Ocean spray and road salt, as well as animal urine, can injure plants. Salts can accumulate on leaves, stems, and buds, or build to toxic levels around the roots. Over time, salt burn weakens the entire plant and causes droughtlike symptoms.

Ozone damage: Ozone is a common air pollutant that can cause a wide range of symptoms in susceptible plants, including withered leaves on citrus and grapes and tipburn on conifers. If you confirm that ozone is a common pollutant where you live, your only recourse is to avoid planting sensitive species.