Curing Common Garden Diseases
Many names of plant diseases describe the symptoms they cause — powdery mildew, leaf curl, and club root diseases, for example. Some diseases attack only one plant part, whereas others can affect the entire plant. The following list describes some of the most common diseases of trees, shrubs, vegetables, flowers, and fruits:
- Anthracnose: This group of fungi can attack many plants (beans, vine crops, tomatoes, and peppers) and trees (dogwoods, maples, ash, and sycamores). Look for small, discolored leaf spots or dead twigs. The disease can spread to kill branches and eventually the whole plant. You can spread it easily by splashing water and walking through wet plants. Many plant varieties are resistant to anthracnose fungi; choose them whenever you can. Prune off affected plant parts, if possible, and dispose of the debris in the trash, not in the compost pile. Fungicides containing copper can help.
- Apple scab: This fungus attacks apple and crabapple trees, producing discolored leaf spots and woody-brown scabs on the fruit. The leaf spots start out olive-colored, eventually turning brown. Plant scab-resistant varieties. Rake up and destroy fungus-infected leaves to prevent the fungus from reinfecting the trees in spring. Spray with copper- or sulfur-based fungicides during wet spring and summer weather.
- Armillaria root rot: This fungus infects and kills the roots and lower trunk of ornamental trees, especially oaks. Symptoms include smaller-than-normal leaves, honey-colored mushrooms growing near the base of the tree (see Figure 9-1), and declining tree vigor. Trees may suddenly fall over when the roots weaken and decay. Keep trees growing vigorously, and avoid damage to their roots and trunks. If you live in an area where the disease is prevalent, plant resistant tree species. Consult local nursery or local extension-office experts for information.
Armillaria root-rot fungus.
Armillaria root-rot fungus.
✓ Black spot: This fungus causes black spots on rose leaves, as shown in Figure 9-2. Yellow rings may surround the spots, and severe infections can cause the shrub to lose all its foliage. The disease spreads easily in splashing water; it overwinters in fallen leaves and mulch around the plant. Remove old mulch after leaf fall in the autumn, and replace it with fresh mulch. Prevent black spot by choosing disease-resistant roses and by cleaning up and destroying any diseased leaves that fall to the ground. Avoid wetting the foliage when you water.
Neem oil (not neem extract) is the best organic fungicide against black spot. Use it at the first signs of the disease, but spray either early or late in the day to avoid harming beneficial insects. Fungicide sprays containing copper, sulfur, or potassium bicarbonate can also offer some protection.
✓ Botrytis blight: This fungus attacks a wide variety of plants, especially in wet weather. It causes watery-looking, discolored patches on foliage that eventually turn brown. Infected flowers — especially roses, geraniums, begonias, and chrysanthemums — get fuzzy white or gray patches that turn brown, destroying the bloom. Strawberry and raspberry fruits in particular develop light brown to gray moldy spots, and the flesh becomes brownish and water soaked. Discourage botrytis by allowing air to circulate freely around susceptible plants, and avoid working with wet plants. Remove and destroy any infected plant parts. Try a citric acid/mint oil fungicide or the bacterial fungicide Mycostop.
✓ Cedar-apple rust: Rust diseases, including this one, often have complicated life cycles in which they infect different plant species and exhibit very different symptoms in each, depending on their life stage. Cedar-apple rust fungus appears as bright orange spots on the leaves and fruit of apples and crabapples. On its alternative hosts — juniper and red cedar — it develops yellow to orange jellylike masses in the spring. The fungus needs both hosts to reproduce and spread.
Prevent the disease by planting resistant apple varieties and keeping the alternative hosts several hundred yards away from susceptible trees. A related fungus, pear rust, affects pears similarly.
- Club root: This fungus infects mainly cole crops (such as cabbage, broccoli, and collards) and grows best in acidic soils. Symptoms include stunted growth, wilting, poor development, and swollen lumps on the roots. Practice good garden hygiene by keeping tools clean and picking up plant debris. Raise the soil pH to 7.2, and avoid planting susceptible crops in infected soil for at least seven years. Some vegetable varieties are immune.
- Corn smut: You can't miss this fungus disease, which causes large, mutant-looking, white to gray swellings on corn ears. When the swellings burst open, the fungus spreads. Prevent the disease by planting resistant corn varieties and rotating crops so that you don't grow corn in the same place year after year. Dispose of infected plant parts in the trash.
- Cytospora canker: Cankers appear as oozing, sunken, or swollen areas on the bark of susceptible trees, such as peaches, apples, maples, spruces, and willows. The new shoots turn yellow, wilt, and then die back. The disease attacks the woody stems of susceptible plants, forming cankers that can kill infected branches. Plant resistant or less-susceptible plants, and keep them growing vigorously. Avoid bark injuries that provide an entrance for infecting fungus. Remove and destroy infected branches, cutting back to healthy wood that doesn't contain any black or brownish streaks.
- Damping off: A problem mostly in young plants and seedlings, this fungus rots stems off near the soil line, causing the plant to keel over and die. Prevent damping off by planting seeds and seedlings only in pasteurized planting soil and avoiding overwatering. Air circulation helps prevent the fungus, too. Clean your tools in isopropyl alcohol to prevent the spread of damping off.
- Fusarium wilt: This fungus is fatal to many vegetable crops. The first symptoms are yellowing leaves and stunted growth, followed by wilting and plant death (see Figure 9-3). In melons, the stems develop a yellow streak, which eventually turns brown. Choose Fusarium-resistant varieties. After plants are infected, no cure is possible. If you build your soil's health so that it contains lots of beneficial microorganisms, you should rarely be bothered with this disease.
- Galls: Galls appear as swollen bumps on leaves, stems, and branches. Gall wasps, aphids, and mites infest oaks and other landscape trees and shrubs, causing unsightly swellings on leaves and twigs. In other cases, bacteria and fungi are the culprits. Usually, the damage is simply cosmetic and not life-threatening to the plant. Control depends on what's causing the problem. Take a sample of the damage to a plant expert at your local extension office, or contact your local Master Gardener program.
- Leaf spots and blights: Several fungi show up first as circular spots on leaves of tomatoes, potatoes, peppers, and other vulnerable vegetables, flowers, and ornamental plants. The spots increase in size until the leaves die and fall off. The fungi spread easily in damp weather and in gardens where overhead watering wets the foliage, especially late in the day. The best control is to remove all plant debris at the end of the gardening season, clean tools between uses, practice crop rotation, buy disease-resistant varieties, and avoid contact with wet plants. Try botanical and biological fungicides first, using copper-based fungicides only as a last resort.
- Mildew (downy and powdery): These two fungi produce similar symptoms: a white, powdery coating on leaves. They infect a wide variety of plants, including roses, vegetables, fruit trees, strawberries, raspberries, and lilacs. A different species of mildew attacks each kind of plant. A mildew that attacks lilacs, for example, won't harm roses. The fungi disfigure plants but may not kill them outright. Instead, they weaken their hosts, making them unattractive and susceptible to other problems. Downy mildew attacks during cool, wet weather. Powdery mildew (shown in Figure 9-4) appears during warm, humid weather and cool nights, especially when the soil is dry.
Many vegetable and flower varieties are resistant to mildew; read package and catalog descriptions carefully. Remove infected plant debris from the garden, and avoid getting the leaves wet. Use potassium bicarbonate, superfine horticultural oil, or neem oil to treat infected plants. Try botanical and biological controls, using copper- and sulfur-based fungicides as a last resort.
✓ Root rot: This broad term covers several fungal root diseases that cause susceptible plants to turn yellow, wilt, and sometimes die. Nearly all plants are susceptible under the right conditions, such as excessive soil moisture, poor soil aeration, and wounding. The fungi can survive in the soil for many years without a host. Prevent root rot by building healthy, well-drained soil. Microbial fungicides can help foil many root-rot diseases.
✓ Rust: Many fungi cause rust, and the symptoms of this disease vary widely, depending on the kind of plant they infect. Usually, the symptoms include yellow to orange spots on the leaf undersides, with white or yellow spots on the upper leaf surface. Susceptible plants include brambles, hollyhocks, roses, pines, pears, bluegrass and ryegrass lawns, wheat, barberry, and beans. Each rust species infects a specific plant species, so the rust on roses can't infect beans, for example. Some rusts, such as white pine blister rust, have complicated life cycles and must infect two different plants — in this case, white pines and Ribes species, such as currant and gooseberry. Symptoms of this disease include yellow, orange, reddish-brown, or black powdery spots or masses on leaves, needles, or twigs.
Rust fungus: Rust fungus, shown in Figure 9-5, forms yellow or orange bumps on leaf undersides and is most prevalent in humid and damp conditions. Provide good air circulation to keep foliage as dry as possible, remove and destroy infected parts, and keep your tools clean. Plant disease-resistant varieties.
✓ Slime flux: This bacterial rot inside infected trees — usually, elms, maples, and poplars — causes oozing and often bad-smelling sap running from old wounds or pruning cuts. No control is possible after the symptoms appear.
- Verticillium wilt: This fungus affects many plants, including tomatoes, eggplant, potatoes, raspberries, strawberries, roses, Japanese maples, olives, and cherries. Look for wilting and yellow leaves, especially older ones. In some plants, the leaves curl up before falling off. Prevent future infections by cleaning up all garden debris, cleaning tools thoroughly with disinfectant, and avoiding susceptible species. Choose resistant varieties, and practice crop rotation.
- Viruses: This group of incurable diseases infects vegetables, brambles, strawberries, trees, and flowering plants. Usually, the leaves develop mottled yellow, white, or light green patches and may pucker along the veins. Flowers may develop off-color patches, and fruit ripens unevenly. Aphids, leafhoppers, nematodes, and whiteflies spread the virus as they move from plant to plant. Viruses often live in wild bramble plants and weeds. Smoking or handling tobacco products around susceptible plants can spread tobacco mosaic virus, which infects tomatoes, eggplants, peppers, petunias, and other plants. Prevention is the only strategy. Buy only virus-free plants and keep pests in check. Eradicate wild brambles near your garden.
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