Herb Gardening For Dummies
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Plant diseases are a concern in any garden. Several different kinds of organisms cause plant diseases. Viruses are the toughest ones because they’re incurable — all you can do is try to prevent them. Bacteria are nearly impossible to eliminate, too, after the plant is infected. Fortunately, fungi cause most plant diseases and they do have effective control chemicals, although prevention is still the best course of action.

The following list describes some of the most common diseases of trees, shrubs, vegetables, flowers, and fruits:

  • Anthracnose: This group of fungi, spread easily by splashing water and walking through wet plants, can attack many plants and trees. Look for small, discolored leaf spots or dead twigs, especially on the youngest ones. The disease can spread to kill branches and eventually the whole plant. Many plant varieties are resistant to anthracnose fungi — choose them whenever you can.

  • 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, 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.

  • 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.

  • Club root: This fungus mainly infects 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. Some vegetable varieties are immune.

  • Cytospora canker: Cankers appear as oozing, sunken, or swollen areas on tree bark. The new shoots turn yellow and wilt, then die back. The disease attacks woody stems on fruit trees, spruces, and maples, forming cankers that can kill infected branches. Plant resistant or less-susceptible plants, keep them growing vigorously, and avoid bark injuries that provide an entrance for infecting fungus.

  • Damping off: Mostly a problem in young plants and seedlings, this fungus rots stems off near the soil line, causing the plant to keel over and die. Avoid overwatering and provide good air circulation to help prevent the fungus. Clean your tools in isopropyl alcohol.

  • 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. In melons, the stems develop a yellow streak, which eventually turns brown. You can’t cure infected plants, so choose Fusarium-resistant varieties.

  • Galls: These appear as swollen bumps on leaves, stems, and branches. Bacteria, fungi, gall wasps, aphids, and mites can all be culprits. Usually the damage is only cosmetic.

  • Leaf spots and blights: Several fungi show up first as circular spots on leaves of susceptible plants. The spots increase in size until the leaves die and fall off. The fungi spread easily in wet conditions. Remove all plant debris at the end of the gardening season, clean tools between uses, buy disease-resistant varieties, and avoid contact with wet plants.

  • Root rots: A number of fungal root diseases cause susceptible plants to turn yellow, wilt, and sometimes die. Nearly all plants are susceptible under the right conditions. The fungi can survive in the soil for many years without a host. Build healthy, well-drained soil to prevent root rot.

  • Slime flux: This bacterial rot inside infected trees, usually elms, maples, and poplars, causes oozing and often bad-smelling sap to run from old wounds or pruning cuts. There’s no control after the symptoms appear.

  • Verticillium wilt: This fungus affects many plants. 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 garden debris, cleaning tools thoroughly, and choosing resistant varieties.

  • Viruses: This group of diseases is incurable, so prevention is your only strategy. 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. Viruses often live in wild bramble plants and weeds; aphids, leafhoppers, nematodes, and whiteflies spread the virus as they move from plant to plant.

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