- Fungi attack many different plants and can have as many causes and symptoms as the number of plants they attack. Fungal diseases can present themselves as leaf spots, mildews or molds; they may also appear as visible fungi such as mushrooms growing on or near your shrubs. Many fungal attacks occur due to poor cultural management. Powdery mildew is one such example. This fungal disease, along with others, occurs when your plant is getting too much moisture and not enough air circulation. Gray mold, Botrytis cinerea, is a fungal disease that attacks woody and herbaceous shrubs and plants. Overhead watering and poor air circulation are also culprits in the cause of this fungal disease.
- As with fungi, various types of bacteria will attack your shrubs due to a number of cultural issues or other problems. Bacteria, though, typically need some other injury to get into the plant and cause issues. Common injuries bacteria enter through include damaged areas from insects, natural openings or another disease. Canker and blight are two common bacterial infections of plants. Canker often affects branches but may also attack leaves and buds. Blight often attacks leaves and causes unsightly reddened, yellowed or otherwise off-colored leaves.
- While insects themselves are not a disease, many insects and other pests can cause similar symptoms in your shrubs. Many insects and other pests feed on various parts of your shrub; leaves, trunks, branches, buds or roots can fall victim to their voracious appetites. Most of these pests cause only aesthetic damage, which can be removed; others may slowly kill your shrub without you realizing it until it's too late. Root nematodes, for example, feed on the roots, and symptoms of the pest may include yellowing of the leaves or wilting. Many beetles and caterpillars simply feed on the foliage and cause an unsightly mess of your shrub, but rarely damage the plant's overall health.
- It is often difficult for an everyday gardener to correctly diagnose the issue with the shrub. In some cases it's obvious; some diseases have identifying symptoms that are relatively obvious, and seeing insects or chewed holes in your foliage is often unmistakable. For fungal diseases, various natural and chemical fungicides can prevent and manage the problem. You can also often remove diseased portions of your plant to prevent the disease from spreading. Bacterial diseases are sometimes harder to diagnose and treat; often the damaged portions of the plant --- or the entire plant itself --- need to be destroyed to prevent the disease from spreading. Insects are among the simplest of issues to diagnose, but not always the simplest to treat. If the pest you have causes only aesthetic damage, hand-picking the pests or using natural repellents can combat the issue. A word of caution: All insecticides are toxic, and many will harm the beneficials --- wasps, honeybees, butterflies, predators --- around your plants. Correct cultural practices can effectively combat a variety of diseases. Make sure your plant is getting the appropriate amount of sunlight, water, air circulation and nutrients it needs. Healthy plants are less susceptible to all types of diseases.
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