Shrubs in your landscape are susceptible to many different kinds of diseases. Determining the exact disease infecting your shrub can be difficult. Many of the presenting symptoms of disease, such as yellowing leaves and stunted plant growth, are common to several different diseases. To help you identify the disease impacting your shrub, take samples of the impacted leaves or stems to your local extension office.
Verticillium wilt is a fungus that attacks the roots shrubs first and then moves upward into the branches and leaves. You may notice a wilting or yellowing in the leaves, sometimes followed by a defoliation of the plant. Often there will also be a discoloration on and inside the wood. Verticillium wilt can cause quick plant death, but more often it takes several years to cause death. If a plant dies quickly, remove and destroy the plant. Otherwise, cut out infected pieces of wood, mulch at the base, and make certain the shrub is receiving adequate water. A fungicide may be applied.
Nematodes are a roundworm that feeds on the roots of your shrub. They can be one of the most destructive pets in a landscaped garden. Unfortunately, you may not know that nematodes are the cause of your shrub's symptoms until it is too late. Nematodes can cause yellowing of the leaves and stunted growth in your shrub. Adjusting your plants water levels and applying fertilizer may have little effect. A soil test may be needed to diagnosis nematodes as the cause of your disease.
Botrytis is also called gray mold. Botrytis is a fungus that thrives in wet conditions. It attacks leaves, new leaf and flower buds, shrub stems and fruit. Botrytis is typically the most visible on shrub leaves, starting in the middle of a leaf as a brown or discolored spot. Another good indication of botrytis is a release of gray spores when you disturb the plant's leaves. Fungicides are rarely effective. Instead, practice good hygiene. Keep shrubs dry. When pruning diseased parts of the shrub disinfect your pruners often with a light rubbing alcohol solution. Destroy the diseased debris you remove.
Powdery mildew is a common fungus infecting many types of plants. Look for a white discoloration on your shrub's leaves. Powdery mildew is more prevalent in warm, dry climates. High humidity increases spore reproduction. To control powdery mildew, remove and destroy all diseased parts of the plant. If you use an overhead watering system, switch to drip irrigation to keep plant leaves drier. Fungicide can also be applied.
Galls are a fungus that form on a shrub's stems. They are round and range in diameter from pea-sized to over 1 inch. Galls can live for years. The best method for controlling galls is to cut out the infected parts of the plants.