Shrubs can suffer from a number of diseases, most fungal or bacterial in origin. In most cases, the diseases only affect the ornamental value of the shrub and are not life-threatening. In addition, you can usually prevent the diseases by following proper cultural practices, such as watering from the base of the plant to avoid wetting the shrub's leaves.
"Leaf spot" is the term used to describe a wide number of fungal infections that can affect the leaves of a shrub. The fungi spreads through water, through rain or aboveground irrigation. The disease differs in appearance, depending on the specific fungus that causes the spots. In some causes, the spots are tiny and appear either orange, yellow or brown. In other cases, the spots are larger, appearing more as lesions with a lighter-colored border or numerous red/brown lesions with light centers and dark margins. The spots may spread, overtaking the leaf and causing parts of it to shrivel up, or the spots may drop out of the leaf, giving it a buckshot appearance. Rarely, infected leaves may drop from the tree.
Chemical control cannot stop the disease, but you can pluck off infected leaves to prevent it from spreading. Rake up any dropped leaves in the fall to prevent the fungi from overwintering in the soil. An application of fungicide in the spring can help prevent leaf spot from developing.
Phytophthora Root Rot
Phytophthora root rot results from a fungus that lives in the soil. This fungus can lie dormant for a long time until it awakens and attacks the roots of the shrub. Phytophthora root rot is serious and difficult to control, according to R.K. Jones, a plant pathologist with North Carolina State University. As the roots die, they lose their ability to intake nutrients, causing the plant to decline. New growth will be deformed or yellow, or it will drop from the shrub before it can fully develop.
Chemicals are usually affective once you notice symptoms of the disease. You can also prevent this condition by planting vigorous, healthy plants in locations with soil that drains well and has not had any sick shrubs previously growing in it. Raised beds can help prevent this disease, and some shrubs, such as the "Caroline" azalea, have been cultivated to resist it.
Powdery mildew is a fungal disease. The fungus spreads through water, usually rain blown about by the wind, which lands on the leaves of the shrub. If the weather is warm enough, the spores of the fungus grow and spread, coating the leaves and twigs with a fine, gray powdery substance. This causes the leaves to turn yellow and drop prematurely. The disease can also stunt new growth, according to R.K. Jones, a plant pathologist with North Carolina State University.
You can prevent and treat powdery mildew by applying a general fungicide. Treat the shrub in the spring, before warm rains begin, to prevent the disease. Euonymus, a popular ornamental shrub, is highly susceptible to this disease; ado not plant it if powdery mildew is an ongoing problem in your area.