Over 200 shrub species make up the Magnolia genus. All share common characteristics, including fragrant and colorful blossoms that make this shrub a prized addition to many backyards and landscapes. Even a well-maintained shrub can be afflicted by one of several common Magnolia shrub diseases. Immediate action is required to keep the disease from killing the Magnolia or spreading to nearby shrubs and plants.
Pseudomonas syringae is the culprit behind bacterial blight. Symptoms include black or brown spots on the Magnolia shrub's foliage and flowers. The problem often arises during extended periods of wet weather. No garden spray or chemical will sufficiently treat this disease. The problem usually rectifies itself once the excessive moisture is removed. Gardeners can help by cutting back surrounding shrubs or trees to increase air movement. Water should always be applied to the shrub's base rather than its leaves. For diseased plants, individuals should trim off foliage and branches that are seriously afflicted.
Powdery mildew, a fungus-based disease, can cover the Magnolia shrub's stems and leaves. Immediate treatment can help ward off foliage loss. All fallen leaves should be collected and discarded to keep the spores from spreading. Sprays formulated with neem oil or sulfur can effectively control the growths and should be sprayed as soon as white spots begin appearing on the shrub.
The Agrobacterium tumefaciens bacteria creates growths on the woody stems of the Magnolia shrub. It doesn't pose a significant health hazard to the plant unless the plant is very young. Gardeners can use a knife to cut off the gall, allowing the diseased wood to dry out.