What Are the Plant Problems for the Mahonia?

Mahonia plants are evergreen shrubs with stiff, leathery leaves that resemble holly leaves. They have fragrant yellow flowers in late winter to early spring, followed by red, blue or purplish-black berries. hinese Mahonia (Mahonia fortunei) has bluish-green foliage and grows 3 to 5 feet wide and tall. Japanese Mahonia (M. japonica) grows 6 to 10 feet tall and Oregon Grapeholly (M. aquafolium) grows 3 to 7 feet tall. Both have dark green leaves.

Environmental Stresses

Mahonia plants grow best in partial shade to full shade and rich, well-drained acidic soil. They are subject to heat stress and leaf burn in full sun. Mahonia plants do not tolerate soils that are alkaline, compacted, wet or clay-based. They are susceptible to winter burn in open, exposed areas.

Diseases

Mahonia rust and other fungal diseases cause leaf spots. Yellow, orange or reddish masses of spores form on the underside of the Mahonia leaves. The upper side of the leaves turns yellow or brown, and may fall off prematurely.

Gray mold caused by the fungus Botrytis, forms a gray fuzzy mass on affected areas. The mold causes blight of stems, leaves, buds, flowers and fruit. Another fungi that grows in dead wood causes wood rot, which is characterized by stemmed or shelf mushrooms. Some affected plants do not show any symptoms.

Pests

Mealybugs and deer brush whiteflies are sap-sucking insects that secrete honeydew. Sooty mold is a fungus that grows in the honeydew. Greenhouse thrips are also sap-sucking insects. Affected leaves appear bleached and have black specks of excrement on the undersides. Finally, bayberry loopers are caterpillars that feed on the leaves.

Mahonia Problems

Mahonia leaves' spiny edges offer no protection against a group of leaf-feeding insects. This honeydew drenches leaves and branches, luring sooty molds that cover the shrubs with velvety, black fungal mats. The night-feeding larvae hatch within four days, feed for two weeks -- leaving nothing but leaf skeletons in their wake -- and drop to the soil to pupate. Rust fungi, including Cumminsiella mirabilissima and Puccinia graminis, target Oregon grape and leatherleaf mahonias (Mahonia bealei). As the disease progresses, brown pustules, called aecia, appear on the undersides of mature leaves. Severely rusted mahonia foliage may distort, pucker and drop prematurely. Spraying with an antitranspirant to limit leaf-moisture loss when conditions favor scorching minimizes the damage. The condition surfaces with small, green-veined, yellow new growth that may develop dead spots as it dries and falls.