Mahonia aquifolium, or Oregon grapeholly, is a small evergreen that grows in mounds and produces faintly fragrant yellow flowers in the spring. Its foliage is leathery and green in summer, resembling holly leaves. They become purple as the weather cools. The plant grows in shade or sun, spreading no more than 6 feet tall and 6 feet wide. The mahonia is also susceptible to several fungal diseases, insects and other garden problems.
Gray mould isn't usually deadly to the mahonia, but it causes blights on branches, buds, flowers, fruit and foliage. It also returns annually to inflict the same damage, since the fungus that causes the disease, Botrytis, is able to survive winters. Wind and water spread the spores, passing the infection to healthy plant tissue. The best strategy to prevent the disease is to keep the plant dry and under its ideal conditions for growth, as this fungus thrives in humid environments and attacks already weakened shrubs.
You'll find many armoured scales---the University of California lists 12, one of which, the greedy scale, might use the mahonia as a host plant. Armoured scales' bodies have a protective shell about 1/8 inch in diameter. Since male scales only live a few hours and don't feed, the injury to the mahonia is caused only by the female of the species, which have the ability to lock onto the plant to suck its fluids. Infested plants wilt, turn yellow and acquire blemishes and deformities that affect their fruit, leaves and stems. Natural predators usually control scale population in gardens, but sometimes they're driven away by pesticide use and other animals that prey on them. In that case, insecticides become a solution to an armoured scale attack.
Rusts are fungal infections that appear as red spores on the mahonia's leaves. As the disease develops, the foliage might die and drop. Sometimes, the plant also develops tissue swellings and bark cankers. A severe infection has the power to kill an entire plant. Water and wind broadcast the spores, which thrive in humidity and warmth, to healthy parts of the plants. Soaker hoses may help reduce the risk of infection, because the plants stay dry while the soil is irrigated.
Weeds, such as broadleaves, sedges, and grasses that sprout close to mahonias compete with them for water and nutrients. They also host insects that may attack the mahonias. The ideal weed control program begins before planting when debris is removed from the planting site. Landscape fabric applied before the mahonia is planted also chokes any weeds that sprout. But if the mahonia is already established, keeping a mulch layer around its base also reduces weed invasion.
- University of Illinois Extension: Oregon Grapeholly
- University of Illinois Extension: Gray Mold
- University of California Integrated Pest Management Program: Mahonia, Barberry, Oregon grape--- Mahonia, Berberis spp.
- University of California Integrated Pest Management Program: Scales
- University of California Integrated Pest Management Program: Rusts