Pest Insects do not bother bog rosemary.
Plant Fails To Thrive means Wrong Location.
As indicated above, these plants have specific environmental requirements. They must have moist, acidic soil, cool summers and good sun. Under these conditions they thrive. If their needs in any one of this respects are not met, they will not do well.
Leaves Coated With White Powder indicates Powdery Mildew.
Powdery mildew, caused by a fungus, sometimes develops on the upper surfaces of bog rosemary leaves and appears as whitish blotches. In late summer, bushes in shadier spots may become infected. This fungus is rarely serious and can safely be ignored if the plant is otherwise healthy. If the infection is serious or unacceptably unsightly in the landscape, spray foliage thoroughly with wettable sulfur once or twice at weekly intervals, starting as soon as the whitish coating of the fungus is visible. Collect and discard all plant refuse in the fall. For more information see file on Controlling Fungal Disease.
Leaves Turn Yellow, Drop Prematurely due to Leaf Spot.
Leaf spot diseases caused by fungi occasionally attack bog rosemary. Leaves of affected shrubs turn straw yellow or brown and are thickly dotted with small black fruiting bodies. These diseases mostly attack foliage already weakened by some other cause. Control leaf spot by shaking out all fallen and diseased leaves from the center of the bush and destroying them, and by pruning out and destroying dead branches in the center of the bush to allow better aeration. Apply copper fungicide before growth starts in the spring. This spray may discolor foliage, but the effect is soon hidden by new growth. Enrich the soil. For more information see file on Controlling Fungal Disease.
Rust Colored Spots on Leaves indicates Rust.
Rusts are caused by various fungi that attack bog rosemary leaves and stems. Rust infections usually appear as numerous rust-colored, orange, yellow or white, powdery, raised localized spots on leaves. Infected leaves wilt and wither and the plants may be stunted. Remove infected leaves as soon as possible. Remove and destroy seriously diseased plants and all debris before growth starts in the spring to prevent the spread of the fungus. See file on Controlling Fungal Disease