Gardening Advice Oak wilt is caused by a fungus carried by sap- feeding beetles and certain other insects that visit wounds in bark. Dead woody twigs and branches, generally in the top of the canopy, are the symptom of dieback in trees and shrubs. Dieback may be the result of decline, canker fungi or winter injury. Root Rots Gardening Advice Thielaviopsis and other soil-inhabiting fungi frequently cause serious root rots, particularly on herbaceous ornamental plants. It is generally found at the soil line or on roots and usually has a rough surface.
Crown galls differ from natural burls.
The spores of needlecast diseases of conifers infect needles when the humidity is high and the leaves are wet. The most severe infections usually occur on the bottom half of the tree and on the shaded northern side. The fungi produce spores in pustules that break through the plant epidermis. The spores are generally reddish brown, orange red to orange, and dry and dusty. Mildew Gardening Advice Powdery mildews are the only fungal leaf disease in which the fungus grows on the surface of the plant.