Cause The absence of disease organisms, insects, or mites associated with this condition suggests the problem is due to stress of some sort. The exact source of stress that results in black flagging is unknown. Water stress, drying winds, high temperatures, injury to the roots, or damage to lower stems are possibilities. In some instances, black flagging has occurred when none of these conditions have been present.
This problem has been widely observed on arborvitae from all regions in Oregon, as well as from Washington, Idaho, and numerous other states across the country. Repeated incubations and isolations from affected tissue onto nutrient media, by multiple diagnosticians across the country, have consistently failed to produce any disease causing microorganisms. The injury is not caused by insects mining in the foliage, nor is it due to girdling twig cankers. Sprays of insecticides and fungicides have no effect on reducing incidence of the discoloration.
Occasionally, the injury can be traced to chemical applications, which will be reflected in the spray pattern. For example, if the trees are growing as a hedge on the border between two properties, and only one side sprayed, then the injury will appear on only the sprayed side.
Symptoms Only younger foliage is affected. Branch tips show a very dark-brown to black discoloration of the foliage-the color is not the brown associated with normal or disease-induced leaf senescence. This can occur on only one side, be evenly distributed over the entire plant, or occur only on some branches. Plants with black flagging are not permanently disfigured or injured. The condition does not always recur on an individual plant.
Cultural control Since this is not a disease or insect/mite problem, no pesticide applications are recommended.
- Prune the affected foliage to remove the unsightly tissue. Pruning allows more light in and stimulates growth of new foliage.
- Water regularly to the base of the plant during dry periods.
- Fertilize in spring to stimulate production of new foliage.