Birch trees, members of the genus Betula, are generally prized as landscape plants for their attractive bark and a variety of forms to choose from. Proper cultural practices, especially providing adequate water to fulfil the birch's high moisture requirements, are key to maintaining birch health. An unhealthy birch is much more susceptible to pests or diseases that can cause brown leaves. Properly identifying the specific cause of brown leaves is necessary to determine the most suitable treatment.
Leafminers are a common insect pest of birch trees. Although leafminers do not kill birches, they affect the tree's aesthetics, turning the leaves brown, and weaken the tree's ability to withstand an attack by bronze birch borers or survive extreme environmental conditions. Control leafminers with an insecticide to prevent further browning and alleviate stress. Insecticides can be applied as a foliar spray or soil application. The first and most critical application should be performed in late spring. Additional applications may or may not be necessary to treat subsequent populations.
Bronze Birch Borer
Unlike the birch leafminer, a birch borer infestation can kill birch trees. The birch borer, a small beetle, feeds on the tree's phloem tissue, leaving a characteristic "D"-shaped hole. This feeding eventually starves the roots, which, in turn reduces water uptake and causes dieback. The earliest dieback occurs on some branches at the top of the tree, where leaves turn yellow and brown. Heavily infested trees do likely not survive even with treatments. Healthy trees don't have problems with bronze birch borer; so, to ensure that the site is suitable for birch, select a variety that is less susceptible to the beetle attack and use good cultural practices. If the tree is not heavily infested, begin a regular deep-watering routine and apply an insecticide to prevent additional attacks.
If the browning on the birch leaves is in the form of spots that may or may not grow and coalesce to give leaves a more complete brown appearance, the tree may be affected by a fungal disease. Several different fungi attack birches. These fungi include Cylindrosporium, Gloeosporium, Gnomonia, Marssonina, Phyllosticta, Physalospore, Septoria and Venturia. Fungal leaf spots are usually not harmful, but, if they appear early in the season (or in successive years), apply the appropriate fungicide.
Other Possible Problems
Several additional problems could cause brown leaves, among other symptoms. If the brown leaves are not the result of leaf miners, bronze birch borer or a fungal infection, the culprit could be any of the following: birch canker, aphids, herbicide damage, girdling and inadequate watering. Maintaining healthy trees by using proper cultural practices is the best preventive measure.