Red Japanese Maple trees, known botanically as Acer palamtum, are deciduous trees grown as ornamentals and prized for their foliage. They are a smaller understory or specimen tree and most varieties range between 6 feet and 30 feet in height at maturity. The growth habit, canopy shape and leaf color vary widely by cultivar contributing to the popularity of the species. They are known to be fairly disease- and pest-resistant but they are subject to a few common diseases brought about or exacerbated by environmental stress or poor cultivation practices.
Leaf scorch is brought on by periods of high heat, drought stress and/or wind. It can also be caused by an ailing or inadequate root system that is not capable of mining and transferring sufficient moisture and nutrients up from the soil. The Japanese red maple will respond to these conditions by the leaves turning brown and desiccating in the field areas between the veins. Scorch can also cause brown or dead patches on young branch tissues, leaf stems and even young trunk tissue.
When the Japanese red maple does not have access to certain trace nutrients in the soil, most commonly Manganese, chlorosis can result causing the leaves to become discolored. The leaves will lose their normal field color turning yellow or pale green and the veins will turn a dark green. On occasion the trace nutrients exist in the soil but the alkalinity or acidity pH of the soil is blocking uptake. Both conditions can be treated with soil amendments.
Tar spot is a fungal infection caused by fungi in the genus Rhytisma that infect the topside of Japanese maple leaves. Not life threatening and rarely requiring intervention, the black spots can however make the tree look unsightly and cause infected leaves to drop. The spots start out yellow in the early summer and age to a black textured spot that looks like a drop of tar by late summer. Rake infected leaves regularly and destroy or discard them, but do not compost them.