Japanese holly (Ilex crenata) is a durable, easy-care evergreen shrub that can find a place in any landscape as a round specimen shrub, hedge or as a ground cover in dwarf varieties. Japanese holly grows well in full sun or light shade in U.S. Department of Agriculture plant hardiness zones 6 to 9 and thrives in a variety of soil types. While not too much bothers, Japanese holly it is prone to several diseases such as black root rot, web blight and botryosphaeria canker.
Examine Japanese holly leaves for small yellow spots that grow into large, round circles with brown centers and dark purple borders. These are symptoms of cylindrocladium leaf spot, a disease common to Japanese holly. Cylindrocladium leaf spot causes Japanese holly to lose its leaves and, in severe cases, it results in a shoot dieback, which is the progressive death of twigs, branches and shoots.
Look for circular and non-uniform tan to brown blotches on Japanese holly leaves, as well as small, pink-orange spore masses. These are signs of anthracnose, which is a fungal disease.
Examine the base of Japanese holly leaves for brown spots that quickly grow into irregularly-shaped brown or black blotches. This disease, known as web blight, often plagues Japanese holly. Warm, humid weather is necessary for web blight to spread, and it may encompass the entire leaf
Look for premature yellowing of Japanese holly leaves, cracked and discolored bark, shoot dieback and cankers, which are open wounds on the stem and trunk. If you find one or more of these symptoms, it may be botryosphaeria canker, also known as bot canker. Bot canker often develops on unhealthy Japanese holly: holly that has recently been exposed to high temperatures, low temperatures or severe drought.
Study young Japanese holly twigs for localized swelling, which looks as though part of the twig is "ballooning" outward, and examine larger branches for knobby galls, which are dark, hard, lumpy growths. These are symptoms of shaeropsis knot, which often causes stunted and leafless shoots, prematurely yellow leaves and a branch dieback.
Watch Japanese holly for yellowing of the leaves followed by slow plant growth, early leaf drop and twig dieback. If you notice these symptoms together, your Japanese holly may have black root rot. Examine the Japanese holly's feeder roots, which are normally small, soft and white, for signs of black bands or rings. Black root rot eventually causes the root system to darken and die.