Although the most common health problem with hemlock (Tsuga canadensis and T. heterophylla) is the Japanese woolly adelgid insect, hemlocks are also susceptible to root diseases. The most common cause are the root fungus, annosus (Heterobasidion annosum), and Phytophthora fungus. 

Annosus root disease

Although the western hemlock (H. heterophylla) is more susceptible to this fungus than the eastern hemlock (T. canadensis) the latter is also susceptible. Two species of fungus, Heterobasidion annosum and Formes annosus, cause this disease. Symptoms of this disease include loose tree bark that can easily be pulled away. Another sign of infection is the presence of fruiting bodies (conks) inside old stumps or on the surface of roots. Annosus root disease can be minimized by keeping trees healthy and avoiding damage by gardening equipment. The fungus can spread from freshly-cut tree stumps. This can be prevented by sprinkling borax on the cut surface of the stump.

Phytophthora root rot

Several fungus species cause this disease in hemlocks - Phytophthora cinnamomi, P. cryptogea and P. dreschleri.  Symptoms include chlorosis, stunting, wilting, browning of the needles and stains on the trunk. Phytophthora root rot is usually found only on trees grown in areas with poor drainage. Improving the drainage will help to prevent this problem. Fungicides can also be used. They do not cure infected trees but can be used as a preventive measure. 

Courtesy of NYBG Plant Information Service