Canker Stain of Planetrees

by JAMES M. WALTER

SINCE its introduction from England about 1900, the London planetree has become one of the most important shade trees of the United States. It has been planted by thousands in the East and Midwest, in a region that includes New York City, Pittsburgh, St. Louis, and Washington, D. C. By 1939 Philadelphia had planted 153,000 of the trees, valued at about 6 million dollars, and Pittsburgh 75,000, valued at 3 million dollars. The London plane (Platanus acerifolia Willd.) is a hybrid between American sycamore (P. occidentalis L.) and Oriental plane (P. orientalis L.). It grows rapidly, is easily propagated, adapts itself to a wide variety of soils, resists smoke and fumes, and withstands the anthracnose disease that often makes the native planetree unsightly. Those virtues explain its meteoric rise to popularity.

But its utility came in question in 1933 when arborists and shade-tree enthusiasts of the western suburbs of Philadelphia recognized that their trees were dying by the score despite good care. Early pathological studies by Dr. L. W. R. Jackson disclosed that the trees were being killed by a previously unknown fungus of the genus Endoconidiophora. Recently we have learned that the fungus occurs occasionally on the native plane-tree, or sycamore, in remote stands in the Appalachians. It seems likely that the organism is native to North America, although it was not recognized until it became a pest among planted trees. Indications are that London plane is more susceptible than American sycamore, but the disease has caused great losses in some plantings of the latter species.

The disease is characterized by blackened, elongate cankers having rather irregular annual zones .5 inch to 2 inches wide. The cankers occur Most commonly on trunks, less frequently on branches, and occasionally On major roots. The most distinctive symptom, however, is the bluish-black or reddish-brown discoloration distributed in the wood in radial patterns beneath the cankers. The distinctiveness of these symptoms allows relatively early diagnosis of the disease and favors its control.

By 1939, an estimated 8,000 of the London planes in Philadelphia had succumbed to the disease; losses were known to be proportionately higher in some of the suburbs; more and more were dying in Baltimore and Washington, and the disease was being reported from new localities with alarming frequency. Congress appropriated $10,000 for an investigation, and organized studies were begun at the Forest Pathology Field Laboratory in Morristown, N. J.

Investigations made during the ensuing 5 years showed that canker stain can be brought under control by simple, inexpensive, and practicable measures. That statement may sound mild enough to the layman, but to the biologist it must smack of dogmatism, and to the student of tree diseases it must be astounding.

Canker stain is unique in that its spread is almost entirely accountable to man, the causal fungus hardly being able to cross a street without the aid of man—or his boys. An inkling of this truth came from early observations on the distribution of cases of the disease. It was noted, for example, that the disease had claimed every tree in the row along a street where the trees had received the best possible care, including regular pruning, while in a row a few feet away, but inside a good fence, the trees had received no attention and none had developed the disease. The problem was tested in every way that we could devise and the answer was always the same—the disease was of no consequence among trees left undisturbed by man. Moreover, tests of dissemination of the fungus by wind and wind-driven rain, common carriers of many disease-producing fungi, resulted in no cases of infection more than 25 feet from the source.

The fungus is spread readily in pruning operations and by other mechanical means. Pruning tools are exceeded in efficiency as transmitters, however, by the commonly used forms of asphalt wound dressing. Wound dressing, besides being a carrier, evidently protects the fungus against weathering, its use in winter resulting in infections at wounds, made with contaminated tools, that would not otherwise become infected.

One of the most important facts concerning the disease is that the fungus is noninfectious in the latitudes of New York and Philadelphia for about three winter months. Between December 1 and February 15, approximately, it is safe to prune, provided the wounds are left bare.

Cooperative tests of local control by sanitary practices were begun in 1940 in two park areas and in two plantings along streets in residential districts. Tree wardens also were encouraged to take precautions against spread of the pest in pruning and to remove dead and dying trees. New information from laboratory and nursery experiments was reported to the tree wardens as it became available, and was taken into account in the tests of local control. Although this work was hampered by changes due to the war, the application of sanitary practices and precautions against further transmission of the fungus quickly checked the losses caused by canker stain in several municipalities.