Measures necessary to control canker stain in localities where it is already established (with brief explanations of important points not emphasized in the foregoing paragraphs) are:

Avoid all unnecessary mutilation of planetrees.

The causal fungus does not enter through undamaged bark, but with enough moisture it enters through the slightest injury.

Remove all diseased trees or parts of trees to eliminate sources from which the pathogen may be transmitted to healthy trees.

Once the infection has developed in the trunk, the tree is doomed; but it usually takes 3 years or longer for the tree to die. Early recognition and prompt removal of infected trees from city streets is important, because the fungus is so readily transmitted by mechanical means. Boys using their knives and hand axes have accounted for many cases of spread. Lawn mowers that were allowed to injure buttress roots have accounted for infections in rows of fine, large trees. Moving vans that scraped trunks and lower branches have accounted for several known cases and are no doubt a hazard for spreading the fungus from city to city.

Some trees having infections definitely limited to branches may be saved by careful pruning. A clearance of 3 feet between the pruning cut and the end of a branch canker nearest the trunk is usually necessary for success in removal of all infected wood, and the surgery must be carefully done to avoid inoculation of the tree at other points.

Disinfect pruning tools, and all other equipment that might have come in contact with an infected tree, before using them in a healthy planetree between February, 15 and December 1.

Saws, other cutting tools, soles of climber's shoes, and ladder parts that must make contact with the tree can be disinfected easily by dipping or thoroughly swabbing them in denatured alcohol of the type commonly used as antifreeze.

It is desirable to avoid the contamination of climbing ropes as far as possible. Ropes may be disinfected by exposing them to vapor from commercial formaldehyde for 3 hours. Effective fumigation of rope is attained with one-fourth pound of formaldehyde spread over about 100 square inches below a false bottom in a tightly capped 10-gallon container. Formaldehyde is poisonous and must be handled with caution.

If wound dressing is necessary, use a gilsonite varnish (type covered by Federal Specification TT–V-51) in which phenylmercury nitrate has been mixed in a 0.2-percent concentration.

When fortified with this powerful germicide, the paint cannot carry the fungus. The commonly used forms of asphalt paints undoubtedly have been the most damaging and insidious means of transmitting the fungus. The paint container, as handled by the arborist, collects sawdust and fragments of bark and wood, much of it carried in by adhesion to the brush. Paint contaminated with such matter from diseased planetrees is highly infectious for a long period, and has regularly given infection at 25 to 50 percent of experimental wounds treated during the winter period when there is no danger of infection at wounds left without paint.

Phenylmercury nitrate is not recommended for use with paints based on petroleum-residue asphalt, because that combination may be too injurious to the cambium. Further, use of paint fortified with phenyl-mercury nitrate will not eliminate the need to disinfect pruning equipment, the tests showing that, despite prompt and careful application, it is not thoroughly effective in preventing infection at wounds made with contaminated tools.

Phenylmercury nitrate is a highly toxic chemical and must be handled with caution. However, a careful workman can safely mix it with the asphalt varnish by first mulling the fine powder in a little linseed oil and then thoroughly stirring it into the varnish. Some people are sensitive to mercurials carried in oils; therefore, anyone using paint fortified with phenylmercury nitrate in this concentration should handle it cautiously and should immediately remove paint spots from his skin.

Restrict pruning of planetrees, as far as possible, to the period from December 1 to February 15.

As previously explained, in the northern half of the range of the disease the chances of spreading it by pruning in midwinter are negligible if no paint is used. All tests on this question have been conducted in the northern part of the involved area, and it is not assured that the results will apply in the South. If paint is used, it is necessary that the pruning tools be disinfected and that the paint be free of contamination or be fortified with phenylmercury nitrate.

Measures necessary to prevent the introduction of canker stain into localities where it is not yet established are :

Require all equipment to be disinfected thoroughly before work is begun on planetrees.

Require that any wound dressing used on planetrees be fortified with phenylmercury nitrate or be from an unopened container and used with new equipment.

THE AUTHOR

James M. Walter served as pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering in the Forest Pathology Field Office at Morristown, N. J. He joined the Department in 1933 and resigned in 1946. His research has included the relations of the corn smut fungus to its host, Dutch elm disease and what causes it, and canker stain of planetrees.