Incidence and Management of Canker Stain in London Plane Trees in Modesto, California

Canker stain, a disease caused by the fungus Ceratoycystis fimbriata f. platani has killed thousands of London plane trees (Plantanus X aceńfolia) in northeastern United States since about 1926 (4). The disease has long been known to destroy plane trees in southern United States as well (3). More recently, canker stain was identified as a serious threat to American sycamores (P. occidentalis) in the Midsouth (6).

In California the disease appears to be confined to Modesto, a community in the San Joaquin Valley. Canker stain was detected by us in 1961 from 35-to 45-year-old London plane trees growing as city street plantings in Modesto. The fungus, C. fimbriata, was pathogenic to London plane and California sycamore, P. racemosa.Since then, the disease has spread to 28 areas in parks and along streets and has destroyed some 600 mature trees.

Both London plane and American sycamore are grown in Modesto, and both have been equally affected. There is no obvious resistance to the disease by American sycamore as has been suggested (7). Symptoms are typical for the disease: trees show sparse foliage and undersized leaves (5); infected trees are often killed in a single summer; however, declining trees in Modesto do not develop trunk and limb cankers as often as do trees in the East (5).

Plane trees in Modesto street plantings are killed in rows as reported for other areas (4), suggesting that root grafts are involved in the spread of the disease. It has been shown that canker stain may spread from tree to tree through root grafts (1). Nitidulid beetles, which are common in the Modesto area, have also been implicated in the spread of the disease (7).

Canker stain also is spread mechanically, and City of Modesto tree crews thoroughly disinfect pruning tools with Lysol Aerosol Spray Disinfectant (contains o-phenyl phenol 0.10%, N-alkyl (C₁₈ 92%, C₁₆ 8%)-N-ethyl morpholinium ethylsulfates 0.03%, alcohol 67.75%, inert 32.2%) after use on each plane tree, regardless of whether the trees are diseased. Pruning and other maintenance of street and park trees is done almost entirely by city crews, limiting mechanical spread of the disease. Private tree crews under contract to prune trees beneath utility lines are also advised to disinfect pruning tools. However, trees in landscaped areas may be injured and possibly infected by lawnmowers and other tools used around the trees by homeowners or, more seriously, by professional gardeners who service a number of yards throughout the city. Also, construction crews repairing sidewalks, curbs and utilities near trees may cause injuries that result in infection.

Despite the City of Modesto’s efforts in containing canker stain by means of sanitation and rapid removal of infected trees, the disease continues to spread at an alarming rate. Because of the serious threat to the entire plane tree population in Modesto, an overall management strategy is being considered. This strategy includes chemical and cultural control techniques.

• © 1988, International Society of Arboriculture. All rights reserved.