Pierluigi Bonello and Maria Bellizzi, Department of Plant Pathology, The Ohio State University, Columbus, Ohio; and Harry A. J. Hoitink, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, Ohio.
In 2002, we received an additional report in midsummer of a similar condition occurring on some urban honeylocusts in the Cleveland area. Throughout this past summer, more reports began coming in from adjoining states. There are now records of the disease from nurseries and landscapes in Ohio, Illinois, Iowa, Kentucky, Maryland, and Michigan.
Initial symptoms observed in nurseries include the development of shepherd's crooks on terminal shoots (Figure 1) and wilt-like leaflet cupping (Figure 2). The condition then progresses to yellowing of the canopy, defoliation, and death of the entire tree.
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| Figure 1. A typical shepherd's crook on a shoot tip. | Figure 2. "Cupping" of leaflets, a wilt-like symptom. |
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| Figure 3. Dieback symptoms. |
Concurrent with the appearance of the shepherd's crooks and wilting symptoms, the terminal shoots often acquire a burnt-like appearance reminiscent of fire blight, except that the shoots defoliate quickly, giving a dieback-like symptom (Figure 3).
The most characteristic symptom of the disease, and the one that suggested the current name, is the appearance of knots, ranging in size from about 5 mm to more than 5 cm in diameter (1/4" to more than 2") on the nodes of twigs and branches. These always appear first as swellings at the base of branches on the main stem, where they can reach a considerable size.
In the early stages and on smaller branches, these galls often appear as two swellings at the nodes, one on each side of the lateral twig or compound leaf (Figure 4). In advanced stages, these swellings can become quite irregularly shaped and deformed (Figure 5).
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| Figure 4. A characteristic double knot on a young shoot near the petiole of a compound leaf. | Figure 5. Advanced stage of coalescing knots resulting in gross deformity near the shoot tip. Note the burnt-like appearance of the tip. |
Occasionally, the swelled area continues down the stem or trunk of the tree to the next branch. This galling activity suggests that the condition alters the hormonal balance of the host, particularly that of indoleacetic acid (IAA = auxin).