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.
We suspect that this disease is not of an abiotic origin, such as improper cultural practices, adverse soil textural and structural properties, pH, application of herbicides and pesticides, unusual climatic events, etc., because the disease has manifested itself under an extremely varied set of conditions in different nurseries and landscape situations across a number of states characterized by different climatic conditions and soil types.
Although several insects can cause galling on many different plants, no evidence of primary insect activity was detected on the diseased honeylocusts. We have observed colonization of older soft tissue galls by larvae but this has been interpreted as a secondary phenomenon that is part of the natural decay process associated with these galls.
Several aspects of the disease suggest that it may be caused by a bacterium. First, similar diseases of woody plants, such as ash trees in Europe and olive, oleander, privet, and jasmine in the United States and elsewhere, are caused by various pathovars of the bacterium Pseudomonas savastanoi (Alvarez et al. 1998; Azad and Cooksey, 1995). P. savastanoi is a known producer of IAA, and the knotting it produces in susceptible trees has been attributed to this trait.
Second, the disease seems to be correlated with pruning activity in nurseries, an aspect reminiscent of one of the main spreading routes for fire blight, a disease caused by a different bacterium, Erwinia amylovora.
Third, the Cincinnati nursery treated some of its trees in 2002 with copper-containing pesticides and reported that symptoms did not develop on the treated trees. Copper-containing pesticides can be effective against bacterial plant pathogens that invade above-ground plant parts.
Based on this information, we embarked on a diagnostic project during the summer of 2002. Initially, we attempted to determine whether P. savastanoi was involved, perhaps as a new pathovar. However, repeated isolation attempts from samples originating in Ohio, Maryland, and Michigan so far have failed to yield strains of that pathogen.
Nevertheless, several unknown and yet uncharacterized strains possessing clear pathogenic properties have been isolated from knots and will undergo proof of pathogenicity tests in early spring of 2003, when the tree hosts are likely to be more sensitive to the pathogen, given their phenological status (resumed active growth, hormonal activity, nutrient mobilization, etc.). It is possible that a new pathogen, or a new pathovar of an existing pathogen, is involved. Considerably more work is necessary before this question can be answered.
While we suspect that the disease is of bacterial origin, we are keeping other possibilities open and will conduct additional electron microscopic studies to determine if other microbes that could, in theory, cause similar symptoms might be involved, either as primary or contributing agents. These possible pathogens are phytoplasmas (a type of simplified bacterium formerly known as mycoplasma-like organisms, or MLOs) and viruses. Initial tests for these microorganisms have been negative, however. No evidence of fungal activity has been found so far.