Potential Sources of Initial Inoculum for Bacterial Speck in Early Planted Tomato Crops in Michigan: Debris and Volunteers from Previous Crops. D. J. Jardine, Former Graduate Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. C. T. Stephens, and D. W. Fulbright. Associate Professors, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 72:246-249. Accepted for publication 29 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0246.

Colonies of genetically marked Pseudomonas syringae pv. tomato were recovered from diseased tomato leaf tissue contained in nylon mesh bags and left to overwinter for 26 wk on the soil surface or buried to depths of 8 or 18 cm. A rifampicin-resistant strain of P. s. pv. tomato used to inoculate a field of tomato plants during the 1983 growing season was recovered from tomato surface debris in April 1984. In 1984 and 1985, however, the organism could not be isolated from the leaves, roots, or rhizosphere soil of 23 different weed species sampled from fields in which the pathogen was present the preceding year. In addition, the pathogen has been consistently isolated from volunteer tomatoes in the spring. Thus, infested crop debris and volunteer tomato plants, rather than weeds, appear to be a source of initial inoculum in northern U.S. tomato fields. The ability of overwintered P. s. pv. tomato to serve as a source of initial inoculum was affected by tillage system used and time of year of the tillage. Leaf infection by P. s. pv. tomato occurred when tomato seedlings were planted in a previously infested field that was either spring-plowed or left untilled. Plants did not become diseased when planted into areas that were fall-plowed.