Relationship of Incidence of Seedborne Xanthomonas campestris to Black Rot of Crucifers

Abstract

SCHAAD, N. W., W. R. SITTERLY, and H. HUMAYDAN. 1980. Relationship of incidence of seedborne Xanthomonas campestris to black rot of crucifers. Plant Disease 64:91-92. The relation of amounts of seedborne Xanthomonas campestris, determined by laboratory assays, to the incidence of black rot in the field was determined. Seeds infested naturally with X. campestris were mixed with healthy seeds, assayed for X. campestris, and seeded in a field in South Carolina. The pathogen was detected in laboratory assays in two of four samples with 0.01% infestation, whereas black rot developed in the field in three of four such samples. In 1976, field plots that initially contained 0.03, 0.07, and 0.14% infected plants resulted in high incidences of black rot. In 1977, the incidences of black rot were high in plots that initially contained 0.05% infected plants but not in plots that initially contained 0.01% infected plants. We conclude that laboratory assays can be used to detect levels of seed infestation that may or may not result in a high incidence of black rot in the field. Xanthomonas campestris is seedborne (3,5,7), and infested seed is an important source of inoculum (2,4,8,14,15). Several methods have been described for detecting the pathogen in or on seeds (1,9,11,13), but the number of infested seeds needed to initiate black rot in the field has not been determined. This information is essential to establish a minimum allowable percentage of seed infestation. This study was designed to determine the relationship of amounts of seed infestation to disease incidence in the field.