Publication no. D-1998-0918-01R

Seed Contamination Thresholds for Development of Carrot Bacterial Blight Caused by Xanthomonas campestris pv. carotae. K. C. Umesh, R. M. Davis, and R. L. Gilbertson, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 82:1271-1275. Accepted for publication 4 August 1998. Copyright 1998 The American Phytopathological Society.

The relationship between levels of carrot (Daucus carota subsp. sativus) seed contamination with Xanthomonas campestris pv. carotae and (i) establishment of populations of X. campestris pv. carotae on carrot leaves and (ii) the incidence and severity of carrot bacterial blight was determined in field plots in Davis, California, in 1995 and 1996. Levels of seed contamination ranged from 0 to 1.5 × 10(^5) CFU/g in 1995 and from 0 to 1.5 × 10(^7) CFU/g in 1996. Seed contamination levels were positively correlated with X. campestris pv. carotae populations detected on leaves and with the incidence and severity of carrot bacterial blight. The size of X. campestris pv. carotae populations on leaves was also directly related to disease incidence. In 1996, yields were significantly reduced in plots established with seed lots having the highest levels of X. campestris pv. carotae contamination. Under the conditions of this study (i.e., a location having low rainfall and relative humidity), the threshold of seed contamination for the establishment of X. campestris pv. carotae populations on leaves and for the development of carrot bacterial blight was unexpectedly high: 10(^4) to 10(^5) CFU/g of seed.