Characterizing the Incidence and Distribution of Bacterial Blight Infestation in Individual Carrot Seeds: Can One Bad Seed Spoil the Whole Seed Lot?

Series/Report Number: COARC2016

Abstract: Bulk samples of carrot seed are tested for Xanthomonas hortorum pv. carotae (Xhc) using a seed wash dilution-plating protocol (Asma, 2005). In this protocol, three 10 gram samples of carrot seed, equivalent to three subsamples of 10,000 seeds each, are soaked in buffer and serial dilutions are plated onto a semi-selective medium that limits the growth of bacteria other than Xhc. Testing protocols for seed-borne pathogens usually assume that infested seeds are fairly uniform (i.e. they follow the normal “bell-shaped curve”) with regards to bacterial populations on individual seeds and that the assay will detect the average number of bacteria for infested seed present in the sample. However, several studies have shown that the number of bacteria found on individual seeds may vary widely and follow non-normal distributions (i.e. they do not follow the normal “bell-shaped curve”) (Dutta et al., 2013). If the distributions of Xhc among infested carrot seeds are non-normal, assay results from bulk samples could result in an inaccurate estimate of the true population number. For example, if a seed lot contains relatively few, highly infested seeds, the bulk seed lot assay will be highly influenced by the number of highly infested seeds that are in a particular sample. On the other hand, a seed wash assay may not detect any infested seeds if only a few seeds in a seed lot are actually infested. The objective of this research is to determine the incidence and level of Xhc infestation among individual seeds in infested carrot seed lots. It is anticipated that this information will be important to the Oregon carrot seed industry, since many countries and markets have a zero-tolerance policy for Xhc in carrot seed. The incidence and level of infestation on individual seeds could also influence inoculum thresholds that are required for the development of bacterial blight in carrot root production. A better understanding of seed infestation in carrot seed lots will enable carrot seed producers to improve the methods used to prevent, detect, and treat infested seeds prior to market.

Chapter Number: 22