UK- Commercial Vegetable Production

 The Case of the Disappearing Bacterial Spot 

2004 

Issue: Bacterial spot of peppers, caused by Xanthomonas axonopodis pv. vesicatoria and other xanthomonads, is a remarkably destructive disease in Kentucky, commonly causing losses of marketable yield of over 90% on susceptible varieties.  Prior to this project, precise diagnosis of this disease could take weeks, well beyond the useful time frame for producers to make a rational management decision for this disease.  Thus, our Plant Diagnostic Laboratories have relied on standard presumptive tests for diagnosing the disease in a short time frame, to provide the producer with our best (although admittedly imperfect) determination in as rapid a manner as possible.  This approach has been based on our understanding that only one bacterial pathogen--X. axonopodis pv. vesicatoria--was responsible for the early-season bacterial leaf spots on peppers in Kentucky. 

What Has Been Done: In June, 2004, we received a sample of pepper from the Quicksand area showing symptoms that we associate with bacterial leaf spot.  Based on our previously standard presumptive diagnostic techniques, we fully expected to confirm the presence of X. axonopodis pv. vesicatoria by PCR in the sample and that an epidemic would develop in that field.  However, the sample repeatedly gave a negative result when tested for xanthomonads using the PCR techniques developed through this IPM project.  Furthermore, to our surprise, the expected epidemic never did develop; in fact, symptoms became very difficult to find later in the season.  Our ongoing work on this outbreak—work which we would have not done without this grant—indicated that Pseudomonas syringae was responsible for the outbreak.  Our experience in 2004 confirmed some apparently overlooked literature dating back to the early 1960’s: that pepper is much, much less susceptible under Kentucky conditions to P. syringae than to X. axonopodis pv. vesicatoria.   

Future Impact: As this situation unfolded, I became profoundly impressed with how useful this real-time PCR diagnostic technique will be in the future for Kentucky farmers.  As a direct result of this project, we now recognize that there are two bacterial leaf spot diseases on pepper in Kentucky that share very similar symptoms.  One—caused by X. axonopodis pv. vesicatoria and related xanthomonads—is extremely aggressive on susceptible cultivars and often leads to abandonment of the field.  The other—caused by Pseudomonas syringae (pathovar as yet uncertain but probably pv. syringae)—is apparently a very minor problem, typically one that does not require any specific control measures.  Although our previously standard presumptive diagnostic techniques would not have permitted us to make this distinction in a time frame useful for a producer, we now have a rapid tool to make this critical diagnostic distinction for pepper producers in Kentucky.

For more information on this project contact:

Dr. Paul Vincelli

201 Plant Science Building

 Department of Plant Pathology

 University of Kentucky

 Lexington, KY 40546-0312

 E-mail: pvincell@uky.edu

Original document: 1 October 2004 Last updated: 29 March 2006