College of Agriculture & Natural Resources
Plant Science & Landscape Architecture

Analysis and Characterization of the Bacterial Pathogen Acidovorax citrulli

The University of Maryland’s Department of Plant Science and Landscape Architecture welcomed Dr. Ronald Walcott from the University of Georgia as a guest speaker on Thursday, October 10th.  A native of Barbados, Dr. Walcott began his career in plant pathology by completing B.S. and M.S. degrees in plant pathology at Iowa State University.  He completed his doctorate at the University of Georgia, where he later was hired as an assistant professor.  Since then, he has dedicated his time to student instruction and to researching the biology of seedborne bacteria and fungi.  Dr. Walcott’s seminar presentation reviewed the history of the bacterial pathogen Acidovorax citrulli, the causal agent of bacterial fruit blotch (BFB) on cucurbits and shared details regarding his latest work on the pathogen.

Bacterial fruit blotch is caused by the seedborne bacterium A. citrulli, affecting the foliage and fruit of a wide range of cucurbits.  Symptoms initially manifest as water soaked lesions on the undersides of seedling cotyledons that can extend to stems and develop into elongated dry reddish-brown lesions on stems and leaf veins.  Symptoms on fruits initially appear as small, olive colored spots which can spread and cover the entire upper surface.  Fruit lesions may develop into sunken spots or deep brown cracks and may penetrate the flesh, causing watery rot within.   

The disease was first reported in 1965, on watermelon at the USDA plant introduction station in Georgia.  Major outbreaks of the disease in the late 1980’s and early 1990’s led to litigation between farmers and seed producers, and garnered even more national attention when Business Week and the New York Times erroneously associated “exploding watermelons” with bacterial fruit blotch symptoms.  Significant changes in the industry led to strict controls on seed production and distribution, limiting further major outbreaks at the time.  Still, this disease remains a prevalent part of cucurbit fruit and seed production in the United States and abroad.  Unchecked outbreaks can be extremely detrimental to plants, making BFB an economically important threat to seed and fruit production. 

Dr. Walcott shared his latest work on Acidovorax, which included efforts to elucidate host virulence and preference of the two distinct pathogenic strains, group I and group II.  Group I strains are known to have a wider range of cucurbit hosts than the more aggressive group II strains, which are predominantly associated with watermelon.  Dr. Walcott reported that this host preference is largely due to a total of 8 DNA differences in their effector protein arsenals and ended his seminar talk by sharing his efforts to find a suitable surrogate host for A. citrulli groups.

Dr. Walcott’s work contributes to an improved understanding of the genetic diversity of A. citrulli and could lead to improved BFB management by detecting new targets for resistance breeding.  Resistant cucurbit varieties could play a key part in increasing yields and decreasing risk for farmers.



Latin, R. X., and D. L. Hopkins. "Bacterial fruit blotch of watermelon." Plant Disease 79.8 (1995): 761-765.         

Silva, Gustavo M., Ricardo M. Souza, Lichun Yan, Rui S. Junior, Falavio H. V. Medeiros, and Ron R. Walcott. "Strains of the Group I Lineage of Acidovorax Citrulli, the Causal Agent of Bacterial Fruit Blotch of Cucurbitaceous Crops, Are Predominant in Brazil." Phytopathology Dec. 2016: 1486-494.

Walcott, R.R. 2005. Bacterial fruit blotch of cucurbits. The Plant Health Instructor. DOI: 10.1094/PHI-I-2005-1025-02

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