BACTERIAL FRUIT BLOTCH: UNDERSTANDING AND MANAGING A GLOBAL THREAT TO CUCURBIT SEED PRODUCTION
Department of Plant Pathology, The University of Georgia, Athens GA 30602
Contact: Ron Walcott, firstname.lastname@example.org
Bacterial fruit blotch (BFB), caused by the Gram-negative bacterium, Acidovorax citrulli, threatens the production of cucurbitaceous crop worldwide. The pathogen is seedborne and seed transmitted, and can survive in melon and watermelon seeds for > 30 years. Infested/infected cucurbit seeds facilitate long distance dissemination of the pathogen and are the most important source of primary inoculum for BFB outbreaks. Under warm and humid conditions, BFB can cause 100% crop yield losses; hence, there is zero tolerance for the bacterium in commercial cucurbit seedlots. Additionally, since A. citrulli is an A1 regulated pest for the European and Mediterranean Plant Protection Organization, BFB is a major constraint for vegetable seed producers. Unfortunately, there are currently no commercially available BFB resistant cucurbit cultivars, and the efficacy of foliar-applied antimicrobial compounds varies with environmental conditions. As a result, BFB management relies on excluding the pathogen from cucurbit production systems by seed health testing and seed treatments. Despite these efforts, BFB outbreaks continue to occur sporadically, particularly in regions with warm and humid growing seasons. To more effectively control BFB, we have studied the mechanisms of seed infection and the molecular basis of host-pathogen interactions. Interestingly, A. citrulli can penetrate female watermelon flowers and localize in the embryos of watermelon seeds without inducing fruit symptoms. This localization might explain the pathogen’s remarkable longevity in seeds. A. citrulli populations can be generally divided into two genetic groups, I and II, that differ in their cucurbit host preference. Group I strains are virulent on several cucurbit hosts, while group II strains are highly aggressive on watermelon, but mild on other cucurbit hosts. More recently, we observed that representative group II and I strains differ in their arsenal of putative type III secreted effector proteins, and that effectors are likely responsible for cucurbit host preference. Using a detached immature melon fruit assay, we showed that fruit tissues are more sensitive to differences in virulence between strains of the two A. citrulli groups. This fruit assay will allow further elucidation of the mechanisms of virulence and host preference that may lead to effective BFB host resistance.