Bacterial spot disease of pepper and tomato is caused by four distinct Xanthomonas species and is a severely limiting factor on fruit yield in these crops. The genetic diversity and the type III effector repertoires of a large sampling of field strains for this disease have yet to be explored on a genomic scale, limiting our understanding of pathogen evolution in an agricultural setting. Genomes of 67 Xanthomonas euvesicatoria (Xe), Xanthomonas perforans (Xp), and Xanthomonas gardneri (Xg) strains isolated from diseased pepper and tomato fields in the southeastern and midwestern United States were sequenced in order to determine the genetic diversity in field strains. Type III effector repertoires were computationally predicted for each strain, and multiple methods of constructing phylogenies were employed to understand better the genetic relationship of strains in the collection. A division in the Xp population was detected based on core genome phylogeny, supporting a model whereby the host-range expansion of Xp field strains on pepper is due, in part, to a loss of the effector AvrBsT. Xp-host compatibility was further studied with the observation that a double deletion of AvrBsT and XopQ allows a host range expansion for Nicotiana benthamiana. Extensive sampling of field strains and an improved understanding of effector content will aid in efforts to design disease resistance strategies targeted against highly conserved core effectors.

Bell pepper plants sprayed with the chemical acibenzolar-S-methyl (ABM, Actigard 50 WG) showed resistance to subsequent infections with the bacterial spot agent Xanthomonas axonopodis pv. vesicatoria. Induction of resistance was independent of the cultivar used, and was expressed as early as 3 days after treatment and continued for at least 2 weeks. In the field, applications of ABM every 2 weeks, alone or in combination with copper, resulted in disease control similar to the standard treatment of copper plus maneb. Yield response was variable, with certain combinations of chemical treatments and cultivars producing yields as large as the copper plus maneb treatment. In contrast, weekly applications during the entire crop season had a negative impact on yield. In plots maintained free of bacterial spot, applications of ABM every 2 weeks caused a reduction in yield for one cultivar of six tested. The use of chemical inducers for the control of bacterial spot on bell pepper, while generally promising, may result in an unpredictable loss in fruit yield.

No abstract

Genetic relationships among 25 isolates of Xanthomonas fragariae from diverse geographic regions were determined by three PCR methods that rely on different amplification priming strategies: random amplified polymorphic DNA (RAPD) PCR, repetitive extragenic palindromic (REP) PCR, and enterobacterial repetitive intergenic consensus (ERIC) PCR. The results of these assays are mutually consistent and indicate that pathogenic strains are very closely related to each other. RAPD, ERIC, and REP PCR assays identified nine, four, and two genotypes, respectively, within X. fragariae isolates. A single nonpathogenic isolate of X. fragariae was not distinguishable by these methods. The results of the PCR assays were also fully confirmed by physiological tests. There was no correlation between DNA amplification product patterns and geographic sites of isolation, suggesting that this bacterium has spread largely through exchange of infected plant germ plasm. Sequences identified through the RAPD assays were used to develop three primer pairs for standard PCR assays to identify X. fragariae. In addition, we developed a stringent multiplexed PCR assay to identify X. fragariae by simultaneously using the three independently derived sets of primers specific for pathogenic strains of the bacteria.

No abstract

No abstract

Populations of Erwinia amylovora bacteriophage greater a spreading translucent halo and a smaller plaque without a than 106 plaque-forming units (PFU) per gram of tissue were halo. Thirty-five bacterial isolates, consisting of nine genera, isolated without enrichment from diseased aerial parts of 18 species, and 15 strains of E. amylovora were typed; the apple trees during the summer of 1975. Three phage isolates phages lysed only E. amylovora. The burst size of the three were selected from different geographical locations. Two isolates was 40 to 50 PFU per cell. The phages could be stored types of plaques were produced; a clear-centered plaque with at 4 C and-20 C but lost titer when stored at 24 C.

A significant portion of harvested produce never reaches the consumer due to, postharvest diseases. Various chemicals have been used to reduce the incidence of postharvest diseases. Many of these materials have been removed from the market in recent years due to economic, environmental, or health concerns. Although somewhat limited in the range of diseases controlled, chlorination is effective when combined with proper postharvest handling practices. Additionally, it is a relatively inexpensive postharvest disease control method that poses little threat to health or the environment. The proper use of chlorination in the management of postharvest diseases in fresh fruits and vegetables is discussed.