Cherry leaf spot disease, caused by Blumeriella jaapii (Rehm) Arx

Fungi isolated from snap bean roots and rhizosphere soil where fungicides are not used included Fusarium oxysporum, Fusarium equiseti, Fusarium subglutinans, Fusarium camptoceras, Fusarium chlamydosporum, Fusarium verticillioides, Fusarium proliferatum, Fusarium acuminatum, Fusarium solani, Peyronellaea pinodella, Macrophomina phaseolina, and Glomerella guttata. Only P. pinodella, M. phaseolina, and F. oxysporum were isolated on symptomatic plants. These soilborne fungi are common pathogens of diverse host plants. Pathogenicity tests under controlled environment demonstrated that these fungi were pathogenic on snap beans. Subsequently, bacterial endophytes isolated from snap bean roots, papaya roots and stems, and dogwood stems were evaluated as potential biological control agents against these diverse fungi. All bacteria isolated, including Bacillus vallismortis (PS), Bacillus amyloliquefaciens (Psl), Bacillus subtilis (Prt), Bacillus thuringiensis (Y and IMC8), Enterobacter sp. (E), Stenotrophomonas sp. (B17A), and Serratia sp. (B17B) suppressed growth of the fungal pathogens in vitro and formed clear inhibition zones in petri dish dual cultures. Growth media taken from the inhibition zones suppressed growth of the fungal pathogens in the absence of the bacterial cells, suggesting that the bacteria released unidentified antagonistic biochemical substances into the media. This study constitutes an initial screening of endophytes as biological control agents against diverse fungal pathogens and forms a basis for the discovery of novel strains that can be further developed and integrated into disease management systems for diverse fungal pathogens. Isolates B. vallismortis (PS), B. amyloliquefaciens (Psl), B. subtilis (Prt), and B. thuringiensis (Y IMC8) exhibited the best performance as potential biological control agents paving the way for larger-scale in vivo studies and characterization of their interactions with fungal pathogens.

No abstract

The gene pool of Mesoamerican common beans (Phaseolus vulgaris L.) includes genotypes in the small-to-medium-size seeded dry beans, as well as some snap beans from hotter environments adapted to the Southeastern United States. However, the warm and humid climate of the Southeastern United States is conducive to diseases such as Common Bacterial Blight (CBB) and Charcoal Rot (CR). The pathogens for these two diseases can survive long periods in infested soil or on seeds and are difficult to control through pesticides. Hence, field-level resistance would be the best management strategy for these diseases. The goals of this study were (1) to evaluate field-level resistance from the various commercial classes and subgroups represented in the Mesoamerican gene pool as sources for breeding beans for the region and (2) to evaluate genome-wide marker × trait associations (GWAS) using genetic markers for the genotypes. A total of 300 genotypes from the Mesoamerican Diversity Panel (MDP) were evaluated for CBB and CR in field experiments for three years. CBB resistance was also tested with a field isolate in controlled greenhouse conditions. The analysis of variance revealed the presence of variability in the MDP for the evaluated traits. We also identified adapted common bean genotypes that could be used directly in Southeastern production or that could be good parents in breeding programs for CBB and CR resistance. The GWAS detected 14 significant Single-Nucleotide Polymorphism (SNP) markers associated with CBB resistance distributed on five chromosomes, namely Pv02, Pv04, Pv08, Pv10, and Pv11, but no loci for resistance to CR. A total of 89 candidate genes were identified in close vicinity (±100 kb) to the significant CBB markers, some of which could be directly or indirectly involved in plant defense to diseases. These results provide a basis to further understand the complex inheritance of CBB resistance in Mesoamerican common beans and show that this biotic stress is unrelated to CR resistance, which was evident during a drought period. Genotypes with good yield potential for the Southeastern U.S. growing conditions were found with resistant to infection by the two diseases, as well as adaptation to the hot and humid conditions punctuated by droughts found in this region.