Scientists have uncovered an unexpected microbial relationship that could help explain differences in the severity of a major disease affecting common beans. The discovery sheds light on how the pathogen evolves and may point to new strategies for breeding disease-resistant crops and reducing reliance on chemical pesticides.
Angular leaf spot, caused by the fungus Pseudocercospora griseola, is a destructive disease of common beans worldwide. The pathogen is known for its variability and ability to evolve alongside its host plant, making disease management particularly challenging.
In a new study , published in Phytopathology and led by Luz M. Serrato-Diaz of the United States Department of Agriculture–Agricultural Research Service (USDA-ARS) Tropical Agriculture Research Station in Mayagüez, Puerto Rico, researchers analyzed 48 isolates of P. griseola collected from Puerto Rico, Guatemala, Honduras, and Tanzania. Using a DNA sequencing method called 3RADseq, the team examined genetic differences among the fungal isolates to better understand how the pathogen evolves and spreads across regions.
"This research highlights the complex hidden relationships between microorganisms and shows how DNA technology can help promote global food security in the context of evolving plant diseases," said Serrato-Diaz.
The genetic analyses confirmed previous findings that the pathogen population is divided into two major groups: Andean and Middle American. The Middle American group is further split into three previously unknown subpopulations corresponding to isolates from Guatemala and Honduras, Tanzania, and Puerto Rico. The findings highlight how geographic separation and local agricultural practices may influence the genetic diversity of plant pathogens.
The researchers also performed pathogenicity tests by infecting 12 different common bean varieties with fungal isolates from Puerto Rico. These tests identified 10 distinct fungal strains, reflecting the pathogen's ability to vary in aggressiveness. However, the observed differences in disease severity were not linked to known virulence genes identified in the genetic analysis.
One of the study's most surprising discoveries was the presence of DNA from an endophytic bacterium, Achromobacter xylosoxidans, within several fungal isolates. This bacterium was found in seven isolates that caused mild disease symptoms; highly virulent isolates did not contain the bacterium, suggesting that it may influence disease severity. These findings represent the first reported potential symbiotic relationship between P. griseola and this bacterial species. If confirmed, this interaction could open new possibilities for biological control using beneficial microbes to naturally suppress plant disease.
The study also identified transposable elements, sometimes called "jumping genes," within the fungal genome. These genetic elements may help the pathogen adapt rapidly to new environments and host plants, contributing to the continuing challenge of controlling angular leaf spot.
Understanding the genetic diversity and evolutionary dynamics of P. griseola is essential for developing durable disease resistance in common beans, one of the world's most important food crops. The researchers note that further work is needed to investigate how the bacterium, the fungus, and the host plant interact and how these relationships shape disease outcomes.
Read " Population Genomics of Pseudocercospora griseola Reveals New Groups in the Middle American Clade and the Presence of the Endophytic Bacterium Achromobacter xylosoxidans " to learn more.
For over 100 years Phytopathology®, published by The American Phytopathological Society, has been the premier international journal for publication of articles on fundamental research that advances understanding of the nature of plant diseases, the agents that cause them, their spread, the losses they cause, and measures used to control them. Articles are characterized by their novelty, innovativeness, and the hypothesis-driven nature of their research.
