USDA-ARS Scientists Identify Potential Bacterial Symbiosis in Fungal Pathogen Causing Global Angular Leaf Spot in Common Beans

USDA-ARS researchers discover a bacterium within angular leaf spot fungus that appears to reduce disease severity, offering new biological control possibilities.

By: AXL Media

Published: Mar 31, 2026, 5:55 AM EDT

Source: Information for this report was sourced from American Phytopathological Society

Mapping the Global Genetic Diversity of a Major Crop Pathogen

Angular leaf spot remains one of the most persistent and destructive diseases affecting common beans, a primary protein source for millions worldwide. The fungus responsible, Pseudocercospora griseola, is notorious for its rapid evolution and ability to bypass plant defenses. To understand this variability, a research team led by Luz M. Serrato-Diaz of the USDA-ARS Tropical Agriculture Research Station utilized 3RADseq DNA sequencing to analyze 48 isolates from Puerto Rico, Guatemala, Honduras, and Tanzania. The results confirmed a deep genetic divide between Andean and Middle American groups, while uncovering three previously unknown subpopulations that correlate with specific geographic regions and local agricultural practices.

The Discovery of an Unexpected Bacterial Symbiote

The most striking revelation of the study was the detection of DNA from Achromobacter xylosoxidans, an endophytic bacterium, embedded within several fungal isolates. This marks the first time such a symbiotic relationship has been reported for P. griseola. This discovery was not merely a biological curiosity, as the presence of the bacterium appeared to correlate directly with the pathogen's impact on the host plant. The team noted that this hidden relationship between microorganisms might be a key driver in the pathogen's evolutionary trajectory, offering a fresh perspective on how fungal diseases develop and spread across different continents.

Bacterial Presence as a Possible Mitigator of Disease Severity

Pathogenicity tests conducted on 12 different common bean varieties revealed a significant spectrum of aggressiveness among the fungal strains. Interestingly, the researchers found that Achromobacter xylosoxidans was present in seven isolates that caused only mild disease symptoms. In contrast, the most highly virulent and destructive isolates lacked the bacterium entirely. This correlation suggests that the bacterium may act as a natural check on the fungus, potentially suppressing its ability to damage the plant. If this interaction is confirmed as a protective symbiosis, it could provide a foundation for new biological control strategies that use beneficial microbes to naturally reduce crop losses.