Soil Bacterium’s Airborne Chemicals Recruit Allies to Fight Apple Ring Rot

Apple ring rot, caused by the fungus Botryosphaeria dothidea, is one of the most damaging diseases facing apple growers, destroying fruit both in the orchard and in storage. Now a team of researchers at Northwest A&F University in Yangling, China, has uncovered a remarkably layered defense strategy that begins with something almost invisible: the airborne chemicals emitted by a common soil bacterium. In a study published in the journal Microbiome, Shengzhi Guo, Arneeb Tariq, Lingfang Zhu, Xihui Shen and their colleagues show that volatile organic compounds released by Streptomyces albidoflavus strain G1 do far more than simply poison the fungus. They also reshape the bacterial community living inside apple tissue and reprogram the fruit’s own defense genes, creating a three-pronged assault on the pathogen.

The team’s starting point was a straightforward observation with important implications. When G1 was grown on one side of a divided petri dish, with B. dothidea on the other, the fungus grew poorly even though the two organisms never touched. The same held true when the bacteria and fungus were separated in assays on detached apple fruit: exposure to the bacterial volatiles alone was enough to reduce lesion development. This confirmed that G1’s protective power travels through the air, a property that makes volatile-mediated biocontrol especially attractive for postharvest settings, where fruits are stored in enclosed spaces and where chemical fumigants face increasing regulatory and consumer resistance.

To find out which molecules were responsible, the researchers used gas chromatography–mass spectrometry, or GC–MS, to profile the full complement of volatiles emitted by G1. The analysis revealed a set of differentially abundant compounds tied to sesquiterpenoid, triterpenoid and phenylalanine metabolism, chemical families that include many known antimicrobial agents. When the team applied pure standards of selected volatiles to fungal cultures, the compounds impaired several aspects of B. dothidea biology at once: they slowed growth, disrupted the integrity of the fungal cell membrane and interfered with biofilm formation, the sticky multicellular structures that fungi often use to establish infection. Membrane disruption is a particularly significant mechanism, because a compromised membrane cannot properly regulate the flow of nutrients and ions, leaving the pathogen physiologically crippled.

But the direct antifungal effect was only the first layer of the story. The researchers next asked what happens to the apple’s resident microbiome when G1 volatiles are present during a pathogen attack. Using 16S rRNA gene amplicon sequencing, a technique that catalogs bacterial species by reading a signature stretch of their genome, they examined the bacterial communities in tissue taken from the margins of lesions on infected fruit. The result was striking: exposure to G1 volatiles under pathogen challenge was associated with a simplified bacterial community dominated by Pseudomonas, and quantitative PCR confirmed that Pseudomonas-specific gene copies had increased markedly in the treated tissues. In other words, the airborne chemicals appeared to be selectively favoring a group of bacteria with a reputation for antifungal prowess.

The team then took the crucial step of culturing. From the volatile-treated lesion-margin tissues they recovered a cultivable isolate, which whole-genome analysis identified as Pseudomonas paracarnis, designated strain aNRL1. This isolate matched ASV_2, the dominant Pseudomonas-assigned sequence variant enriched by the volatile treatment, providing a direct functional link between the community-level signal and a living organism the researchers could test. In laboratory assays, aNRL1 strongly inhibited B. dothidea, and when applied to apple fruit it reduced lesion development. The researchers also screened the aNRL1 genome for toxin-associated biosynthetic loci, and their experiments showed that selected G1-associated volatiles exerted compound-specific effects on aNRL1’s own growth, biofilm formation and motility, suggesting a nuanced chemical dialogue between the two bacteria rather than simple one-way stimulation.

The third layer of the defense system emerged from the apple itself. By profiling the fruit’s transcriptome, the complete set of genes being actively expressed, the researchers found that the volatile treatment triggered a pathogen-dependent remodeling of brassinosteroid signaling, a plant hormone pathway involved in growth, stress responses and immunity. Alongside this, they observed increased expression of genes associated with mitogen-activated protein kinase signaling, a conserved cellular alarm system that plants use to mount rapid defenses, as well as genes involved in lipid metabolism and in the biosynthesis of cuticle and wax, the fruit’s outermost physical barrier against invading pathogens. The pattern suggests that G1’s volatiles prime the apple to reinforce its own armor and activate internal defense circuits precisely when the fungus is present.

Taken together, the findings support a multilayered model of volatile-mediated biocontrol. First, the volatiles act directly on B. dothidea, damaging its membranes and disrupting biofilms. Second, they coincide with a restructuring of the internal lesion-margin bacterial community toward Pseudomonas dominance, effectively recruiting an antifungal reinforcement corps within the infected tissue. Third, they prompt the host to upregulate barrier-building and defense-related genes. Each layer reinforces the others: a weakened pathogen faces not only hostile chemistry but also a newly hostile microbial neighborhood and a fruit whose defenses are on high alert. This kind of multi-mechanism action is valuable in agriculture because pathogens that can evolve resistance to a single compound find it much harder to circumvent three simultaneous pressures.

The study is notable for the breadth of methods it integrates. Volatilome profiling identified the chemical mediators; microbiome sequencing revealed the ecological consequences; interaction assays connected individual compounds to effects on both the pathogen and the recruited ally; and transcriptomics illuminated the host’s contribution. Few biocontrol studies trace a single intervention through all three biological compartments, the antagonist, the resident microbiome and the host plant, in the same experimental system. The use of absolute quantification, including plasmid-based standard curves for measuring pathogen and Pseudomonas loads, added rigor by allowing the team to track changes in actual gene copy numbers rather than only relative proportions.

Apple ring rot causes substantial production and postharvest losses worldwide, and control options have relied heavily on fungicides whose residues and resistance problems grow more problematic each year. A volatile-based approach offers distinct practical advantages: volatiles can reach hidden infection sites inside fruit, they leave no direct contact residue on the surface, and they are well suited to the enclosed environments of storage facilities. The identification of P. paracarnis aNRL1 as a cultivable partner opens the possibility of combining volatile treatments with deliberate application of the enriched bacterium, a formulation strategy that could amplify the community-level effect observed in the study.

There remain questions to resolve before such a strategy reaches the packinghouse. The experiments were conducted in plate, detached-fruit and controlled-challenge settings, and field-scale performance, dose optimization and interactions with commercial storage conditions will need further testing. The compound-specific effects of G1 volatiles on aNRL1 also hint that the composition of the volatile blend, not just its total antifungal punch, matters for shaping the recruited community. Still, the work provides a compelling proof of concept that a single soil-dwelling Streptomyces strain can orchestrate a coordinated defense spanning chemistry, ecology and plant physiology. As the search for sustainable alternatives to synthetic fungicides intensifies, the idea that farmers might fight fruit rot by letting beneficial bacteria speak in airborne chemical signals, and by persuading the fruit’s own microbes to join the fight, is moving from speculation toward science.

Subject of Research: Volatile-mediated biocontrol of apple ring rot through microbiome enrichment and host defense activation

Article Title: Volatile-mediated microbiome enrichment by Streptomyces albidoflavus G1 enhances suppression of apple ring rot pathogen Botryosphaeria dothidea

Article References: Volatile-mediated microbiome enrichment by Streptomyces albidoflavus G1 enhances suppression of apple ring rot pathogen Botryosphaeria dothidea. (n.d.). https://doi.org/10.1186/s40168-026-02540-4

Image Credits: AI Generated

DOI: 10.1186/s40168-026-02540-4

Keywords: apple ring rot, Botryosphaeria dothidea, Streptomyces albidoflavus, volatile organic compounds, Pseudomonas paracarnis, microbiome, postharvest biocontrol, lesion-margin microbiome, brassinosteroid signaling, transcriptomics, GC-MS, antifungal