Keystone Pathogens Open the Door for Plant Disease Complexes

Plant diseases have long been diagnosed through the lens of Robert Koch’s “one microbe, one disease” paradigm, but a new review argues that this framework is fundamentally insufficient for many of the world’s most damaging crop diseases. Writing in the journal Discover Plants, researchers Laxmi Kanta Paudel and Anita Puyam propose that within polymicrobial disease complexes, primary pathogens act as keystone drivers—organisms that may be present at low abundance yet exert a disproportionately large influence on the entire disease process. Their central claim is that secondary pathogens depend mainly on host modifications made by these primary initiators, and that recognizing this hierarchy could transform how plant diseases are diagnosed, managed, and bred against.

The review builds on the pathobiome concept, which defines disease as the product of interactions among multiple microorganisms within a host rather than the action of a single pathogen in isolation. Many diseases once considered monomicrobial are now recognized as complexes, in which pathogens with distinct biologies and life cycles occupy the same niche. The authors argue that previous frameworks have treated co-isolated pathogens as equal participants, failing to distinguish primary initiators from secondary colonizers—a conceptual gap they say has misled diagnosis, management, and resistance breeding for complex pathosystems.

The first mechanism by which primary pathogens “open the door” is physical barrier disruption. The cuticle and cell wall constitute a plant’s first line of constitutive defense, but plant-parasitic nematodes overcome these barriers through two synergistic modes: mechanical rupture and physiological reprogramming. Migratory nematodes such as Pratylenchus species physically lacerate root cortical tissues with stylet thrusting, while sedentary nematodes like Meloidogyne species induce nutrient-rich giant cells that serve as feeding sites. These actions create direct entry pathways for soil-borne fungi such as Fusarium oxysporum, Verticillium dahliae, and Rhizoctonia solani, and for bacteria that cannot independently penetrate intact barriers.

Timing, the review emphasizes, decisively governs whether such complexes turn synergistic or antagonistic. When the cyst nematode Heterodera avenae was inoculated one month before the fungus Fusarium culmorum, yield reduction reached 44 percent, compared with 33 percent when the fungus was applied first. Early nematode penetration creates wounds, converts root tissue into nutrient-rich sinks, alters root exudates, and suppresses plant defenses. When the fungus arrives first, however, the relationship inverts: early Fusarium colonization disrupts nematode feeding sites and lowers populations of Heterodera filipjevi. The nematode functions as the keystone initiator only when host viability persists long enough for secondary pathogens to exploit its wounds.

Fungal pathogens breach barriers through their own arsenal. Necrotrophs such as Botrytis cinerea, described as the world’s second most destructive fungal pathogen, kill host cells with cell wall-degrading enzymes, oxalic acid, and immunity-suppressing effectors, converting intact tissue into necrotic, nutrient-rich lesions that admit soft-rot Erwinia and Pseudomonas species. In the summer bunch rot complex of grape, Botrytis cracks berry skin and leaks sugars, opening the way for Alternaria, Cladosporium, Rhizopus, Penicillium, and Aspergillus. In the Styrian oil pumpkin complex, Didymella bryoniae breaks down tissue and even provides continuous liquid films along its mycelium—a “fungal highway” across which motile bacteria such as Pectobacterium carotovorum move across dry gaps to reach healthy plant parts.

Biotrophic pathogens work differently, establishing intimate associations with living tissue while suppressing immunity rather than destroying it. A striking example comes from Arabidopsis thaliana, where the biotrophic oomycete Albugo candida secretes effectors that suppress salicylic acid-mediated defenses and alter tryptophan-derived secondary metabolites, enabling colonization by Phytophthora infestans—a pathogen that normally cannot infect Arabidopsis at all. The authors interpret this as a keystone driver converting a non-host plant into a susceptible host for an aggressive secondary pathogen. Viruses contribute through viral suppressors of RNA silencing, which interfere with DICER-like enzymes and Argonaute proteins, suppressing both pattern-triggered and effector-triggered immunity and shifting the host into a state of effector-triggered susceptibility that favors subsequent fungal and bacterial infection.

The third mechanism is nutritional niche creation. Root-knot and cyst nematodes induce multinucleate feeding structures—giant cells or syncytia—that act as metabolic sinks, increasing carbohydrates and nitrogenous compounds in root exudates and attracting Fusarium solani more effectively than non-infested roots. In the Fusarium-Monosporascus melon root rot complex, Fusarium oxysporum f. sp. melonis secretes cell wall-degrading enzymes that release simple sugars and amino acids, creating nutrient-rich zones for Monosporascus cannonballus; the co-infection produces severe root rot that neither pathogen achieves alone. The review also describes how a pathogenic Colletotrichum tofieldiae strain activates host abscisic acid signaling in Arabidopsis roots, suppressing nitrogen and iron uptake genes and raising root sucrose—a carbon-rich niche supporting extensive fungal growth.

From these mechanisms, the authors derive a hierarchical classification of secondary pathogens. Obligate secondary pathogens cannot infect healthy hosts under any conditions; Pantoea agglomerans, Erwinia toletana, and Erwinia oleae cannot cause disease independently but colonize olive knots formed by Pseudomonas savastanoi. Facultative secondary pathogens can infect intact hosts but benefit enormously from prior modification—Magnaporthe oryzae is highly aggressive on its own, yet prior infection with Rice necrosis mosaic virus markedly enhances fungal growth by suppressing the host’s RNA-silencing machinery, a synergy lost in rice mutant lines deficient in RNA-dependent RNA polymerases. Co-primary pathogens, the third category, show mutualistic synergism: Burkholderia glumae produces the antifungal toxin toxoflavin, promoting colonization of Fusarium graminearum, which is itself toxin-resistant, while Fusarium conidia carry Burkholderia spores aloft and shield them from UV damage.

The review is careful to document where the hierarchy breaks down. Antagonistic interactions, priority effects, and temporal succession can reverse or complicate primary-secondary roles. In the wheat leaf blotch complex, early establishment of Pyrenophora tritici-repentis facilitates later colonization by Parastagonospora nodorum, whereas prior infection by P. nodorum consistently suppresses P. tritici-repentis regardless of host genotype. In the western gall rust system, the secondary pathogen Nectria fuckeliana eventually kills the rust’s galls, starving the biotrophic rust and reducing local rust inoculum. Keystone status, the authors stress, is not an inherent property of a species but emerges from ecological context: a pathogen may be a keystone in one environment or host genotype and minor in another.

The practical implications are substantial. Traditional diagnostics that culture a single pathogen can misattribute causation—the review cites a maize “mysterious disease” case that took over two decades to resolve, in which isolated bacteria proved to be saprophytes and the true cause lay in the maize variety’s own tb1 gene. Resistance breeding against one pathogen can fail when another breaks it: nematode infection can overcome tobacco resistance to Ralstonia and Xanthomonas. The authors advocate targeting dominant keystone pathogens, integrating multi-omics strategies, network-level analyses, and synthetic microbial communities, and accounting for temporal dynamics and climate extremes—which they note can shift pathogen composition and disrupt established hierarchies over time. As molecular tools continue to reveal the polymicrobial reality of crop disease, the hierarchy of who strikes first may prove as important as the pathogens themselves.

Subject of Research: The hierarchy of keystone pathogens driving polymicrobial plant disease complexes

Article Title: Plant pathobiome and the hierarchy of keystone drivers in disease complexes

Article References: Paudel, L. K., & Puyam, A. (2026). Plant pathobiome and the hierarchy of keystone drivers in disease complexes. Discover Plants, 3(1), Article 508. https://doi.org/10.1007/s44372-026-00969-3

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00969-3

Keywords: plant pathology, pathobiome, keystone pathogens, disease complexes, polymicrobial infection, secondary colonizers, nematodes, fungal pathogens, immune suppression, resistance breeding, multi-omics, plant microbiome