Phytophthora blight, caused by the oomycete Phytophthora capsici, is one of the most destructive soil-borne diseases of red pepper worldwide, capable of wiping out entire fields within a single growing season. For decades, growers have relied heavily on chemical fungicides and soil fumigants to keep the pathogen in check, an approach that carries mounting costs for soil health, beneficial organisms, and environmental quality. Now, a team of researchers at Shanxi Agricultural University in Taiyuan, China, has reported that two strains of the beneficial fungus Trichoderma harzianum, applied together, can cut blight incidence in red pepper by an extraordinary 90.9 percent compared with pathogen-infected controls. The study, published in the journal Plant and Soil, goes beyond simply documenting disease suppression and dissects, at the molecular and ecological level, how the fungal partnership protects plants and even helps them grow better.

The research, led by Zhifen Guo with corresponding authors Qiang Zhang and Min Yan, focused on two Trichoderma harzianum strains designated T891 and T58. Trichoderma species are among the most widely studied biological control agents in agriculture, known for their ability to colonize root surfaces, parasitize pathogenic fungi, and stimulate plant defenses. What sets the new work apart is its systematic comparison of single-strain and combined-strain applications, combined with a deep dive into the rhizosphere, the narrow zone of soil surrounding the roots where plants, microbes, and pathogens interact most intensely. Synthetic communities of complementary microbial strains are an emerging strategy for managing soil-borne disease, but the authors note that research on Trichoderma combinations specifically for red pepper Phytophthora blight had remained scarce.

In greenhouse pot experiments, the team inoculated pepper plants with the pathogen and then treated them with T891 alone, T58 alone, or the two strains together. Disease incidence dropped significantly in the T58 single-inoculation treatment and in the co-inoculation treatment relative to the pathogen-infected control. The most dramatic result came from the combined application: the T891-T58 pairing produced the lowest disease incidence of all treatments, a reduction of 90.9 percent. Beyond disease control, Trichoderma treatments significantly promoted plant growth, suggesting the fungi deliver a double benefit, shielding plants from attack while simultaneously boosting their vigor and biomass.

To understand the physiological basis of this protection, the researchers measured the activity of three key antioxidant enzymes in the plants: peroxidase, or POD; superoxide dismutase, or SOD; and catalase, or CAT. These enzymes form the front line of the plant’s defense against oxidative stress, a damaging cascade of reactive oxygen species that pathogens trigger during infection. When a pathogen invades, plant cells produce a burst of reactive oxygen that, if left unchecked, damages membranes, proteins, and DNA. Antioxidant enzymes neutralize these molecules, and their activity levels serve as sensitive indicators of how well a plant is coping with attack. The team also measured malondialdehyde, or MDA, a breakdown product of lipid peroxidation that accumulates when oxidative damage overwhelms cellular defenses, making it a classic marker of stress severity.

The measurements revealed that Trichoderma treatment reshaped this antioxidant landscape in ways that favored plant survival. Most strikingly, the T891-T58 combination produced the highest catalase activity of any treatment. Catalase is a ubiquitous antioxidative enzyme that catalyzes the degradation of hydrogen peroxide, one of the most abundant and dangerous reactive oxygen species generated during pathogen assault. By keeping hydrogen peroxide in check, the co-inoculated plants appear to have limited the self-inflicted tissue damage that often accompanies infection, while still retaining enough of the oxidative burst to signal and contain the invader. The interplay between pathogen-triggered stress and Trichoderma-primed antioxidant capacity thus emerges as a central mechanism linking the fungal treatment to reduced disease incidence.

But the physiological story is only half of the picture. Using amplicon sequencing, a technique that reads marker gene regions to profile entire microbial communities, the researchers mapped the bacterial and fungal inhabitants of the rhizosphere across all treatments. The sequencing revealed that Trichoderma treatments enriched beneficial microbial taxa and shifted the network topology of the rhizosphere community toward what the authors describe as a healthy state. In microbial ecology, network topology refers to the pattern of correlations and interactions among species; healthy, disease-suppressive soils tend to show more complex, stable, and cooperative networks, whereas diseased soils often exhibit simplified or pathogen-dominated interaction structures. The finding that a fungal inoculant can actively restructure these ecological networks adds a sophisticated dimension to biocontrol that goes far beyond the simple idea of one organism killing another.

Among the taxa reshuffled by the treatment, one bacterium stood out. Sphingomonas, detected as the amplicon sequence variant ASV30598, was found exclusively in the co-inoculation treatment and was negatively correlated with disease incidence in the team’s key taxa analysis. In other words, the more Sphingomonas present, the less disease the plants suffered. Sphingomonas species are increasingly recognized as plant-beneficial bacteria with capacities for degrading pathogen cell walls, producing protective metabolites, and priming host immunity. Its appearance only when both Trichoderma strains were present suggests that the fungal partnership created ecological conditions, perhaps through altered root exudation, competition, or signaling, that specifically welcomed this protective bacterium into the root zone, adding a recruited ally to the defense system.

To move from correlation to mechanism, the researchers deployed two complementary analytical tools. Partial Least Squares Path Modeling, or PLS-PM, is a statistical framework that tests hypothesized chains of influence among latent variables, allowing the team to quantify how strongly different factors drive disease outcomes. Random Forest analysis, a machine learning method, ranks predictors by their power to explain variation in a response. Together, these analyses indicated that bacterial and fungal community structures and oxidative stress markers were linked to Phytophthora incidence, while fungal community structure, oxidative stress markers, and growth indicators were associated with plant biomass. The modeling thus ties the two halves of the study together, showing that the microbiome-driven modulation of the host’s antioxidant responses underlies both the suppression of blight and the promotion of growth.

The implications for agriculture are considerable. Phytophthora capsici has a broad host range, attacking peppers, tomatoes, cucurbits, and other solanaceous and cucurbit crops, and it produces long-lived oospores that persist in soil for years, making crop rotation only partially effective. Chemical controls face increasing regulatory pressure and resistance problems, and the pathogen community itself is the subject of ongoing research on fundamental biology and disease management. A biological strategy that combines two compatible Trichoderma strains, primes the plant’s own enzymatic defenses, and recruits beneficial bacteria such as Sphingomonas offers a multi-layered, self-reinforcing form of protection that is harder for the pathogen to circumvent than any single-acting fungicide. The approach also aligns with a broader shift in plant pathology toward viewing disease suppression as an ecosystem property rather than a one-on-one battle.

The study builds on a growing body of evidence that rhizosphere microbiomes can be deliberately steered toward disease suppressiveness. Previous work by some of the same authors showed that Trichoderma harzianum prevents red kidney bean root rot by increasing plant antioxidant enzyme activity and regulating the rhizosphere microbial community, and other research groups have demonstrated that microbial inoculants drive disease suppression and rhizosphere modulation in pepper Phytophthora blight. What the new study adds is a quantitative, path-modeled link between community structure, oxidative stress physiology, and both disease and growth outcomes, along with the identification of a strain-combination effect that outperforms single-strain applications. The raw sequencing data have been deposited in the NCBI Sequence Read Archive under accession numbers PRJNA1345668 for bacteria and PRJNA1345672 for fungi, allowing other researchers to verify and extend the findings. If the results translate from pots to fields, the humble pairing of two Trichoderma strains could become a cornerstone of sustainable pepper production, turning the root zone itself into the front line of disease defense.

Subject of Research: Biological control of red pepper Phytophthora blight using combined Trichoderma harzianum strains that modulate plant antioxidant defenses and rhizosphere microbiome structure

Article Title: Trichoderma harzianum lowers red pepper Phytophthora blight incidence and promotes plant growth by inducing enzymatic antioxidant defenses and modulating rhizospheric microbial community structure

Article References: Guo, Z., Wang, R., Hou, X., Yin, J., Meng, Q., Yang, Z., Zhang, Q., & Yan, M. (2026). Trichoderma harzianum lowers red pepper Phytophthora blight incidence and promotes plant growth by inducing enzymatic antioxidant defenses and modulating rhizospheric microbial community structure. Plant and Soil. https://doi.org/10.1007/s11104-026-08518-3

Image Credits: AI Generated

DOI: 10.1007/s11104-026-08518-3

Keywords: Trichoderma harzianum, Phytophthora blight, red pepper, biological control, antioxidant enzymes, catalase, rhizosphere microbiome, Sphingomonas, soil-borne disease, amplicon sequencing, plant growth promotion, microbial co-inoculation