The use of synthetic fungicides to control fungal diseases has growing limitations due to eco-toxicological risks. Therefore, it is necessary to replace or integrate high risk chemicals with safer tools for human health and environment. Consequently, research on the selection, evaluation, characterization, and use of biocontrol agents (BCAs) has consistently increased in the last decades. BCA formulates, particularly in some countries, are still scarce in coping with the growing demand for their use in sustainable agricultural management. To foster development and utilization of new effective bioformulates, there is a need to optimize BCA activity, to share knowledge on their formulation processes and to simplify the registration procedures. Studies based on new molecular tools can significantly contribute to achieve such objectives. The present review provides the state of the art on biocontrol of fungal plant diseases with special emphasis on (i) features of the most studied BCAs; (ii) key strategies to optimize selection and use of BCAs (iii); mechanisms of action of the main BCAs; (iv) molecular tools and metagenomic studies in the selection and use of BCAs; (v) main issues and constraints in the registration and commercialization of BCAs, and (vi) perspectives in the biocontrol of fungal plant diseases.

Fusaric acid (FA) is one of the first secondary metabolites isolated from phytopathogenic fungi belonging to the genus Fusarium. This molecule exerts a toxic effect on plants, rhizobacteria, fungi and animals, and it plays a crucial role in both plant and animal pathogenesis. In plants, metal chelation by FA is considered one of the possible mechanisms of action. Here, we evaluated the effect of different nitrogen sources, iron content, extracellular pH and cellular signalling pathways on the production of FA siderophores by the pathogen Fusarium oxysporum (Fol). Our results show that the nitrogen source affects iron chelating activity and FA production. Moreover, alkaline pH and iron limitation boost FA production, while acidic pH and iron sufficiency repress it independent of the nitrogen source. FA production is also positively regulated by the cell wall integrity (CWI) mitogen-activated protein kinase (MAPK) pathway and inhibited by the iron homeostasis transcriptional regulator HapX. Collectively, this study demonstrates that factors promoting virulence (i.e., alkaline pH, low iron availability, poor nitrogen sources and CWI MAPK signalling) are also associated with increased FA production in Fol. The obtained new insights on FA biosynthesis regulation can be used to prevent both Fol infection potential and toxin contamination.

The aim of the work was to study the biological interference of the spontaneous colonization of pathogenic and saprophytic endophytes on the nitrogen assimilation of mycorrhized wheat plants cultivated in soils deficient in N and P. The nitrogen assimilation efficiency of mycorrhized plants was determined by measuring the activities of nitrate reductase assimilatory and glutamine synthetase enzymes and free amino acid patterns. Mycorrhizal plants at two different sites showed an assimilative activity of nitrate and ammonium approximately 30% greater than control plants. This activity was associated with significant increases in the amino acids Arg, Glu Gln and Orn in the roots where those amino acids are part of the inorganic nitrogen assimilation of mycorrhizal fungi. The nutrient supply of mycorrhizal fungi at the root guaranteed the increased growth of the plant that was about 40% greater in fresh weight and 25% greater in productive yield than the controls. To better understand the biological interaction between plant and fungus, microbiological screening was carried out to identify colonies of radicular endophytic fungi. Fourteen fungal strains belonging to nine different species were classified. Among pathogenic fungi, the genus Fusarium was present in all the examined roots with different frequencies, depending on the site and the fungal population present in the roots, providing useful clues regarding the principle of spatial conflict and fungal spread within the root system.

The active regulation of extracellular pH is critical for the virulence of fungal pathogens. Penicillium expansum is the causal agent of green-blue mold on stored pome fruits and during its infection process acidifies the host tissues by secreting organic acids. P. expansum is also the main producer of patulin (PAT), a mycotoxin found in pome fruit-based products and that represents a serious health hazard for its potential carcinogenicity. While it is known that PAT biosynthesis in P. expansum is regulated by nutritional factors such as carbon and nitrogen and by the pH, the mechanistic effects of biocontrol on PAT production by P. expansum are not known. In this work, we assessed how optimal and suboptimal concentrations of the biocontrol agent (BCA) Papiliotrema terrestris LS28 affect both extracellular pH and PAT biosynthesis in P. expansum. In wounded apples, the optimal and suboptimal concentrations of the BCA provided almost complete and partial protection from P. expansum infection, respectively, and reduced PAT contamination in both cases. However, the suboptimal concentration of the BCA increased the specific mycotoxigenic activity by P. expansum. In vitro, the rate of PAT biosynthesis was strictly related to the extracellular pH, with the highest amount of PAT detected in the pH range 4–7, whereas only traces were detectable at pH 3. Moreover, both in vitro and in apple wounds the BCA counteracted the extracellular P. expansum-driven acidification maintaining extracellular pH around 4, which is within the pH range that is optimal for PAT biosynthesis. Conversely, in the absence of LS28 the pathogen-driven acidification led to rapidly achieving acidic pH values (<3) that lie outside of the optimal pH range for PAT biosynthesis. Taken together, these results suggest that pH modulation by LS28 is important to counteract the host tissue acidification and, therefore, the virulence of P. expansum. On the other hand, the buffering of P. expansum-driven acidification provided by the BCA increases the specific rate of PAT biosynthesis through the extension of the time interval at which the pH value lies within the optimal range for PAT biosynthesis. Nevertheless, the antagonistic effect provided by the BCA greatly reduced the total amount of PAT.

The nematicidal potential of the yeast Papiliotrema terrestris strain PT22AV (YSY) was investigated against the root nematode (RKN) Meloidogyne incognita in in vitro bioassays on infective juveniles (J2) and experiments on tomatoes in pot and greenhouse conditions. The J2 nematodes were exposed to YSY solutions for 19 days, using abamectin (ABA), fosthiazate (FOS) and distilled water as controls. In the experiments on potted and greenhouse tomatoes, 0.5 and 1 kg ha⁻¹ doses of YSY were tested in comparison to ABA, biocontrol agents Purpureocillium lilacinus strain 251 (PUL) and Bacillus firmus strain 1-1582 (BAF), a plant biostimulant/fertilizer (ERG) and the nematicide Fluopyram (FLU). J2’s viability was affected by YSL after 7 days, decreasing to zero on the 15th exposure day, while ABA and FOS resulted in 83 and 100% J2 mortality within 24 h. Only the 1.0 kg ha⁻¹ dose of YSY was able to significantly reduce the final nematode population in soil and gall formation on tomato roots, without significant differences from PUL and BAF. All treatments in comparison also resulted in a significant increase in tomato growth and crop yield, except for 0.5 kg ha⁻¹ of YSY. Data indicated that YSY could represent an additional tool for organic and integrated RKN management.

Management of Xylella fastidiosa subsp. pauca (Xfp) presents significant challenges because of the lack of genetic resistance in major crops, pathogen variability and resistance development against the main antimicrobial compounds, and the lack of effective chemical compounds for control. Traditional bactericides in the EU are restricted, and copper-based products are increasingly limited because of environmental impacts. In this study, the antibacterial side effects of some commercial fertilizers against Xfp were evaluated. These products, containing metal ions (mainly copper and/or zinc) complexed with phosphites and bioavailable silicon, show antimicrobial and bactericidal activities in vitro, which suggest their possible use to control the pathogen, as well as enhanced plant resistance to biotic and abiotic stresses. Greenhouse and open field trials showed that they can significantly reduce disease severity and pathogen population, thus improving fruit yield. The integrated pest and disease management strategy, which combines these treatments with agronomic and phytosanitary vector-control measures, leads to a reduction of disease symptoms in treated plants compared with untreated ones. These findings highlight the potential of systemic fertilizers to mitigate Xfp symptoms by providing antimicrobial collateral effects and probably enhance plant defenses, since similar compounds are known to be plant defense inducers in other pathosystems. Therefore, these products offer a sustainable solution for managing plant pathogenic bacteria and improving crop health and yield. This approach is crucial for the sustainability of olive production in Xf-affected areas. However, further research is needed to optimize these strategies for long-term effectiveness under field conditions. Overall, the results emphasize the value of the collateral effects of some mineral fertilizers in a comprehensive strategy aimed at mitigating the symptoms of Xfp and safeguarding the agricultural productivity as well as the invaluable heritage of centuries-old Apulian olive trees. [ ] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .