5-Aminolevulinic acid against strawberry Fusarium wilt: Bidirectional regulation of biocontrol agents and pathogens

Strawberry Fusarium wilt (SFW) is a systematic soil-borne disease caused by Fusarium oxysporum f. sp. fragaria (Fof), which infects the vascular bundles, blocking water and nutrient transport from roots to the aboveground. It is a severe pathogen which spreads rapidly and destroys strawberry production. Finding a way to control this disease is of great scientific value and practical importance. In this study, three fungi were isolated from the vascular tissues of sick strawberries in the field. After DNA sequencing, they were identified as Fof, Aspergillus fumigatus and Trichoderma harzianum, respectively, among which the first two are pathogens and the third is a probiotic. All fungi were controlled by thiophanate-methyl (TM), a commercial fungicide. On PDA medium, 20 mg·L⁻¹5-aminolevulinic acid (ALA), a natural non-protein amino acid, promoted T. harzianum proliferation, but inhibited Fof and A. fumigatus. In confrontation test, the growth of Fof or A. fumigatus was inhibited by T. harzianum and exogenous ALA promoted T. harzianum growth but significantly inhibited the pathogen growth. When three species of fungi were separately or combinedly inoculated on healthy strawberry plants, T. harzianum promoted plant growth and development while Fof or A. fumigatus caused growth retardation, where Fof directly caused leaf yellowing and plant wilting. When the plants inoculated with different fungus were treated with ALA, the results turned out that ALA alleviated SFW symptoms by bidirectionally promoting T. harzianum proliferation and inhibiting Fof and A. fumigatus. Thus, ALA might be used in comprehensively controlling SFW in strawberry industry.