Phenylalanine Treatment Suppresses Necrotrophic and Biotrophic Pathogens in Mono- and Dicotyledonous Plants

L-phenylalanine (Phe) is converted via the phenylpropanoid pathway into phenolic compounds, and elevated phenolic/flavonoid levels are generally associated with enhanced plant resistance. We previously showed that exogenous Phe suppresses the necrotrophic fungus Botrytis cinerea in petunia, chrysanthemum, determinant tomato, and several postharvest diseases. Here, we expanded evaluation to a broad range of pathosystems, including an indeterminate greenhouse tomato cultivar, multiple dicot species, and the monocot wheat. Phe, applied as spray or drench across concentrations, was consistently effective at [≥]4 mM. It reduced disease severity in diverse systems: Sclerotinia sclerotiorum (tomato, sweet basil, cucumber, lettuce), Leveillula taurica and Oidium neolycopersici (tomato), Podosphaera xanthii (cucumber), wheat foliar pathogens (Blumeria graminis f. sp. tritici, Zymoseptoria tritici, Puccinia triticina, P. striiformis f. sp. tritici), the oomycetes Pseudoperonospora cubensis (cucumber leaves) and Pythium aphanidermatum (roots), bacterial pathogens (Pseudomonas syringae pv. tomato and Clavibacter michiganensis subsp. michiganensis) and Tomato brown rugose fruit (ToBRFV) that belongs to the Tobamovirus genus. Phe was effective on both young and mature leaves and often performed comparably to chemical fungicides; combinations rarely improved control, except for enhanced activity with pyrimethanil against B. cinerea in tomato. Synergistic effects were observed when Phe was combined with an adjuvant against tomato powdery mildews. A formulated product (NaturaFend 550 SP) was more effective than non-formulated Phe in B. cinerea (tomato) and P. xanthii (cucumber). Application timing also influenced efficacy, with treatment 5 days before infection providing superior control compared with 0, 3, or 7 days. Overall, Phe effectively controlled biotrophic and necrotrophic fungi, oomycetes, bacterial pathogens and a virus across diverse crops under experimental greenhouses and commercial like conditions.