Potential of Three Forms of Ascorbate as Fungicides Against the Blast Pathogen, <i>Pyricularia oryzae</i>

ABSTRACT Reactive oxygen species (ROS) are important as signal molecules in the pathogenicity of the blast fungus Pyricularia oryzae ; therefore, the ROS signalling pathway could provide potential targets for developing fungicides. Ascorbates can degrade ROS produced in fungal cells, and so three forms of ascorbate, l ‐ascorbic acid (AsA), dehydroascorbic acid (DHA) and sodium ascorbate (AsA‐Na), were assessed for antifungal effects. All three inhibited mycelial growth in vitro at 20 mM, and AsA and DHA disrupted typical linear growth of hyphae. Because polarised growth of hyphae is regulated by Tea genes, the expression of three PoTea genes in mycelium exposed to AsA and DHA was analysed. These analyses showed that (1) all PoTea genes in nontreated hyphae had a circadian‐like pattern of expression and (2) circadian‐like regulation of each PoTea gene was considerably disrupted after exposure to AsA and DHA. Expression of the circadian clock gene PoFrq was similarly affected, suggesting that the circadian rhythm in P. oryzae was disrupted by 20 mM AsA and DHA, resulting in inhibition of hyphal growth. As expected, the three ascorbates significantly suppressed infection of barley. DHA was also shown to act as a plant defence activator against the blast pathogen.