Biological Activity And Action Mechanism Of Physcion Against Magnaporthe Oryzae

Rice blast,caused by Magnaporthe oryzae,is one of the most destructive diseases and a serious threat to global food security.At present,the control of rice blast relies mainly on the use of synthetic fungicides.The excessive use of synthetic fungicides has resulted in fungus resistance and environmental pollution problems.It urgently needs efficient biofungicides and other measures to control rice blast.Physcion is one of anthraquinone derivatives extracted from various plants,such as rhubarb and giant knotweed.It is effective on powdery mildew and has been developed as a botanical fungicide to control cucumber powdery mildew.The most of previous studies on this biofugicide focus mainly on obligate parasitic fungi,such as Blumeria graminis,but little on facultatively parasitic fungi.The purposes of this study were to investigate bioactivity and action mechanism of phycion on facultatively parasitic phytopathgen M oryzae,results of which would be helpful for the safe and effective application of the biofungicide.The result of bioassay in vitro showed that physcion significantly inhibited the mycelium growth of M.oryzae.The EC50 valus was 6.72 mg L-1.When physcion was applied at the concentration of 20 mg L-1 on detached leaves or 60 mg L-1 on whole plants,the control efficacy was above 90%against M.oryzae in greenhouse.The field experimental results indicated that the biofungicide could effectively control rice leaf blast and neck blast.At the tillering stage,appling physcion once at the dose of 11.25 g a.i.ha-1 or twice at 7.5 g a.i.ha-1 provided about 80%control efficacy on rice leaf blast,which was similar with the control efficacy of tricyclazole or isoprothiolane.At the panicle stage,applying physcion at the dose of 11.25g a.i.ha-1 provided 77%control efficacy on neck blast,which was significantly higher than that of tricyclazole treatment(71%).The results of simulating secondary infection in vitro tests showed that physcion could prevent the secondary infection of M Oryzae when it was applied at a certain concentration.When physcion was applied at the concentration of 40 mg L-1,it could inhibit mycelium conidiation.Even though the conidia produced from physcion-treated mycelia at this concentration,they could not germinate and then lost pathogenicity ability.When physcion was applied under the concentration of 40 mg L-1,it promoted mycelium conidiation and did not inhibit conidial gemimation and pathogenicity.Physcion directly inhibited the conidial germination,germ tube elongation and appressoriumformation in vitro,leading to deformation and decreased diameter of appressoria.In vivo,it was found that physcion seriously inhibited the conidial adhesion ability,germination and appressorium formation of M oryzae on crop leaves,which led that the conidia could not successfully penetrate leaf cuticle.In physcion treatment,membrane permeability,chitinase activity and malondialdehyde content of M.oryzae mycelia increased significantly,which indicated disruption of its cell walls,and finally resulted in the reductions of soluble protein,sugar,and pyruvate content.Moreover,physcion enhanced activities of various defense enzymes,which might induce rice resistance to the disease.There was a synergistic effect on M.oryzae when physcion was combined with tricyelazole at ratios of 9:l-5:5 and an additive effect at ratios of 5:5-1:9.Similar results were observed for the combinations of physcion and difenoconazole.These results indicated that combining physcion with triazole fungicides could increase control efi-icacy on rice blast.Additiing adjuvants methylated seed oil and nonionic surfactant Beichuang also significantly increased control effect of physcion on this disease.