Studies On Inhibitory Effects Of Pyraclostrobin On Penicillium Italicum

Blue mold caused by Penicillium italicum is one of the most serious postharvest diseases of citrus.At present,resistance of P.italicum to widely used fungicides prochloraz and imazalil has been reported.Therefore,fungicides with different mode of action from are needed to control blue mold.Pyraclostrobin is a quinone outside inhibitor（Qo I）fungicide and has not been registered for the control of citrus blue mold.In this thesis,we determined the effects of pyraclostrobin on the permeability and integrity of cell membranes,reactive oxygen species content and antioxidant enzyme activity of P.italicum,and explored the risk and molecular mechanism of resistance to pyraclostrobin.The main results are as follows:1.The effective concentration for 50%inhibition(EC₅₀)values of pyraclostrobin against 131 strains of P.italicum ranged from 0.0018 to 0.0132μg/m L,and the average EC₅₀value was 0.0067±0.0021（SD）μg/m L.These EC₅₀values fitted a normal distribution（P=0.053）,and no field-resistant strains were found.2.Pyraclostrobin significantly（P<0.01）reduced the permeability of cell membrane of P.italicum.However,experiments with propidium iodide showed that pyraclostrobin destroyed the integrity of cell membrane of P.italicum spores.3.Pyraclostrobin at 0.02μg/m L increased the content of O₂^-and H₂O₂in P.italicum mycelia by 413%（P<0.001）and 69.4%（P=0.047）,respectively,whereas reduced the activities of superoxide dismutase（SOD）and peroxidase（POD）by 49%（P=0.001）and73.9%（P=0.002）,respectively.4.After inducing with pyraclostrobin and ultraviolet for 10 consecutive generations,resistance factors of 397-1466 were attained.However,after culturing on potato dextrose agar（PDA）for 5 successive generations,resistance factors declined to 42-652,indicating that pyraclostrobin resistance could not be inherited stably.The target gene Cytb was sequenced and results showed that the base guanine（G）at the point 155 downstream of the coding region was mutated to thymine（T）in the resistant mutants.5.There was negative cross resistance between pyraclostrobin and prochloraz on PDA plate（R=-0.520,P=0.004）;but pyraclostrobin was less effective against prochloraz resistant strains than against sensitive strains,so there was no negative cross resistance on citrus.At the same concentration,the control efficacy of pyraclostrobin on prochloraz resistant strains was significantly higher than that of prochloraz,indicating pyraclostrobin could be used to control prochloraz high resistant strains of P.italicum.In summary,pyraclostrobin has a high biological activity against P.italicum.These studies have important implications for in-depth understanding the mode of action of pyraclostrobin and for future registration and scientific application of this fungicides to control blue mold of citrus.