Transformation of BG2 Gene into Populus×euramericana cv.‘Neva’and Resistance of the Transgenic Poplar to Canker Disease Transformation of an Antimicrobial Peptide Gene LJAMP2 into Populus tomentosa and Resistance of the Transgenic Poplar to Canker Disease Type Ⅲ secretion system is an essential pathogenic factor in poplar canker pathogen Lonsdalea quercina