A Study on Measuring the Resistance of the Plus Tree to the Canker of Poplar

11 plus trees were selected in 1984 from canker disease regions of the natural hybrids of P. simonii × P. italica. In order to test the effects of plus selection and supply new resistant clones to disease regions as soon as possible, the differences of the plus trees' resistance to Dothiorella gregaria Sacc. were evacuated according to their pathogenic rate, pathogenic index and pathogenic velocity index in laboratory for three years by inoculating the disease on the artificially wounded bark. The results suggests that all of the plus tree are more resistible to the disease than P. pyramidalis × P. cathayana cv. 'Beijingensis', the most resistant clones of which are Liaoning No.5 and Liaoning No.11.