Selection and Evaluation of Interspecific Hybrids of Pear Highly Resistant to Venturia nashicola

Pear scab (Venturia nashicola Tanaka et Yamamoto or Venturia pirina Aderhold) is one of the most important diseases of pears. Two hybrids, 03‐04‐034 and 03‐08‐080, highly resistant to pear scab were selected from interspecific reciprocal crosses of Pyrus × bretschneideri cv. Yali and Pyrus × ussuriensis cv. Jingbaili. The content of salicylic acid (SA) and polyphenols in the leaves of 03‐04‐034 and a highly susceptible hybrid individual were tested by high‐performance liquid chromatography. The two scab‐resistant individuals 03‐04‐034 and 03‐08‐080 were backcrossed with both of their parents, and the progenies were used for inheritance analysis of the resistance trait. The results indicated that diseased and non‐diseased progenies segregated qualitatively and that diseased was dominant over non‐diseased. The ratio of diseased/non‐diseased progenies in all of the backcross populations was not significantly different from 1 : 15 indicating the segregation of four major gene loci was involved in the variation of resistance in these hybrid populations. When susceptible subpopulations from the two crosses were assessed for the severity of infection as measured by the ratio of lesion area to total leaf area, resistance was identified as a quantitative trait. Endogenous SA content in the leaves was extensively induced as early as 3 h after inoculation of the highly resistant individual 03‐04‐034, but no significant change in leaf SA content was found in the highly susceptible individual 03‐19‐136. Polyphenolic compounds, such as phlorhizin, catechol, quercetin and rutin, began to accumulate in leaves of 03‐04‐034 several hours earlier than that 03‐19‐136. The lignin content increased 45 h after inoculation in the resistant individual 03‐04‐034, but not in the susceptible individual 03‐19‐136. These observations indicated that the genetic resistance to pear scab of this interspecific population was correlated to SA induction, earlier polyphenol accumulation and the subsequent lignification of leaf tissue.