POWDERY MILDEW RESISTANCE IN PYRUS GERMPLASM

Powdery mildew (PM), caused by the fungus Podosphaera leucotricha, occurs in most pear-growing areas of the world. PM causes economic losses due to reduced market value of russeted fruit as well as the increased need for fungicides. A recent study of PM resistance was done using the core Pyrus germplasm collection at NCGR-Corvallis, which consists of about 200 cultivars and species selections identified to represent most of the genetic diversity present of this crop. It includes 29 Asian cultivars (ASN), 107 European cultivars (EUR) as well as hybrids and pear species selections. Trees were evaluated for PM symptoms from natural field infections during 2003, by counting the number of leaves with symptoms on ten current year shoots. In 2001-2003, three trees of each core accession were grafted on potted seedling rootstocks, artificially inoculated in a greenhouse, grown under suitable PM conditions and evaluated for symptoms. EUR were overall more susceptible to PM than ASN, with 47% of EUR and 25% of ASN (field) and 93% of EUR and 43% of ASN (greenhouse) being infected with PM. Average PM incidence in the greenhouse (8% for ASN and 31% for EUR) was much higher compared to field infections (2% for ASN and 6% for EUR). In the field, 33% ASN and 38% of EUR with PM symptoms had a mean PM value of >10%. Symptoms were more severe in the greenhouse, with 62% ASN and 80% of EUR with PM symptoms having a mean PM value of >10%. Cultivars with consistent low PM ratings may have good promise for developing improved PM resistant cultivars in future pear breeding programs.