Submitted to: HortScience
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/20/2005
Publication Date: 6/20/2006
Citation: Sardani, M., Spotts, R.A., Calabro, J.M., Postman, J.D., Qu, A.P. 2006. Evaluation of the USDA National Clonal Pyrus Germplasm Collection for Resistance to Podosphaera leucotricha. HortScience. 41(3):717-720. Interpretive Summary: Powdery mildew is a disease that causes economic damage in pear orchards throughout the world. Fruit from infected trees may be unmarketable due to russeting of the skin, and control of the disease using fungicides can be expensive. The use of mildew resistant cultivars can provide an environmentally sustainable method of control. Genetic resistance can reduce the cost of fruit production. This project sought to identify sources of mildew resistance in a diverse group of pears at the USDA-ARS National Germplasm Repository in Corvallis. Potted trees were inoculated in a greenhouse during the first year, and placed in an infected orchard the next year. Mildew symptoms were more severe in the greenhouse than in the orchard. European pears were more susceptible to mildew than Asian pears. Symptoms were more severe when in the greenhouse, than in the field. Asian, European and hybrid cultivars that remained completely free of powdery mildew in the greenhouse and field were identified. These cultivars may be useful for commercial fruit production or may be promising as parents for breeding disease resistant pears.
Technical Abstract: Powdery mildew (PM) occurs worldwide and is prevalent on susceptible cultivars wherever pears are grown. It causes economic losses due to russeted fruit and increases the need for fungicides. A core subset of the Pyrus germplasm collection at the USDA National Clonal Germplasm Repository in Corvallis, Ore., was evaluated for resistance to Podosphaera leucotricha, the causal agent of PM, using greenhouse and field inoculations of potted trees. The core collection consists of about 200 cultivars and species selections, representing most of the genetic diversity of pears. The core includes 31 Asian cultivars (ASN), 122 European cultivars (EUR), 9 EUR × ASN hybrids and 46 pear species selections. Three trees of each core accession were grafted on seedling rootstocks. In 2001–02, trees were artificially inoculated in a greenhouse, grown under conditions conducive for PM, and evaluated for symptoms. The same trees were subsequently evaluated for PM symptoms from natural field infections during 2003 and 2004. In the greenhouse, 95% of EUR and 38% of ASN were infected with PM. Average PM incidence (percent of leaves infected) in the greenhouse was much higher than incidence in the field during 2003. In the field, 42% of EUR and 23% of ASN were infected with P. leucotricha in 2003. Incidence in 2004 was about half that of 2003. Percent of leaves infected in ASN and species selections in the field was significantly less than EUR. Accessions with consistently low PM ratings should be very useful as parents in pear breeding programs to develop improved cultivars with high PM resistance.