Submitted to: Acta Horticulturae
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/1/2004
Publication Date: 4/20/2005
Citation: Sardani, M., Spotts, R.A., Calabro, J.M., Postman, J.D. 2005. Powdery mildew resistance in pyrus germplasm. Acta Horticulturae. 671:609-613. 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 controlling this disease, and when combined with resistance to other diseases, genetic resistance can reduce the cost of fruit production. This project sought to identify sources of mildew resistance in a genetically diverse group of pears at the USDA-ARS National Germplasm Repository in Corvallis. Potted trees were inoculated in a greenhouse 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 found to be more susceptible to mildew than Asian pears. About half of the European and 75% of the Asian cultivars showed resistance to mildew in the field. Following greenhouse inoculations, only 7% of European but almost half of Asian cultivars were resistant to the disease. In addition to the many resistant Asian cultivars, six European cultivars and several hybrid cultivars were identified that remained completely free of powdery mildew following both greenhouse and field inoculations. Some cultivars with low powdery mildew ratings may be useful for commercial fruit production and others may be promising as parents for breeding new disease resistant pears.
Technical Abstract: 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.'