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Title: NATURAL OCCURRENCE OF FIRE BLIGHT IN USDA APPLE GERMPLASM COLLECTION AFTER 10 YEARS OF OBSERVATION

Author
item Forsline, Philip
item ALDWINCKLE, HERB - CORNELL UNIVERSITY

Submitted to: Acta Horticulture Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/10/2002
Publication Date: 5/1/2002
Citation: FORSLINE, P.L., ALDWINCKLE, H. NATURAL OCCURRENCE OF FIRE BLIGHT IN USDA APPLE GERMPLASM COLLECTION AFTER 10 YEARS OF OBSERVATION. THE 9TH INTERNATIONAL WORKSHOP ON FIRE BLIGHT, NEW ZEALAND. ACTA HORTICULTURE PROCEEDINGS 590:351-357. 2002.

Interpretive Summary: The objectives of this study were three fold: 1) record natural incidence of fire blight on shoots of 2351 clonal accessions over a 10-yr period; 2) a separate study on 708 accessions that were observed for incidence of blossom blight in the severe blossom blight epidemic of 2000; and 3) record of fire blight in the four-year old M. sieversii seedling block. For shoot blight severity, 2351 accessions of Malus have been rated and nearly 50% were very susceptible. Over 60% were rated from moderate to very heavy and 25% have shown no infection in the 10-yr period. For the on-yr observation of blossom blight on 708 accessions, 44% showed moderate to extremely susceptible levels and 22% showed no infection. Although 1/3 of the collection showed no infection, this does not indicate resistance. These results determine a baseline level for further studies to determine if those accessions with no symptoms are resistant. The Kazakhstan seedlings of M. sieversii have shown infection on 25% of the collection after four years in the field. The results presented in this paper have been used to learn more about how to most effectively manage the Malus germplasm collection. We have used three approaches to improve management of the collection since our major challenge has been minimizing fire blight in the collection: 1) cryogenic storage to ensure against losses of accessions; 2) use of fire blight resistant, semi-dwarf rootstocks to better manage the field collection; and 3) application of a growth regulator (prohexadione calcium 'Apogee') to reduce vegetative growth which also reduces the incidence of shoot blight.

Technical Abstract: The USDA, ARS Plant Genetic Resources Unit (PGRU) apple collection consists of 2351 clonally-propagated accessions planted in orchard sites with duplicates of each accession grafted on domestic apple seedling and on EMLA 9 rootstocks. There is often a 5-6 week interval from early to late blooming cultivars that presents challenges for early season control of fire blight in blossoms. In addition, shoot blight throughout the season i very prevalent in susceptible accessions grown on vigorous seedling root- stock. We recorded natural incidence of shoot blight on the entire collection from 1990 to 2000. These records are in the USDA/ARS GRIN database system with the fire blight descriptor: (http://www.ars-grin.gov/gen/apple.html). Annually, each accession is given a rating of 1-5 (zero infection to very heavy). Records are listed for 2351 accessions with 46% rated as 5 and 25% rated as 1. In addition to the clonal accessions, 1151 seedlings of Malus sieversii from Kazkhstan were rated over a 4-yr period: 25% had shoot blight (rating of 2-5) and 5% of trees died of fire blight. Because streptomycin has been used regularly at bloom, blossom blight incidence is a less reliable indicator of resistance. Nevertheless, in 2000 a severe blossom blight infection occurred when a streptomycin application was missed. For 708 accessions that had heavy bloom and were also rated 1-5: 45% were rated 4-5 and 37% were rated 1-2. Cryogenic storage of dormant apple buds now serves as a back up to the collection. Survival of the orchard collection has also been improved with phased repropagation on fire blight-resistant EMLA 7 rootstock.