Natural occurrence of fire blight in USDA apple germplasm collection after 10 years of observation.
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. All of the available control methods for fire blight (Erwinia amylovora) have been used diligently to keep the orchards as healthy as possible, but given the diversity of the collection, this has not always been possible. For example, in managing blossom blight, there is often a 5- to 6-week interval from early to late blooming cultivars. In addition, shoot blight is very prevalent in susceptible accessions grown on vigorous seedling rootstock. 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). Each accession is given a rating of 1-5 (1: no occurrence of infection, to 5: consistent heavy infection annually). 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 collected from eight ecosystems in Kazakhstan were rated over a 4-year period: 25% had shoot blight (rating of 2-5) and 5% of trees died of fire blight. Fire blight incidence in the eight Kazakhstan populations ranged from 0 to 43%. Although a rating of 1 for shoot blight from natural infection does not prove that an accession is resistant, it is a good indicator for further studies. 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 rated on a different scale from 1-5 (1: no infection, to 5: very heavy infection), 45% were rated 4-5 and 37% were rated 1-2. Cryogenic storage of dormant apple buds now serves as a back-up to 85% of the orchard collection, avoiding complete loss of accessions due to fire blight. Survival of the orchard collection has also been improved with phased re-propagation on fire blight-resistant EMLA 7 rootstock.