Fire blight incidence on Malus sieversii grown in New York and Minnesota

Malus sieversii (Lebed.), a wild apple species native to Central Asia, has been recognized as the major progenitor of the domestic apple. This investigation summarizes the natural infection by Erwinia amylovora (fire blight) on 2590 M. sieversü seedlings grown as own-rooted trees. At a USDA orchard in Geneva, New York (NY), 1180 seedlings (106 populations) representing germplasm from eight sites in Kazakhstan were planted in 1997 and 1998. At a University of Minnesota (MN) orchard in Excelsior, MN, 1410 seedlings representing 124 populations from two climatically diverse sites in Kazakhstan were planted in 1998. Individuals in each population are half-sibs of each other from seeds collected from the same mother tree of wild M. sieversii. One of the Kazakh sites (site 9) selected for the MN trial was a mesic site at 47° N latitude chosen for its potential for extreme cold hardiness. The other group was collected from site 6, a relatively warm, xeric site. To compare results, 32 of the populations grown in MN were also grown in NY including 10 from site 6 and 22 from site 9. Natural infection by fire blight was recorded annually using a 5-point scale (1= no infection to 5= very severe). This rating was cumulative over years. In both MN and NY, we observed a 20% greater incidence of fire blight in the trees from Kazakh site 9 than from site 6, with many infected trees dying outright. Populations grown only in NY from the other six Kazakh sites had results similar to those from site 6. Where infection was medium to severe (3-5 rating), we defined seedlings as susceptible. We determined that 535 (45%) of the seedlings grown in NY were rated 1 or 2 and potentially resistant. To determine if these genotypes were truly resistant, scions from the field trees were grafted to seedling rootstock, and shoot tips of the resulting grafted plants were inoculated in a greenhouse. Of 289 of the genotypes checked to date, nearly 60% of this group that escaped infection in the orchard were resistant.