A primary focus of the apple rootstock breeding and evaluation program at USDA-ARS/Cornell Univ. has been to develop screening protocols to identify genotypes resistant to the fire blight bacterium (Erwinia amylovora). Direct inoculation is a simple technique, but does not represent the only mode by which rootstocks become infected in the orchard. Selection based on direct inoculation screens may, however, enrich the population for resistant genotypes. Large breeding populations from controlled crosses are shoot-tip inoculated with E. amylovora, and the fraction showing the highest levels of resistance are retained for further evaluations. These survivors are again screened through direct inoculation in the field, and the less-resistant genotypes are discarded. Following selection for other pathogen tolerance and horticultural characters, elite genotypes are multiplied through asexual propagation. Replicated tests using direct inoculation with multiple strains of E. amylovora are then used to estimate the level of fire blight resistance of elite genotypes. A final screen utilizes mature, grafted orchard trees to verify that the resistance of rootstock genotypes to fire blight is maintained under conditions simulating natural infection. Direct inoculation screening and selection have resulted in a high frequency of strong resistance to severe fire blight epidemics in recent orchard inoculation trials.
In 1998, the USDA-ARS and Cornell Univ. instituted a cooperative agreement that mobilized the resources for a jointly managed apple rootstock breeding and evaluation program. The program is a successor to the Cornell rootstock breeding program, formerly managed by Emeritus Professor of Horticultural Sciences James N. Cummins. The agreement broadens the scope of the program from a focus on regional concerns to address the constraints of all the U.S. apple production areas. In the future, the breeding program will continue to develop precocious and productive disease-resistant rootstock varieties with a range of vigor from fully dwarfing to near standard size, but there will be a renewed emphasis on nursery propagability, lodging resistance, tolerance to extreme temperatures, resistance to the soil pathogens of the sub-temperate regions of the U.S., and tolerance to apple replant disorder. The program draws on the expertise available at the Geneva campus through cooperation with plant pathologists, horticulturists, geneticists, biotechnologists, and the curator of the national apple germplasm repository. More than 1000 genotypes of apple rootstocks are currently under evaluation, and four fire blight- (Erwinia amylovora) resistant cultivars have been recently released from the program. As a service to U.S. apple producers, rootstock cultivars from other breeding programs will also be evaluated for productivity, size control, and tolerance to a range of biotic and abiotic stress events. The project will serve as an information source on all commercially available apple rootstock genotypes for nurseries and growers.
Three studies were conducted to evaluate the effect of post-infection sprays of prohexadione-calcium on the severity of naturally occurring fire blight infections on 3- and 4-year-old 'Gala' apple trees on blight-susceptible or blight-resistant rootstocks. Although post-infection prohexadione-calcium reduced the dry weight of fire blight strikes removed by pruning in one commercial orchard site, this treatment did not reduce mortality of young 'Gala' trees on M.9 or M.26 rootstocks, and did not reduce the incidence of scion or rootstock cankers on any of the rootstocks tested. We conclude that post-infection treatment with prohexadione-calcium is of no practical value in reducing fire blight symptoms on apple. Our results suggest that resistant apple rootstocks will be very valuable in increasing orchard survival in a fire blight epidemic.