results of screening on 89 apple cultivars for fire blight resistance using usda system in karaj conditions

fire blight disease caused by erwinia amylovora is a destructive disease of pome fruits including apple worldwide. use of resistant cultivars is the most efficient method of disease control. in the present research, we investigated genetic resistance of 89 local and imported commercial apple cultivars in natural contaminated conditions to the bacterial disease using beltsville scoring system (usda) in horticulture research station of kamalshahr, karaj (iran), during triennial observations, 2007-2009. apple cultivars differed significantly (p<1%) in their resistance to fire blight. among 89 apple cultivars under study, 19 (21%) were scored as highly resistant, 51 (57%) moderately resistant, 17 (19%) intermediate, 2 (2%) moderately susceptible and none highly susceptible. local commercial cultivars of mashhad nouri, sheikh ahmad and new released cultivar sharbati, registered in national cultivar registration office, along with imported cultivars of northern spy, hi-early, cooper spur, early red one, red chief, prime gold and red spur were scored as highly resistant. the most disease severity (p<0.01) occurred in year 2008.