Resistance of fear cultivars in Oregon to natural fire blight infection

In April 1994, severe fire blight infection occurred in a planting of 119 p ear cultivar/rootstock combinations. Infection varied from 0 to 59 strikes per tree. Of four replicate trees of each cultivar/rootstock combination, 3 4, 34, 34, 15, and 2 combinations had infection in 0, 1, 2, 3, and 4 trees, respectively. 'Jules d'Airolles' and 'Worden Seckel' on OH x F 333 had fir e blight strikes in all trees, with an average of 6.7 and 5.0 strikes per t ree, respectively. Over 80% of all fire blight strikes were visible by 9 Ma y. Most of the highly infected cultivars were at full bloom between 16 and 20 April, the time that coincided with extreme risk weather conditions. Whi le some of the noninfected cultivars were well past full bloom when risk wa s high and may have escaped infection, others were in full bloom and may po ssess genetic resistance.