Abstract
Three pear and three apple cultivars with different susceptibility to fire blight were used in this experiment. Artificial inoculations were carried out on orchard trees during 1991 in south-west Germany. Disease assessment was made over time to obtain disease progress curves. Symptomp development on pear cultivars was faster than on apple cultivars and reached to a higher asymptote. The monocyclic experimentation with Erwinia amylovora on pear and apple shoots resulted in disease progress curves that fit well by the monomolecular growth model. In this experiment Ymax, Yo and r were decreasing together with the area under the disease progress curve (AUDPC) as resistance was increasing in cultivars. The longest latent period was associated with resistant cultivars. Because AUDPC summarizes plant disease development, it should be used in the screening programs as one of the criteria for determining the varietal level of resistance in relation to standard cultivars. We suggest this epidemiological approach, especially in the later stages of the screening program where a few number of varieties are used, for blight resistance in pear and apple breeding programs. From this more accurate determination of relative resistance, selections and better comparisons of varieties for resistance to blight will result.
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