Comparison of Efficiency of Indices for Fire Blight Susceptibility Evaluation in Quince (Cydonia oblonga Mill.) in Orchard Condition

This research was conducted in 2009 and 2010 to survey the efficiency of indices used for fire blight resistance evaluation on 30 genotypes from national quince collection of Kamalshahr, Karaj, Iran in orchard condition. Disease susceptibility was monitored by three indices including index of Beltsville, developed by USDA (IUSDA), index of susceptibility of varieties (ISV) and index of frequency (IF), and the results were compared then with total percentage of fire blight damage on trees (IT). IF had the highest correlation with IT, but each single index also demonstrated various efficiency for evaluation of real disease damage and severity on the quince trees, therefore, due to the variant results, the genotypes were classified differently in resistant classes. In order to obtain more reliable judgment on the susceptibility of quinces by accumulated indices, cluster analysis of the results was found to be more effective scale for assessment of susceptibility compared to a single index. Based on these results, quince genotypes derived from Guilan province showing higest level of fire blight resistance were identified as the most suitable genotypes for using as source of resistance in breeding programs.