Scab resistance in progenies of varieties of the cultivated pear

Abstract

• 1. Several selections from progenies from crosses between pear varieties were found to be free from scab (Venturia pirina) in one locality but susceptible when tested at another location. It is suggested that biotypes of the fungus are sharply restricted in their range of distribution at any given time.

• 2. Due to the restricted distribution of biotypes of the pathogen, varieties are often considered to be resistant when in fact they have merely escaped the biotypes to which they are susceptible.

• 3. Resistance to pear scab is shown to be inherited quantitatively and pear varieties vary in the resistance contribution which they pass on to their progeny.

• 4. A statistical analysis of the data, using methods applicable to an incomplete diallel cross, has enabled the contribution of each of ten parent varieties to the scab resistance of its progenies to be calculated.

• 5. The occurrence of high percentages of slightly susceptible seedlings suggests that moderately resistant selections of good fruit quality should not be difficult to obtain if the parents are carefully selected. It is unfortunate that both Durondeau and Fertility, the two best parents from the standpoint of scab resistance, are not particularly suitable for breeding types with high fruit quality. Of the parents in the present series of tests Doyenn\'e du Comice and Williams' Bon Chr\'etien offer one of the best combinations, bearing fruit quality in mind, but because of the low number of resistant types given by this cross a large number of progeny would have to be grown to give worthwhile results.