OBSERVATIONS ON PYRUS COMMUNIS RESISTANCE TO PSYLLA PYRI

Spring growth, as well as growth rate during the season, influenced susceptibility much more than leaf morphology. We did not find immunity to Psylla. However, preferences for oviposition and various degrees of host resistance were pointed out by differences in nymph/egg ratio and in the intensity of honeydew, sooty mould and defoliation.

Unfortunately, these factors were not grouped within the same tree. This fact calls for a breeding programme in P. communis, using the available material.

Psylla pyri is now the major pest of pear. Several expensive sprays are yearly requested in order to keep it under control and these are becoming less successful as insect resistance to chemicals is progressing /l/.

Some Asiatic pear for species appear to be less atractive than Pyrus communis for insect oviposition. P. fauriei and P. calleryana appear the most resistant on nymph/egg ratio ratings /8/. When P. ussuriensis was crossed with P. communis 60 % of the seedlings were resistant /4/ and oviposition was reduced and nymphal development retarded /3/. Resistance could be also related to physiological or morphological characteristics /5, 9/. It would be useful to determine the mechanisms of low susceptibility or resistance in P. communis for breeding purposes. The insect showed no preference for oviposition on "Spina Carpi", an old Italian cultivar, when other cultivars were present /6/. Two of us have already reported /7/ on pear seedlings with low susceptibility to Psylla which were selected from a population of 5498 individuals within 18 progenies. During the selection period /1970–78/ the trees did not receive any spray; they were evaluated for the traditional agronomic traits/early bearing, yield, fruit characters/ and, in the last four years, for their resistance to scab /Venturia

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