MICROSCOPIC STUDIES OF SUSCEPTIBILITY AND RESISTANCE SYMPTOMS EXPRESSED ON TWO CULTIVARS OF PYRUS COMMUNIS INOCULATED WITH VENTURIA PIRINA

Scab is an important disease in pear orchards caused by an ascomycetous fungus, Venturia pirina. Young leaves, stems and fruits at all stages of their development are susceptible. Breeding has been undertaken to obtain new hybrids with a good level of resistance to V. pirina. In this context, a better understanding of the host-parasite relationship is necessary. A microscopic study of susceptibility and resistance symptoms on leaves and stems was undertaken on a susceptible cultivar (‘Angelys’) and a resistant cultivar (‘Navara’). Cultivars were grafted onto the rootstock ‘Kirchensaller’, grown in greenhouse conditions and inoculated with a mixture of local strains of V. pirina. The samples were prepared for confocal micoscopy, scanning electron microscopy and histological studies. On ‘Angelys’ and ‘Navara’, the early stages of host-parasite relationships observed by confocal microscopy are similar. A subcuticular primary stroma develops slowly on the susceptible host, ‘Angelys’. Often hyphae of the fungus progress deeply between epidermal cells in leaves and stems. The leaf and stem tissue cells show no important changes. Sporulation occurs on leaves 12 to 14 days and on stems 18 to 21 days after inoculation. On the resistant host ‘Navara’, conidia germinate and form an appressorium, but only a reduced primary subcuticular stroma is observed on the leaf. The cultivar ‘Navara’s’ resistance reaction is of the hypersensitive type. Microscopic analysis shows collapse and necrosis of the epidermal cells; the underlying parenchyma cells undergo important changes. The susceptibility and the resistance symptoms observed on pear leaves with V. pirina are very similar to the symptoms observed with Venturia inaqualis on apple trees but severe symptoms are observed on pear stems with destruction of superficial tissues and deep changes in the cortical parenchyma. The symptom of resistance on stems appears later and is very different from those observed on leaves.