RESPONSE OF F₁ INTERSPECIFIC HYBRIDS, BELONGING TO DIFFERENT ORNAMENTAL APPLE SPECIES, TO APPLE SCAB AND POWDERY MILDEW ATTACK

Ornamental species are important genetic resources for the improvement of disease resistance in apple. The response to apple scab and powdery mildew attack of F1 interspecific hybrids, belonging to different apple ornamental species, used as either maternal or paternal genitors and crossed with several cultivars was studied to establish their resistance or susceptibility. The seedlings were noted for their response to disease attack over four successive years both in natural conditions of infection and in a hybrid field where no treatments with fungicides were applied. A scale of notation ranging from zero (no attack) to five (very strong attack) was used. The mean value for apple scab infection in a group of combinations in which the ornamental Malus species were used as maternal genitors, ranged between 0.38 for M. coronaria × ‘Reinette Baumann’ F₁ offspring and 1.87 for M. zumi × ‘Reinette Baumann’ crosses respectively. The mean value for powdery mildew infection, varied between 0.42 for M. coronaria × ‘Reinette Baumann’ and 1.47 (M. zumi × ‘Reinette Baumann’ respectively. These differences were not significant among hybrid families. When Malus species were used as paternal genitors, progenies with significant resistance to apple scab in the following combinations: ‘Cluj 218/2’ × M. niedzwetzkyana (1.13), ‘Frumos de Voinesti’ × M. niedzwetzkyana (0.53), ‘Golden Delicious’ × M. prunifolia (1.00) were obtained. Totally different responses were noted in progenies derived from ‘Cluj 218/2’ × M. floribunda (3.52), ‘Frumos de Voinesti’ × M. floribunda (3.23) and ‘Reinette Baumann’ × M. floribunda (2.51). These results suggest that cytoplasmic genetic effects might be involved in the inheritance of apple resistance to powdery mildew.