Histological and cytological studies of the interaction between apple selections carrying the resistance Vm and compatible and incompatible Venturia inaequalis strains

1994 
Resistance overcome by a new race of pathogen is today one of the greatest preoccupations of breeders. The resistance of Vm gene was overcome and the Vf gene was also broken down under certain conditions. The resistance of Vm gene, present in a Malus micromalus clone, was overcome by race 5 of Venturia inaequalis; this gene constitutes outstanding material for studying host-parasite interactions. Consecutive inoculation of resistant and susceptible hybrids with race 1 and 5 of Venturia inaequalis enables rigorous histological and cytological studies. When the resistance is overcome, susceptible and resistant symptoms coexist. At symptom expression, the conidiogenesis is restricted in comparison to the susceptible control. Host reactions are limited to the upper epidermis with abundant subcuticular stroma and very deep erosion of epidermal walls. Epidermal cells collapse irregularly and wall or organelle modifications vary from one cell to another. The resistance’s mechanisms still operate. It is possible to envisage that Venturia inaequalis has acquired sites compatible with those of the receiving host.
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