Observations on cytological and ultrastructural changes in Plasmopara viticola oospores were carried out during the overwintering period. Three types of oospores were observed. Type I, characterized by a thin inner oospore wall (IOW), large lipid globules and two nuclei, was recovered only in samples collected in October. These oospores were considered to be immature. Maturation occurred during November and involved a noticeable increase in thickness of the IOW, fusion of nuclei, formation of an ooplast and break up of large lipid globules into smaller ones (type II oospores). A few oospores (type III) showed abnormal organization with very large lipid globules and less frequently discernible nuclei. IOW solubilization, dissolution of the ooplast and lipid globules and nuclear division were the first detectable events during oospore germination. Germinating oospores produce a germ tube which was terminated by a sporangium. In its young stage, the sporangium had a thick wall and an unusual multi-layered membrane. During this phase, nuclear divisions took place in the sporangium. While sporangium development progressed, the ribosome density in the cytoplasm decreased and mitochondria, initially roundish with evident cristae, became their usual tubular profile. The plasma membrane had a typical structure and storage organelles, such as finger print vacuoles and lipid globules, became more numerous in the cytoplasm. Larger vacuoles contained the flagella of differentiating zoospores.

Gallus cultures of two susceptible Castanea sativa cultivars, 'Garrone rosso' and 'Glone 71' were grown on culture medium supplemented to 50% with culture filtrates (GEs) from E4 virulent (E4-V) or E13 hypovirulent (E13-H) Cryphonectria parasitica strain, respectively. E13-H GEs caused a reduction in fresh weight and an increase in dry weight on calli of both genotypes. E4-V GEs did not induce any modification of these parameters compared with the control, but fresh weight of 'Glone 71' was reduced. Eight and electron microscopy observation showed the presence of large osmiophilic aggregates in the vacuoles and, sometimes, the ruptured tonoplast in the E4-V treated calli. Galli grown on E13-H GEs had a meristematic-like appearance, with small, scarcely vacuolated cells containing altered mitochondria and plasmaletnma proliferations.