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.