Zoosporangia form and size were studied on a collection of 94 strains of Plasmopara halstedii (sunflower downy mildew). Both oval and round forms were present in all strains analysed. The proportion of two forms varied significantly according to strain and plant age but more especially to host plant genotype. Whatever the strain or host genotype, oval zoosporangia were larger than round ones, but there was no relation between the proportion of the oval form and mean zoosporangia size. There was no relation between zoosporangia form or size and race virulence profiles or aggressiveness criteria, with the possible exception of zoosporangia size and sporulation density. It is concluded that, for this obligate parasite, although form and size of zoosporangia depend on pathogen strain, these characters also vary according to growth conditions of Plasmopara halstedii, in particular to the genotype of the plant host.
