Variation in form and size of Plasmopara halstedii (sunflower downy mildew) zoosporangia

Abstract

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

Relationship between aggressiveness and zoosporangia viability inPlasmopara halstedii(sunflower downy mildew)

“…Virulence has been defined as specific disease-causing abilities and aggressiveness as non-specific diseasecausing abilities (Van der Plank 1968). In our previous studies, Sakr et al (2008) found no relation between morphology of zoosporangia and their race virulence profiles or aggressiveness criteria. However, zoosporangia viability is an important criterion that contributes with the primary and secondary P. halstedii infection processes (Meliala et al 2000;Tourvieille de Labrouhe et al 2000;Spring 2009).…”

“…This line does not carry Pl gene, but it is known to have high level of quantitative resistance (Tourvieille de Labrouhe et al 2008). Latent period was defined as the number of days of incubation necessary to obtain sporulating 1586 N. Sakr pathogen on 80% of the plants.…”

“…16, October 2011, 1585-1594 Labrouhe et al 2000). The second is quantitative resistance which is controlled by minor genes and tends to impact the rate of disease development (rate reducing) rather than producing a disease-free plant (Tourvieille de Labrouhe et al 2008).…”

“…In this pathosystem, there have been several studies on virulence (Tourvieille de Labrouhe et al 2000;Spring et al 2006;Gulya 2007;Sakr 2010) and more recently on aggressiveness (Sakr et al 2008Sakr 2009Sakr , 2010. Virulence has been defined as specific disease-causing abilities and aggressiveness as non-specific diseasecausing abilities (Van der Plank 1968).…”

Pathogenic groups identified amongPlasmopara halstediiisolates belonging to several races

“…Values of latent period and sporulation density for 50 P. halstedii isolates presented by Sakr (2009) and Sakr et al (2008), respectively.…”