| Summary: | The blastocladialean fungus <i>Paraphysoderma sedebokerense</i> parasitizes three microalgae species of economic interest: <i>Haematococcus pluvialis</i>, <i>Chromochloris zofingiensis</i> and <i>Scenedesmus dimorphus</i>. For the first time, we characterized the developmental stages of isolated fungal propagules in <i>H. pluvialis</i> co-culture, finding a generation time of 16 h. We established a patho-system to compare the infection in the three different host species for 48 h, with two different setups to quantify parameters of the infection and parameters of the parasite fitness. The prevalence of the parasite in <i>H. pluvialis</i> and <i>C. zofingiensis</i> cultures was 100%, but only 20% in <i>S. dimorphus</i> culture. The infection of <i>S. dimorphus</i> not only reached lower prevalence but was also qualitatively different; the infection developed preferentially on senescent cells and more resting cysts were produced, being consistent with a reservoir host. In addition, we carried out cross infection experiments and the inoculation of a mixed algal culture containing the three microalgae, to determine the susceptibility of the host species and to investigate the preference of <i>P. sedebokerense</i> for these microalgae. The three tested microalgae showed different susceptibility to <i>P. sedebokerense</i>, which correlates with blastoclad’s preference to the host in the following order: <i>H. pluvialis</i> > <i>C. zofingiensis</i> > <i>S. dimorphus</i>.
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