Temperature and light effects on in vitro germination of Peronospora effusa sporangia

Spinach downy mildew, caused by the biotrophic oomycete Peronospora effusa, is an economically important disease that is found in all spinach growing regions of the US. To effectively predict disease risk we need to understand the response of P. effusa to different environmental conditions. We conducted several in vitro germination assays, exposing P. effusa sporangia to different temperature and lighting conditions. Between 5 and 25 °C under constant darkness, germination of P. effusa sporangia on water agar declined with increasing temperatures. These results were qualitatively different from a previous study of P. effusa germination that found a bimodal response curve, with increased germination at lower and higher temperatures. Time course studies revealed that sporangia consistently germinated within the first 12 h of plating, regardless of incubation temperature. Blue light exposure significantly reduced sporangial germination when compared with sporangia exposed to red, yellow, or no light. Light intensity and color significantly impacted germination, although the effect of color varied by light intensity.