Repeated emergence, motility, and autonomous dispersal by sporangial and cyst derived zoospores of Phytophthora spp

The purpose of this study was to examine the biology of Phytophthora zoospores in irrigation water. The effect of temperature on asexual life cycle stages of Phytopthora spp. was investigated. The ability of selected common water-borne Phytophthora spp. to produce additional generations of zoospores (re-emergence) after vortex encystment and the influence of temperature on this process was also assessed. Biological attributes of zoospores released from sporangia were assessed and compared to zoospores derived from cysts by re-emergence. These attributes included motility, autonomous dispersal in model mazes and linear troughs, and location of infection sites on roots.; Asexual developmental life cycle stages of Phytophthora spp. were influenced differently by temperature. Phytophthora cysts are able to germinate either by re-emergence, or germ tubes. Re-emergence may be a common mode of cyst germination for water-borne Phytophthora spp. Temperature influences the mode of cyst germination differently for each isolate. Phytophthora zoospores released from sporangia were able to move autonomously to distances of 215 mm (in model mazes) and 320 mm (in linear troughs) with or without seedlings. Cyst derived zoospores were also able to move the same distances and cause seedling infections, demonstrating that they may serve as dispersal and infection agents of Phytophthora spp. Zoospores of P. cryptogea isolates (FDM51 and FWDM4) had different capacities for motility retention. For both isolates, motility was longer at the lowest water temperature (15 C) than at the highest water temperature (25 C). Motility retention by cyst derived zoospores was less than by sporangial derived zoospores for both isolates. These studies of the biology of Phytophthora zoospores have important implications for managing these plant pathogen in irrigation water.