PRESERVATION OF AIR-DRIED DOWNY MILDEW SPORANGIA IN LIQUID-NITROGEN WITHOUT CRYOPROTECTANTS OR CONTROLLED FREEZING

Zoosporangia of Plasmopara halstedii, causal agent of sunflower downy mildew, were infective after storage in liquid nitrogen for up to 4.75 yr, using a dry spore, fast-freeze (DSFF) technique. The DSFF technique consists of vacuuming air-dried zoosporangia with a cyclone collector, placing them in a polypropylene screw-cap tube without any cryoprotectant, and immediately placing them in a liquid nitrogen storage tank. Addition of a cryoprotectant or the use of a controlled freezing rate did not enhance infectivity of zoosporangia stored in liquid nitrogen over that of the basic DSFF method. The ability of air-dried zoosporangia to withstand liquid nitrogen storage without cryoprotectants appears to be due to their lower water content compared to that of spores collected by washing and filtering. Spores stored in liquid nitrogen by the DSFF technique were infective after five freeze/thaw cycles. The technique was also successful with Peronospora parasitica and Pseudoperonospora cubensis sporangia, but not with Peronosclerospora sorghi.