Impact of Thermal History on Tolerance of Meloidogyne hapla Second-stage Juveniles to External Freezing.

Low temperature induced physiological changes that increased the ability of second-stage juveniles of Meloidogyne hapla to survive external freezing. Second-stage juveniles in polyethylene glycol solution were exposed to -4 , 0, 4, or 24 C, and then their survival was determined after ice-induced freezing of the suspensions at - 4 C for 24 hours. Survival was greatest for juveniles exposed to 4 C before freezing. Some juveniles were killed by exposure to - 4 C before freezing of the suspensions. The percentage of juveniles surviving freezing increased from about 30% to 80% within 12 hours of exposure to 4 C. This tolerance of external freezing was lost during subsequent exposure to 24 C. Longer exposures, of 1 to 15 days, to low temperature did not increase the percentage surviving external freezing, as compared to the 12-hour exposure, but reduced the tolerance of external freezing lost during subsequent exposure to 24 C for 48 hours.