Induction and inhibition of germination in Neozygites fresenii (Entomophthorales: Neozygitaceae) zygospores by various time-temperature stimuli

Neozygites fresenii is the most prevalent entomopathogen of the aphid pest of citrus in Israel, Aphis citricola. Observations in nature and germination experiments in vitro with zygospores of N. fresenii corroborated the indications of our previous report in 1979, that their overwintering is by an endogenous dormancy mechanism, and that after dormancy breaks, germination accumulates logarithmically in a steep, sigmoidal curve. Germination measured following various time-temperature regimes of dry storage and moist incubation indicated a sequence of requirements: (i) a “maturation” period of ca. 20 days at 5°–23°C; (ii) a “vernalization” period of at least 14–15 days at 5°–14°C, which can overlap or coincide with maturation; (iii) and “activation” period after which germination starts, which in nature takes 200 or more hours if the time-temperature ratio is at least 4:1 (hours above 10°C:hours below 10°C); activation was shorter in vitro when the temperature was held constant above 14°C (e.g., 72–96 hr at 23°C). Temperatures around 5°C deactivated the process, causing a reversible return to dormancy. Other findings were as follows: (i) Insufficient vernalization or maturation at temperatures above those needed for vernalization caused germination to accumulate as a linear slope rather than as a steep, sigmoidal curve drawn by normally vernalized spores. Additional vernalization of such spores returned germination to a steep sigmoidal accumulation; (ii) germination accumulated faster when after vernalization the temperature was raised gradually rather than abruptly; none occurred above 27°C. A possible model of dormancy breaking in N. fresenii zygospores is presented. Reversible deactivation is assumed to be a built-in safeguard against premature activation and germination of zygospores during periods of several days of warm day time weather which occur from time to time in mid-winter in Israel.