Limits to the negative logarithmic relationship between moisture content and longevity in conidia of Metarhizium flavoviride

Conidia of Metarhizium flavoviride were hermetically stored at 50 °C and 14 moisture contents between 2.5 and 31.8 % (fresh weight basis) for up to 146 d, and tested for germination on Sabouraud Dextrose Agar at 25 °C for 24 h. Survival of conidia conformed to cumulative negative normal distributions and all 14 survival curves could be constrained to a common origin. There was a negative logarithmic relation between longevity and conidia moisture content, but limits to the relation were detected: the lower-moisture-content limit was 4.6% [in equilibrium with 10.7% relative humidity (RH) at 20 °C], below which value further reduction in moisture content did not increase conidia longevity; and an upper-moisture-content limit between about 21.2 and 31.8 % moisture content (between 77 and 90.0 % equilibrium RH at 20 °C) above which conidia longevity no longer decreased. The observations could also be described by a negative semi-logarithmic relation between conidia longevity and equilibrium relative humidity. In this model, each reduction in equilibrium relative humidity by 11.2% within the range 10.7 to 80% RH doubled conidia longevity. The similarities in these relations, and the limits to these relations, between the conidia of this entomopathogenic fungus and the orthodox seeds of higher plants are discussed.