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 degrees 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 degrees 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 degrees 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 degrees 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. (C) 1998 Annals of Botany Company.