Variation in conidial morphology among geographic isolates of Sphaeropsis sapinea

Abstract

In the U.S.A., Sphaeropsis sapinea is believed to occur in at least two distinct forms (type A and B) that can be distinguished by their conidial wall ornamentation. In this study, mature conidia of 50 monoconidial isolates of S. sapinea from a worldwide collection were examined using SEM to determine whether these two forms occurred more widely. Twenty S. sapinea isolates consistently had conidia with smooth outer surfaces typical of type A. The remaining 30 isolates had typical pits or similar indentations occurring on between 6 and 38% of conidia examined. Pits, or indentations, either occurred uniformly on the outer surface or only on some parts of individual conidia. Authentic S. sapinea isolates, categorized as type A and type B in previous studies, were chosen for detailed examination of the effect of spore age, nutrition, and pigmentation on surface ornamentation. No distinct differences in conidial morphology between the two types were discernible between conidia of different ages, or those produced on media containing different carbon or nitrogen sources. The melanin inhibitor, tricyclazole, was used to compare melanin-deficient conidia with pigmented conidia. Although mature, melanin-deficient conidia were hyaline, this did not appear to influence the occurrence of pits on conidia. The results of these studies strongly suggest that conidial morphology of S. sapinea is a variable characteristic and, therefore, a poor taxonomic criterion.