Roles of low pH, carbon and inorganic nitrogen source use in chlamydospore formation by Fusarium solani

Citrate and malate were poorer sources of exogenous carbon than several hexose, pentose, or disaccharide sugars for supporting macroconidial germination by Fusarium solani at high conidial density (1 × 105 conidia/ml). Only citrate, however, failed to block chlamydospore morphogenesis to a degree comparable to glucose or other readily used sugars. Mostly immature chlamydospores were formed in the presence of citrate. At low conidial density (5 × 103 conidia/ml), exogenous carbon-independent macroconidial germination and subsequent rapid chlamydospore formation on germ tubes was not inhibited by ammonium or nitrate nitrogen. The citrate–phosphate buffered, low pH (4.0) medium of Cochrane induced more immature chlamydospore formation by F. solani than a pH 6.0 medium, but few mature chlamydospores were formed in either medium. Condensation of hyphal cytoplasm into developing chlamydospores, a character typical of chlamydospore formation, did not occur extensively and macroconidia, hyphae, and immature chlamydospores stained deeply with Sudan III, suggesting lipid biosynthesis. This inhibition of chlamydospore maturation may be due partly to nitrogen deficiency imposed by the high C: N ratio of the medium and to the presence of citrate. Only vesiculate hyphal cells were formed by F. solani f. sp. phaseoli in both media.Field soils to which the clone of F. solani used is indigenous had mean pH values ranging from 5.2 to 6.0.