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On soft substrata conidia of Colletotrichum graminicola germinated by the formation of germ tubes, whereas on hard, hydrophobic substrata they germinated by the formation of appressoria. Regardless of the substratum, conidia did not germinate at high conidium concentrations, suggesting the presence of an endogenous self-inhibitor. Bioassays to test for the inhibition of germination demonstrated the presence of a low-molecular-weight self-inhibitor in the mucilage which surrounds conidia as they are produced in acervuli. The inhibitory material exhibited an absorbance maximum of 310 nm, indicating that it is a member of the mycosporine family of compounds. The inhibitor was purified to homogeneity and shown by mass spectrometric analysis to have a molecular mass consistent with the structure corresponding to mycosporine-alanine.

We devised a procedure to propagate selectively the vegetative and asexual-reproductive states of Helminthosporium carbonum so that we could characterize morphological and subcellular events associated with the onset of conidiation. Solidified agar media were uniformly inoculated with an overlay of conidia suspended in molten agar. After the overlay solidified, it was covered with a sheet of Miracloth. When incubated in the dark, cultures produced abundant aerial hyphae that grew through the Miracloth layer and conidiation was suppressed for 48 to 50 h. Hyphae were easily harvested from the surface of the Miracloth with a spatula. When cultures were placed in the light after 38 h of growth in the dark, differentiation was detected in 90% of the hyphal tips within 8 to 10 h. The initial response of the hyphal tips, comprising early stages in conidiophore development, was rapid and highly synchronized. The behavior of nuclei during conidiogenous cell development and the initiation of conidia was similar to that reported for other fungi that form blastic conidia. One-dimensional gel electrophoresis of in vitro translation products confirmed differences in poly(A) RNA populations from dark-grown and light-induced cultures.

The release of esterase enzyme activity by conidia of Erysiphe graminis is described as a tropic response to the stimulus of contact with leaf or cellophane surfaces. The release of esterase was completed within 30 minutes of the contact stimulus, and activity was released in two stages. The first began within 2 minutes of conidium contact and the second began between 10 and 15 minutes after contact. A portion of the esterase activity was solubilized in aqueous buffer and a portion remained bound to the conidial wall. Preparations of soluble proteins containing esterase activity were separated by sodium dodecyl sulfate-polyacrylamide and native gel electrophoresis which revealed the presence of several proteins and three zones that possessed esterase activity. The release of enzyme activity was correlated with changes in surface morphology described to occur within the first 30 minutes of initiation by the contact stimulus. The results are discussed in the context of cuticular degradation and preparation of the infection court by the fungus prior to the time of conidium germination.