Secretion of oxalic acid in the infection process of <i>S. sclerotiorum</i>.

<p>Light micrographs of the early, advanced, and late stages of infection after histochemical staining for calcium oxalate of fresh epidermal strips that resulted in a yellow-brownish colour of tissue and a brown-black colour of the compact oxalate crystals. <b>A</b>: At the early infections stage (12-24 hpi) the epidermal layer around one infections cushion (cu) is bleached (arrow), while most of the infection cushions (circles) do not show any influence on the epidermal layer. <b>B</b>: In the advanced stage (36-48 hpi) there is bleaching of the epidermal layer (arrows) around all infection cushions (cu) and only a single dark stained particle of oxalate was visible (arrowhead). <b>C</b>: At the late stage (72 hpi) there is not only bleaching, but calcium oxalate crystals accumulate around the infection cushions (arrow). <b>D</b>: Detail of a late stage at the infection front. The bleaching effect (stars) did not become visible at the hyphal tips (arrows), but along the older parts of the subcuticular hypha (shy).