Abstract

Alternaria helianthi conidia have been shown to cause disease on common cocklebur. Alternaria helianthi conidia grown at 18 °C are more virulent than those grown at 28 °C, and adhere to the leaves of the treated plants and causes necrotic lesions, stunting and mortality in common cocklebur at the 6- to 12-leaf stage. Using confocal laser scanning microscopy (CLSM) the virulent conidia produced multiple branched germ tubes. The distribution of the adhesive material on the conidial surface was varied, being evenly distributed on some conidia while appearing as globules on others. Examination by transmission electron microscopy (TEM) showed that virulent conidia had dense ribosomes and abundant endoplasmic reticulua indicating actively synthesizing cytoplasm. Adhesive vesicles, which appear to be the means of export of the adhesive from the cytoplasm, were often arranged along external cell walls. An osmophilic material, possibly the adhesive substance itself, was seen between the cell membrane and the cell wall. This substance may play an important role in the virulence of A. helianthi to common cocklebur and survival of this pathogen.

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