Scanning electron microscopy of germinated ascospores of Monosporascus cannonballus

Ascospore germlings of Monosporascus cannonballus attached to melon roots were examined with the scanning electron microscope. Results showed that ascospores germinated with one to three germ tubes. Sixty percent of the ascospores germinated with two germ tubes. No germ pores were observed. Instead, germ tubes emerged from a single linear fissure on each ascospore. The largest fissure measured 25 μm in length and 6 μm at its greatest width. Germ tubes of ascospore germlings were firmly anchored to melon roots. However, no structures resembling appressoria were observed. Rather, the tips of germ tubes, upon contact with the epidermis, appeared to have penetrated the epidermis directly. When the germ tube at the point of attachment was dislodged, no infection peg was present at the germ tube tip.