The Role of Melanin and Periods of Dryness on Germination of Conidia and Virulence of Alternaria alternata on Amaranthus retroflexus

Alternaria alternata (isolate 423) has been identified as a promising mycoherbicide for the biocontrol of redroot pigweed {amaranthus retroflexus). transmission electron micrographs of a. alternata conidia showed the presence of pigments enmeshed within the cell wall. light microscopy showed that conidia grown on media containing tricyclazole at 50 or 100 jug ml1, which inhibits melanin biosynthesis, lacked pigmentation. after long term storage (5-12 months) at 25 °c and 15% relative humidity (rh), conidia on semi-permeable discs were placed on top water agar at 25 °c for 5 h. germination of non-pigmented conidia was significantly higher than that of pigmented ones. when seedlings of redroot pigweed were transferred to a plant propagator at 25 °c with 92±2% rh, 2 - 6 h after being inoculated with conidial suspension (3><106 conidia ml1), the disease symptoms were significantly more severe than those transferred to the propagator immediately or 24 h after treatment.