Effects of <i>Myrothecium verrucaria</i> on Two Glyphosate-Resistant <i>Amaranthus palmeri</i> Biotypes Differing in Betacyanin Content

Abstract

Previously we found two biotypes of Amaranthus palmeri (Palmer amaranth) in a population of this economically important weed that were resistant to glyphosate but differed with respect to pigmentation. One biotype was typically red-pigmented (betacyanin) while the other was green, with no visual appearance of red hue on any plant part at any growth stage. We have also reported that a strain of Myrothecium verrucaria (MV) exhibited bioherbicidal activity against several important weeds including glyphosate-resistant Palmer amaranth. In greenhouse tests, MV was applied to these two biotypes (red and green) at two ages (3-week- and 6-week-old) and effects of this fungus monitored over a 5-day time course. Initial symptoms of MV (16 to 24 h after inoculation) were: epinastic curvature, wilting and development of lesions on leaves and stems. Generally, the younger plants tended to be more sensitive to MV than older plants. Bioherbicidal damage increased with time leading to necrosis and plant mortality and increasing disease progress. Severe loss of fresh weight occurred in both biotypes as compared to untreated plants. Results indicated that MV was effective on both biotypes, but effects on growth reduction and disease progression were more rapid and generally greater in the green biotype, suggesting that compounds responsible for red pigmentation may be more potent as defense against pathogen attack.