Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: February 5, 2007
Publication Date: February 5, 2007
Citation: Moran, P.J. 2007. Use of Fungal Bioherbicides and Vinegar to Control Pigweeds in South Texas. Weed Science Society of America Meeting Abstracts. (Abstract) Technical Abstract: Palmer amaranth (Amaranthus palmeri S. Wats.), redroot pigweed (A. retroflexus L.), and smooth pigweed (A. hybridus L.) are weeds in cotton and other crops in South Texas. Two plant pathogenic fungi (Phomopsis amaranthicola and Microsphaeropsis amaranthi) were applied singly or in combination as potential bioherbicides to 4-10-leaf pigweed seedlings in a greenhouse maintained between 25-35 °C, or in field plots (M. amaranthi only). Spore suspensions (1 x 106 -107 spores per ml) contained water only, or a formulation of algal polysaccharide, citrus oil, and detergent. A dew period of 12-14 hours was used in all experiments. Inoculations with single pathogens in the greenhouse caused leaf lesions on 48%-94% of the leaves on smooth and redroot pigweed plants, reduced leaf gain in smooth pigweed, and induced stem rot in 25-100% of plants of both pigweed species, but caused no symptom development in Palmer amaranth. The use of the formulation increased stem rot development in smooth pigweed and Palmer amaranth. Inoculation with P. amaranthicola killed 25%-66% of smooth and redroot pigweed plants but did not kill Palmer amaranth. Inoculation with M. amaranthi killed 25% of redroot pigweed seedlings. Nine accessions of Palmer amaranth from South Texas showed equally high resistance to the two fungi. Combining the two bioherbicides did not significantly increase symptom development or mortality in any of the three pigweed species. Concentrating the inoculum to 107 spores per ml increased leaf necrosis in Palmer amaranth, but did not increase stem rot or mortality. The addition of 2 mM valine to the formulation, to mimic the effects of application of amino acid overexcretor pathogen strains, did not increase symptom development or mortality. In field plots (fine-loamy, mixed hyperthermic Udic arguistolls), M. amaranthi inoculation caused stem rot and reduced growth in smooth pigweed, but caused no mortality in any of the three pigweeds. Soluble peroxidase activity was elevated two to three-fold in Palmer amaranth leaves two weeks after inoculation with P. amaranthicola alone or combined inoculum, consistent with the high resistance of Palmer amaranth. As an alternative control method for this weed, vinegar (9% acetic acid) was applied at a rate of 2400 L per ha, and killed 100% of 5-10-leaf and 80% of 10-20-leaf Palmer amaranth seedlings, but did not kill mature, reproductive plants. Improved formulations and application strategies are needed for fungal bioherbicidal control of pigweeds under the hot, dry field conditions of South Texas.