Development of a bioherbicide for the control of waterhemp, Amaranthus tuberculatus (Moq.) Sauer.

Daljit Singh, Purdue University

Abstract

Weeds in the genus Amaranthus are troublesome in agronomic and horticultural crops in many parts of the world. Numerous populations of Amaranthus spp, especially waterhemp [Amaranthus tuberculatus (Moq.) Sauer.] have evolved resistance to herbicides, including resistance to glyphosate. Moreover, waterhemp has a prolonged emergence pattern and a high level of genetic diversity, which makes its control difficult in corn-soybean cropping systems in United States. Microsphaeropsis amaranthi is a candidate bioherbicide for the control of weedy Amaranthus spp. because it has restricted host range, is easily grown in culture and can cause damaging infections. Microsphaeropsis amaranthi performance in greenhouse conditions is encouraging, but needs significant improvement in its virulence for a reliable field performance. The experiments in this thesis addressed the enhancement of the bioherbicide and its utilization in different cropping systems and niche markets. The fungus grown on sterilized corn stover media produced highly virulent conidia and resulted in enhanced disease severity on waterhemp seedlings. The conidia produced on corn stover were thick walled and germinated and caused infection earlier on waterhemp leaves than those produced on V-8 agar. The enhanced virulence was observed due to early germination and appressorium formation in conidia produced on corn stover. The most critical factor in the performance of the bioherbicide is availability of prolonged leaf wetness period. An experiment was conducted to address this issue, wherein M. amaranthi was cultured on waterhemp seedlings in a multigenerational experiment with reduced dew period of 8 h. It was observed that selecting lesions and spraying with a reduced dew period enhanced the performance M. amaranthi in terms of disease severity and dry matter reduction of waterhemp seedlings, in subsequent generations. Experiments were conducted on delivery methods, and it was observed that granular application caused more stem infections when applied preemergence. The most suitable substrate to grow fungus for granular applications was barley grains, which caused maximum reduction in emergence or survival and dry matter accumulation of waterhemp seedlings. In greenhouse experiments, when glyphosate and conidia of M. amaranthi were sprayed onto common waterhemp seedlings, the herbicide predisposed plants to infection by M. amaranthi. To confirm this interaction in field conditions, experiments were conducted at West Lafayette, Indiana and Macomb, Illinois, during 2007 in Roundup Ready soybean fields infested with natural stands of common waterhemp and redroot pigweed (Amaranthus retroflexus L.). At both locations, a synergistic interaction was found between M. amaranthi and glyphosate resulting in enhanced disease severity and weed control. Field experiments were also conducted during 2006 and 2007 on tomatoes to evaluate the potential of M. amaranthi in horticulture production system. Multiple applications of the bioherbicide caused high disease severity and resulted in good weed control and higher tomato yield. Microsphaeropsis amaranthi continues to show promise as a candidate bioherbicide as evident by good weed control under optimum growing conditions. Its failure in sub-optimal conditions, however, shows that significant improvements still need to be made. These experiments demonstrate that there is potential for the enhancement of the efficacy of M. amaranthi and for the integration of the bioherbicide into production systems.

Degree

Ph.D.

Advisors

Hallett, Purdue University.

Subject Area

Agronomy|Plant Pathology|Plant sciences

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