Abstract
Colletotrichum truncatum, grown on rice grain (3 to 4 weeks, 22°C to 24°C) produced a fungus-infested rice mixture of microsclerotia and conidia (spores) in a ratio of ~9:1, respectively. Greenhouse tests of this formulation (0.4 to 50 mg finely-ground fungus-rice product) which applied pre-emergence to 5 cm2 of soil surface, caused 22% to 96% hemp sesbania plant mortality, after 14 days. Post-emergence treatment (fungus-rice aqueous formulation; 2.4 × 105 microsclerotia ml-1, 30% unrefined corn oil and 0.2% Silwet L-77 surfactant) of weeds surviving the pre-emergence application, resulted in 93% mortality, after 14 days. Based on greenhouse results, field tests were undertaken: 1) pre-emergence treatment (fungus-rice formulation at 2.4 × 105 microsclerotia cm-2), 2) post-emergence (fungus-rice product in 30% unrefined corn oil, 0.2% Silwet) only treatment, applied 15 days after planting and 3) pre-emergence treatment followed by post-emergence treatment (fungus-rice product in 30% unrefined corn oil, 0.2% Silwet) applied 15 days after planting to surviving weeds. Control treatments were: 1) autoclaved rice product sans fungus, 2) unrefined corn oil (30% unrefined corn oil, 0.2% Silwet in water) and 3) untreated plants. Planting dates were: early season (April-May), early-mid season (June-July), late-mid season (July-August), and late season (September-October). Weed mortality was recorded at 15 days for the pre-plus post-treatment, and at 30 days after planting for the pre-emergence only and the post-treatment only. The early season, pre-emergence treatment caused 67% hemp sesbania mortality (3-yr average) within 15 days and the post-emergence treatment caused 91% mortality of the surviving weeds. In the late-mid-season, pre-emergence treatment caused minimal (80%. Results suggest that seasonal environmental conditions are important in the efficacy of this C. truncatum-rice product formulation when applied pre- or post-emergence to this onerous weed.
Highlights
Concerns regarding environmental contamination, food safety, and the evolution of herbicide-resistant weeds, brought about by the use of chemical herbicides, have culminated in the need for alternative weed management tools and strategies
With regard to the development of herbicide resistance in weeds, for example, in 1996 there were no reports of weed species resistant to the herbicide glyphosate in the United States, but currently 24 weeds are documented as resistant to this compound and 218 species of weeds have become resistant to various herbicides in 61 countries [8]
A single-spore strain of C. truncatum (NRRL No 18434) previously isolated from infected hemp sesbania colled near Stoneville, MS, USA [14] was used in all experiments
Summary
Food safety, and the evolution of herbicide-resistant weeds, brought about by the use of chemical herbicides, have culminated in the need for alternative weed management tools and strategies. Ex A.W. Hill (hemp sesbania), is avigorous, nodulating leguminous weed in soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L.) and rice (Oryza sativa L.), often reaching heights of 3 m at maturity [9]. Hemp sesbania ranks as one of the 10 most troublesome weeds in the three southern US states of Arkansas, Louisiana, and Mississippi [10], reducing crop yield by shading and competition [11]. Hemp sesbania is a prolific seed producer, yielding up to 21,000 seed per plant [12] and can reduce quality and market value due to black seed contamination in harvest rice grain
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