Biological control of the weed Sesbania exaltata using a microsclerotia formulation of the bioherbicide Colletotrichum truncatum

Authors: Boyette, Clyde; Abbas, Hamed; Johnson, Bobbie; Hoagland, Robert; Weaver, Mark

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/5/2014
Publication Date: 8/18/2014
Citation: Boyette, C.D., Abbas, H.K., Johnson, B.J., Hoagland, R.E., Weaver, M.A. 2014. Biological control of the weed Sesbania exaltata using a microsclerotia formulation of the bioherbicide Colletotrichum truncatum. American Journal of Plant Sciences. 5:2672-2685.

Interpretive Summary: Colletotrichum truncatum spores have been previously shown effective in controlling the weed hemp sesbania, a problematic weed in the southern U.S. that exhibits tolerance to glyphosate. C. truncatum also produces other infective structures called microscelerotia. We found that large quantities of microsclerotia could be inexpensively produced using solid state fermentation on rice grain. Greenhouse tests revealed that 0.4 to 50 mg/cm2 of this finely-ground product, applied pre-emergence caused 22 to 96% hemp sesbania mortality, 14 days after treatment. A post-emergence treatment of this product, formulated in 30% unrefined corn oil and 0.2% Silwet L-77 surfactant, caused 93% mortality 14 days after application to weeds surviving the pre-emergence treatment. In field tests over four distinct periods from mid-April to mid-October, the early season (April-May) pre-emergence treatment provided 67% of hemp sesbania, at 15 days, and post-emergence treatment caused 91% mortality of surviving weeds. Results were similar in the early-mid-season (June-July), with 73%, 93, and 98% mortality of weeds occurring in the pre-emergence, post- emergence, and pre- plus post-emergence treatments, respectively. However in the late-mid-season (July-August), pre-emergence treatment caused only about 2% mortality within 15 days, while post-emergence treatment caused 75%, and pre- plus post-emergence treatments provided 80% control. We hypothesize that high soil and air temperatures during this period adversely affected disease development. These results show that a microsclerotia formulation of C. truncatum is highly efficacious for control of hemp sesbania under favorable environmental conditions. This research also demonstrates the utility of solid-state fermentation to produce microsclerotia and suggests that this technique may also be applicable to certain other bioherbicidal weed pathogens.

Technical Abstract: The fungus Colletotrichum truncatum, grown on rice grain for 3 to 4 weeks at 22 to 24°C, produced a fungus-infested rice containing mostly microsclerotia, rather that conidia (spores) in a ratio of ~9:1, respectively. Greenhouse tests showed that this formulation at various concentrations, from 0.4 to 50 mg of finely-ground fungus-rice powder applied pre-emergence to 5 cm2 of soil surface, caused 22 to 96% hemp sesbania plant mortality, within 14 days after treatment. Weeds surviving this pre-emergence application were treated with a post-emergence treatment of a fungus-rice aqueous formulation containing 2.4 x 105 microsclerotia ml-1, 30% unrefined corn oil and 0.2% Silwet L-77 surfactant), which resulted in 93% mortality, 14 days after treatment. Based on these greenhouse results, the following tests under field conditions were undertaken: 1) pre-emergence treatment (fungus-rice formulation at 2.4 x 105 microsclerotia cm-2; 2) post-emergence (fungus-rice formulation in 30% unrefined corn oil, 0.2% Silwet) only treatment, applied to plants 15 days after planting; and 3) pre-emergence treatment followed by a post-emergence treatment (fungus-rice formulation in 30% unrefined corn oil, 0.2 Silwet) applied 15 days after planting to surviving weeds. Control treatments were: 1) autoclaved ground product sans fungus, 2) unrefined corn oil (30% unrefined corn oil, 0.2% Silwet in water) and 3) untreated plants. Four field planting dates were utilized: early season (April to May), early-mid season (June to July), late-mid season (July to August), and late season (September to October). Weed mortality ratings in field studies were recorded at 15 days for the pre- plus post-treatment, and at 30 days after planting for the pre-emergence only treatment and the post-treatment only. The early season, pre-emergence treatment resulted in 67% of hemp sesbania mortality (averaged over a three-year testing period) 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 (<5%) mortality within 15 days, while mortality in the post-emergence treatment was > 80%. These results suggest that seasonal environmental conditions can play an important role in the efficacy of this C. truncatum-infested rice formulation when applied pre-emergence or post-emergence to this onerous weed.