Submitted to: Plant Pathology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2012
Publication Date: 6/24/2013
Citation: Boyette, C.D., Hoagland, R.E. 2013. Adjuvant and refined corn oil formulation effects on conidial germination, appressorial formation and virulence of the bioherbicide, Colletotrichum truncatum. Plant Pathology Journal. 12(2):50-60.
Interpretive Summary: The weed hemp sesbania, which is relatively tolerant to glyphosate, is a serious problem of soybean, rice, and cotton in the southern U.S., reducing quality and yield of these crops by as much as 40, 50, and 80%, respectively. The bioherbicidal fungus Colletotrichum truncatum can effectively control this weed if spores (conidia) are properly formulated. The objective of this project was to examine several adjuvants (surfactants and plant extracts) for possible augmentation of conidial germination and virulence of this organism, because components of bioherbicides should maintain or enhance conidial germination, growth, and infectivity on their target weeds. We found that some surfactants stimulated germination and virulence when formulated with conidia and applied to hemp sesbania leaves. Conidia formulated in water, a surfactant, or an emulsion of refined corn oil, were ineffective in the absence of dew or when dew was delayed. But, formulations of conidia plus surfactant in emulsified refined corn oil exhibited bioherbicidal activity when sufficient dew or free-moisture was available. This is important since previous reports indicated that refined corn oil did not enhance bioherbicidal activity, whereas unrefined corn oil promoted pathogen germination and efficacy. Aqueous extracts of several plant species also stimulated germination. This research is important since results show that formulations containing an emulsion of conidia, a surfactant and refined corn oil can improve C. truncatum efficacy, and suggest that similar formulations may also be applicable to other weed pathogens used as bioherbicides.
Technical Abstract: Several surfactants, plant extracts, and fatty acids were tested for stimulation of conidial germination and appressorial formation of Colletotrichum truncatum, a bioherbicide of the weed, hemp sesbania (Sesbania exaltata). The commercial surfactants (Tweens® 40, 60, 80, 85 and Myvatex® 60) at concentrations of 0.25 to 1.0 % (v/v), stimulated germination, but these effects were not exclusively related to their hydrophylic-lipophylic balance values. The stimulatory surfactants were also tested for germination and virulence of C. truncatum when formulated with the conidia and applied to hemp sesbania seedlings. Conidia formulated in either water, a surfactant or, an emulsion of refined corn oil were ineffective on the plant in the absence of dew or when dew was delayed. However, formulations of conidia combined with surfactant in emulsified refined corn oil did exhibit bioherbicidal activity when adequate dew or free-moisture was unavailable. This is important since previous reports indicated that refined corn oil did not enhance bioherbicidal activity, whereas unrefined corn oil promoted pathogen germination and efficacy of C. truncatum. The fatty acids tested had little or no effect on conidial germination but, aqueous extracts of several plant species including pathogen hosts and non-hosts, stimulated germination. Appressorial formation influenced by the surfactants did not necessarily reflect the disease rating on hemp sesbania seedlings. Overall, results show that formulations containing an emulsion of conidia, a surfactant and refined corn oil have potential for enhancement of the efficacy of C. truncatum