Potential of Phomopsis amaranthicola and Microsphaeropsis amaranthi, as bioherbicides for several weedy Amaranthus species

Several plants in the genus Amaranthus are weeds in cropping systems throughout the world, and some biotypes have developed resistance to a number of herbicide families. In an effort to develop alternative, biologically based weed management tactics, studies were initiated to quantify the selective ability of two fungal organisms to several Amaranthus species. Response of weed seedlings to Microsphaeropsis amaranthi (3 x 10(6) conidia.ml(-1)), Phomopsis amaranthicola (I x 10(7) conidia.ml(-1)), and a mixture of the two organisms (1.5 x 10(6) + 4 x 10(6) conidia.ml(-1), M. amaranthi and P. amaranthicola, respectively) were tested Linder controlled and field conditions at Urbana, I L in 2004. Weeds included Amaranthus rudis; A. palmeri; A. powellii; A. retroflexus; A. spinosus; A. hybridus; A. albus; and A. blitoicits. Seeds of each species were sown in the greenhouse, and conidial suspensions were applied at the 2- to 4-leaf stage, and then pots were placed either in a dew chamber (24 h) and back in the greenhouse, or in the inter-row of a soybean field. Treatment with fungal organisms infected most weeds, reducing growth and survival, although responses in the greenhouse were less than those observed in the field. Percent seedling mortality for A. albus and A. blitoides were between 80 and 100%, 14 to 15 DAT for the mixture or M. amaranthi alone, in greenhouse and field trials. In the greenhouse, the mixture of two organisms and M. amaranthi alone significantly reduced A. albus and A. blitoides height. Fungal treatments reduced biomass of A. powellii, A. albus and A. blitoides. In field experiments, ail eight weed species treated with M. amaranthi or the mixture of both organisms had severe disease ratings 15 DAT, and mortality ranged from 74% to 100%. In addition, these treatments reduced biomass of A. rudis, A. retroflexus, A. spinosus, A. hybridus, and A. albus. Height of A. rudis, A. hybridus, and A. albus was reduced by all fungal treatments. This research indicates seedlings of several Amaranthus species are susceptible to conidial suspensions of P. amaranthicola and M. amaranthi in both controlled and field environments. (c) 2005 Elsevier Ltd. All rights reserved.