Control of seven grasses with a mixture of three fungal pathogens with restricted host ranges

Three fungi indigenous to Florida, Drechslera gigantea, Exserohilum longirostratum, and E. rostratum, were isolated from large crabgrass (Digitaria sanguinalis), crowfootgrass (Dactyloctenium aegyptium), and Johnsongrass (Sorghum halepense), respectively. In greenhouse trials, these pathogens controlled the following seven grasses: crowfootgrass, guineagrass (Panicum maximum), Johnsongrass, large crabgrass, southern sandbur (Cenchrus echinatus), Texas panicum (Panicum texanum), and yellow foxtail (Setaria glauca). Four-week-old seedlings were inoculated with spore suspensions of each pathogen (2 x 10(5) spores/ml) or a mixture of the three pathogens (1:1:1 by volume; total 2 x 10(5) spores/ml). The spore suspensions as well as the control (water only) were amended with 0.5% Metamucil and the seedlings were sprayed, incubated in a dew chamber for 12 h at 28 degreesC, and then held in a greenhouse. All seven grasses were susceptible to each of the pathogens and the pathogen mixture, each with 100% disease incidence. Two weeks after inoculation, disease severity ranged from 83 to 100% for the different grasses. In a host-range trial, 52 plant species were tested, and all nontarget plant species were safe from the individual pathogens and the pathogen mixture inoculum amended with 0.5% Metamucil. An emulsion-based inoculum preparation of the pathogen mixture was also not injurious to the crop species tested (21 species). Thus, D. gigantea, E. longirostratum and E. rostratum and a mixture of these pathogens were nonpathogenic to the crop species tested, including Poaceae members. It is preferable to have more than one or even two pathogens in the grass pathogen mixture to achieve a broad spectrum of weed control and provide insurance against any possible failure of one or even two of the pathogens in any given combination under field conditions. More pathogens (at least three instead of two) with different modes of action and/or sites of action in a combination have a much greater chance of reducing potential risk of development of resistance in weed populations than a single pathogen used repeatedly. The use of a mixture of these pathogens appears to have the potential to be developed further as a bioherbicide system to control weedy grasses. (C) 2001 Academic Press.