A Multiple-Pathogen Bioherbicide System for the Control of Grass Weeds UF# 1730 A Synopsis

Concept:   To use two or more fungal pathogens that share overlapping host ranges as a broad-spectrum bioherbicide cocktail to control several weedy grasses. We call this "the multiple-pathogen strategy." An essential element of the cocktail is an emulsion that acts as an adjuvant, predisposing the grasses to the pathogens. The fungal cocktail may not be used in crops where injury to the crop may result from the emulsion. Therefore, the primary markets for this bioherbicide system are expected to be in tree crops (citrus, apples, etc.), home and industrial landscape maintenance (to clear grass along driveways and pavements, around shrubbery, etc.), and power-line right-of-ways. The citrus growers in Florida, who funded our research, is very interested in using our system.

Pathogens:   Three fungi, Drechslera gigantea, Exserohilum longirostratum and E. rostratum, discovered from different locations in Florida. They were isolated from three different grass weeds.

Susceptible hosts:   Several weedy grasses. An extensive host-range study has been completed with each of these fungi as well as a fungal cocktail. Each fungus is pathogenic at different levels to one or more grass species. Some graminaceous crops develop mild damage when sprayed with these fungi, singly or as a cocktail. This bioherbicide system is recommended for use around tree trunks, between tree rows, and for landscape maintenance, but not for over-the-top use in crops. When used in this manner, it is considered a safe and acceptable bioherbicide system.

Symptoms:   All three fungi cause leaf spots, leaf lesions, and leaf blights and kill the photosynthetic tissues. Symptoms appear in about one week after the fungus is sprayed on the foliage and the disease progresses steadily over the following two to three weeks. Grasses are killed completely without regrowth in treated areas and the control lasts for 10 weeks or more. These fungi do not appear to spread outside the treated areas, possibly due to 1) confinement of the inoculum to treated sites by the emulsion; 2) poor dissemination of the spores which are relatively quite large; and 3) lack of secondary inoculum production on rapidly dying foliage. The lack of secondary spread is considered a desirable safety feature in this bioherbicide system.

Biocontrol efficacy:   Field trials, repeated over the past three years, have confirmed that these fungi are highly effective under Florida field conditions.

Cultural characteristics:   The fungi can be grown easily on common laboratory media such as V8-juice agar, tomato juice, potato-dextrose agar, and several inexpensive natural substrates. A standard procedure we use consists of growing the fungi in a biphasic system: first in liquid culture and then in solid phase. In the latter phase, blended culture, derived from the liquid phase, is spread in a tray and allowed to dry gradually. Two or three crops of spores can be collected from the trays. The fungi are stable with respect to sporulation, virulence, or cultural characteristics. They are also fairly easy to grow on various agar media and natural substrates.

Research and development needs:   Studies on large-scale inoculum production and formulation are needed. Preferably this aspect of the R&D should be done in collaboration with an industrial partner using commercially feasible methods. Efficacy trials under commercial conditions (e.g., growers' fields, natural areas, home lawns and gardens, etc.) can then be undertaken with commercial prototype formulations.