In citrus, weedy grasses compete for moisture, nutrients, and light and can inhibit the growth of young trees and delay fruit production. These weeds are difficult to control, either because of their tolerance to available herbicides or due to growth habits that enable them to resist other control practices. Control of seven such weedy grasses (southern sandbur, large crabgrass, crowfootgrass, guineagrass, Texas panicum, johnsongrass, and yellow foxtail) with a mixture of three fungal pathogens, termed the multiple-pathogen strategy, was field tested in 1996 and 1998. Three fungi indigenous to Florida, Drechslera gigantea, Exserohilum longirostratum, and E. rostratum, isolated from large crabgrass, crowfootgrass, and johnsongrass respectively, were used. Two separate field studies were conducted: one study with seven grasses transplanted and grown within each plot (grass mixture field trial) and another study on a population of guineagrass alone present in a naturally infested field (guineagrass field trial). The objectives of this study were to (1) evaluate the field performance of D. gigantea, E. longirostratum, and E. rostratum individually and in a mixture to control the seven transplanted weedy grasses (grass mixture) and a population of guineagrass in a naturally infested field, respectively, and (2) compare the effectiveness of three carriers (water, Metamucil®, and an invert emulsion) on the bioherbicidal efficacy under field conditions. The fungi were applied as foliar sprays, each pathogen alone or in a mixture of the three fungi (1:1:1, v/v/v, for a total of 5 × 105 spores ml –1) in water, 0.5% aqueous Metamucil®, or an emulsion containing Sunspray® 6E. During the 14-wk experimental period, one or two additional sprays of all treatments were applied. Disease severity was recorded weekly for 4 to 6 wk after the initial spray (WAI). Maximum disease severities were obtained in emulsion-inoculum treatments, and were higher than those in the water-inoculum and the Metamucil-inoculum treatments. The pathogen mixture was equally effective as the individual pathogens in controlling the weeds tested. In the 1996 trial, 6 WAI, disease severity on grasses inoculated with D. gigantea spore suspensions in emulsion ranged from 78 to 100%, with E. longirostratum 90 to 100%, E. rostratum 79 to 100%, and the mixture 74 to 100%. In the 1998 trial, 4 WAI, disease severity on grasses inoculated with D. gigantea spore suspensions in emulsion ranged from 45 to 98%, with E. longirostratum 45 to 98%, E. rostratum 34 to 98%, and the mixture 32 to 98.5%. Thus, it was possible to manage all seven weedy grasses under field conditions using an emulsion-based inoculum preparation with the individual pathogens as well as the mixture of pathogens. The same three fungal pathogens were field tested for their ability to manage populations of guineagrass in a naturally infested field. The experimental design and treatments were identical to the field testing with the seven transplanted grasses. Two applications of an emulsion-based inocu...
A novel approach, termed the "multiple-pathogen strategy", in which three or more host-specific fungal plant pathogens are combined and applied inundatively for the control of several weeds was evaluated. In a model system, the following weed species were tested: pigweed {Amaranthus hybridus), sicklepod {Senna obtusifolia), and showy crotalaria {Crotalaria spectabilis). The pathogens tested were Phomopsis amaranthicola (pigweed pathogen), Alternaria cassiae (major host, sicklepod; alternative host, showy crotalaria), Colletotrichum dematium f. sp. crotalariae and Fusarium udum f sp. crotalariae (showy crotalaria pathogens). Spore suspensions of each pathogen (10® spores per ml) and a mixture of the four pathogens (1 : 1 : 1 : 1 v/v; total 1 0® spores per ml) were tested on the weed seedlings grown together in pots. The pathogens completely killed their respective weed hosts, when VI 11 used alone or as a mixture. The growth pattern of each pathogen was conditioned by specific interactions with the leaf surfaces of the different hosts, whether the pathogen was applied alone or in a mixture. The pathogens did not lose their efficacy and host-specificity when used as a mixture, and therefore, the pathogen mixture could be used to control several weeds simultaneously.Similarly, the bioherbicidal control of several weedy grasses with a pathogen mixture was evaluated using three fungi indigenous to Florida: Drechslera gigantea, Exserohilum longirostratum, and Exserohilum rostratum isolated from large crabgrass (Digitaria sanguinalis), crowfootgrass (Dactyloctenium aegyptium), and johnsongrass {Sorghum halepense), respectively. In trials conducted in a greenhouse, each pathogen (2x10ŝ pores/ml) as well as a mixture of the pathogens (1:1:1 v/v; total 2x10^spores/ml) caused 82.5-100% disease severity on large crabgrass, crowfootgrass, johnsongrass, southern sandbur {Cenchrus echinatus), guineagrass {Panicum maximum), Texas panicum {Panicum texanum), and yellow foxtail {Setaria glauca). In a host-range trial, crop plants tested were either immune or resistant to each pathogen and the pathogen mixture.In separate field trials conducted in Florida, an emulsion-based inoculum of D.gigantea, E. longirostratum, and E. rostratum (5x10^spores/ml) and a mixture of these pathogens (1:1:1 v/v; total 5x10^spores/ml) almost completely killed the seven weedy grasses mentioned above, and also, a natural population of guineagrass, a major weed in citrus in Florida. The weed control lasted for more than 10 weeks. Thus, D. gigantea, E. longirostratum, and E. rostratum have potential to be developed as bioherbicides for the management of weedy grasses. The multiple-pathogen strategy is a novel and effective means of weed control. IX a mixture on different weed hosts, (3) develop a technique to mass produce fungal inoculum for field studies, (4) field-test the "Multiple-Pathogen Strategy" for bioherbicidal control of several weedy grasses, and (5) field-test the pathogen mixture for bioherbicidal control of a natural population of guineag...
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