Based on field surveys and evaluations in the greenhouse, two fungal pathogens, Bipolaris sacchari and Drechslera gigantea, were identified as promising biological control agents for cogongrass. In greenhouse trials, the application of spore suspensions of these fungi containing 105 spores/ ml in a 1% aqueous gelatin solution to cogongrass plants and their incubation in a dew chamber for 24 h resulted in disease symptoms that ranged from discrete lesions to complete blighting of leaves. Disease severity (DS), based on a rating scale for southern corn leaf blight with 50% as the maximum DS rating, ranged from 42 to 49%. In greenhouse experiments, the application of spores formulated in an oil emulsion composed of 4% horticultural oil, 10% light mineral oil, and 86% water resulted in higher levels of foliar blight with no dew exposure or shorter periods of dew exposure (4, 8, or 12 h) as compared with the application of spores formulated in 1% gelatin. Field trials demonstrated that under natural conditions, the application of a spore and an oil emulsion mixture containing 105 spores/ml of either fungus could cause foliar injury from disease and phytotoxic damage from the oil emulsion. Depending on the application rate (100 or 200 ml/plot), the level of foliar injury ranged from 40 to 86% (based on a field assessment scale of 0 to 100% foliar injury) with B. sacchari as the test fungus. However, with D. gigantea as the test fungus, foliar injury ranged from 9 to 70% depending on the application volume and the oil concentration used. Although B. sacchari and D. gigantea were capable of causing foliar blight on cogongrass, the regenerative ability of the rhizomes allowed cogongrass to recover from the damage caused by these fungi. However, the level of injury caused by these fungi is sufficient to support their use as components for integrated management of cogongrass.
Nomenclature: Cogongrass, Imperata cylindrica (L.) Beauv. #3 IMPCY; corn, Zea mays L.
Additional index words: Bioherbicide, biological control, Bipolaris sacchari (E.J. Butler) Shoemaker, Drechslera gigantea (Heald & F.A. Wolf) Ito.
Abbreviations: CRD, completely randomized design; DAI, days after inoculation; DS, disease severity; RH, relative humidity; SOE, spore and oil emulsion mixture; WAI, weeks after inoculation; WM, weed mortality.