|Devalerio, J. T.|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2005
Publication Date: 12/1/2005
Citation: Rosskopf, E.N., Ables, C.Y., Devalerio, J., Charudattan, R. 2005. Influence of epidemiological factors on the bioherbicidal efficacy of phomopsis amaranthicola on amaranthus spp.. Plant Disease. 89:1295-1300. Interpretive Summary: The efficacy of a weed biological control agent can be influenced by environmental factors as well as host condition. The biological control agent, Phomopsis amaranthicola, is a host-specific pathogen of pigweeds (Amaranthus spp.). The ability of the pathogen to cause high levels of plant mortality is influenced by the age of the plant; younger plants are more affected than older plants. Highest levels of plant mortality are achieved when the plants are exposed to 8 hours of dew or more with temperatures ranging from 25-35 C. The use of an invert emulsion enhanced the level of mortality possibly through phytotoxicity resulting from the invert emulsion alone. Efficacy was not increased by increasing the number of spores applied to greater than 106 conidia/ml.
Technical Abstract: Greenhouse experiments, conducted with Amaranthus hybridus as the test plant, determined the effect of dew period, temperature, plant growth stage, conidial concentration, and the addition of various adjuvants on the efficacy of Phomopsis amaranthicola as a biological control agent for Amaranthus spp. The dew period and temperature significantly influenced mortality levels of inoculated A. hybridus plants. Higher mortality resulted when inoculated plants are exposed to '8h of dew at temperatures between 25-35 C. Mortality was higher in younger plants (< 2-leaf stage to 2-to-4 leaf stage) than in older plants (4-to-6 leaf stage to flowering). Increasing the conidial concentration from 105 to 106 or 107 conidia/ml did not result in higher mortality levels. The type of adjuvant used in the formulation influenced conidial germination; conidia formulated with an organosilicone surfactant, a food grade algin, and a non-ionic surfactant had lower germination than conidia formulated with an invert emulsion, psyllium mucilloid, polysorbate, xanthan gum or water. The type of adjuvant also had a significant effect on biocontrol efficacy; application of P. amaranthicola conidia formulated with the invert emulsion resulted in the greatest proportion of dead plants (0.74). However, application of invert emulsion alone also caused plant death (0.33) due to phytotoxicity. The level of weed suppression through the use of P. amaranthicola could be enhanced by timing the application of the biocontrol agent with the early stages of weed growth, with periods in the day when there is sufficient moisture and warm temperature, and by applying the conidia with an additive, such as an invert emulsion.