Phomopsis amaranthicola, Broad-Spectrum Bioherbicide to Control Amaranthus spp. UF#1095 and UF#1343 A Synopsis

Concept: To develop an indigenous, host-specific fungal pathogen, Phomopsis amaranthicola, as a broad-spectrum bioherbicide for several susceptible species of Amaranthus. Various Amaranthus spp. are among the most important weeds in crops. Although chemical herbicides are commonly used to control these weeds, a bioherbicide would be accepted by organic growers and for crops where chemical herbicides lack selectivity or have satisfactory efficacy (e.g., sugar beets). Based on our contacts, we feel that organic growers and vegetable growers in Florida will be interested in using a P. amaranthicola-based bioherbicide. Furthermore, the bioherbicide may be used alongside chemical herbicides to manage herbicide-resistant Amaranthus spp. Amaranthus spp. are among the most common herbicide-resistant weeds found in herbicide-intensive crops.

Pathogen: The fungus was discovered in Gainesville, FL and described as a new species (publication enclosed). Its distribution outside Florida is unknown at this time because of lack of reports. A related fungus, P. amaranthophila, has been described from Brazil, but without valid examination of our material (Fitopathologia Brasileira 24(2): 185-189, 1999). Our preliminary examination of the Brazilian isotype leads us to believe that the two fungi belong to different species. A molecular analysis of the two species is yet to be done.

Susceptible hosts: Several Amaranthus species and regional biotypes of some amaranth species have been tested and found to be susceptible to P. amaranthicola; some are highly susceptible, while others are less susceptible. Some members of economically important Amaranthus spp. (e.g., those cultivated as edible or ornamental species) are susceptible, but this is not considered a major issue because the bioherbicidal use of P. amaranthicola would be site-specific. Furthermore, we have conducted an extensive host-range study (publication enclosed) and no plant outside of Amaranthaceae has been found to be susceptible. Hence, P. amaranthicola is considered a highly host-specific, safe biocontrol agent.

Symptoms: P. amaranthicola causes foliar lesions, leaf blight, and stem lesions. Stem lesions are important to the efficacy of this fungus; these lesions cut off the vascular flow and cause the tissues above the affected points to be killed. The plant then topples and wilts. Symptoms appear in about one week after the fungus is sprayed on aerial portions of Amaranthus plants and the disease progresses steadily over the following weeks. The fungus produces secondary inoculum on the foliar and stem lesions.

Environmental factors: A minimum dew period of 8 h is needed for disease development; the dew duration can be shortened by using suitable formulations. The fungus is infective over a wide range of temperatures (25-35 C). Both spores (conidia) and mycelium are infective, although conidia are more effective in initiating disease. Optimum inoculum level for disease establishment in the field is 106 conidia per ml, applied in 900 liters of water per ha. Reductions in application volume and inoculum level are possible through suitable formulations, but further research is needed on these aspects.

Biocontrol efficacy: The efficacy of the fungus has been confirmed in field trials repeated over the past six years in Florida. Results from a recent trial in a vegetable production field yielded highly successful results (presented at the 25th Annual Meeting of the Florida Weed Science Society and the Weed Science Society of America; abstracts enclosed).As stated above, the bioherbicidal efficacy of P. amaranthicola is host-species and host-age dependent; some Amaranthus spp. are more resistant and, in general, younger seedlings are more susceptible than older plants. For best results, the fungus should be applied in the field before the Amaranthus plants flower and set seeds.

Cultural characteristics: P. amaranthicola can be grown easily on common laboratory media such as V8-juice agar, tomato-paste agar, potato-dextrose agar, and several inexpensive substrates. The fungus can be grown in liquid culture to produce mycelial mass but not spores. The fungus has been highly stable with respect to sporulation, virulence, or cultural characteristics.

Research and development needs: Studies on large-scale inoculum production and formulation should be done, preferably with an industrial partner committed to commercial development of this bioherbicide agent. Commercial prototype formulations can then be tested in growers' fields to develop efficacy and other data needed for EPA registration.