Influence of Epidemiological Factors on the Bioherbicidal Efficacy of Phomopsis amaranthicola on Amaranthus hybridus

The genus Amaranthus L. comprises numerous species of broadleaf plants that are commonly called pigweeds or ama- ranths. These species are predominantly herbaceous or sometimes woody annuals with a well-developed taproot that allows them to thrive under dry conditions. Al- though some species are considered to be excellent grain crops or leafy vegetables (7,22), some are regarded as agronomic pests in many countries of the world, caus- ing significant reduction in the yield of crops such as sugar beet (Beta vulgaris (L.) Beauv.), sugar cane (Saccharum offi- cinarum L.), potato (Solanum tuberosum L.), wheat (Triticum aestivum L.), soybean (Glycine max (L.) Merr.), cotton (Gos- sypium hirsutum L.), coffee (Coffea spp.), oat (Avena spp.), peanut (Arachis hypogea ABSTRACT Rosskopf, E. N., Yandoc, C. B., Charudattan, R., and DeValerio, J. T. 2005. Influence of epide- miological factors on the bioherbicidal efficacy of Phomopsis amaranthicola on Amaranthus hybridus. Plant Dis. 89:1295-1300. Greenhouse experiments were performed to determine the effect of dew period temperature and duration, plant growth stage, conidial concentration, and the addition of adjuvants on the bioher- bicidal efficacy of Phomopsis amaranthicola on Amaranthus spp., using Amaranthus hybridus as test plant. P. amaranthicola infected A. hybridus at 20, 25, 30, and 35oC but the disease level achieved at 20oC may not be sufficient to cause high plant mortality. Plant mortality was also significantly lower in plants that were exposed to 4 h of dew. Plants at less than two- to two- to four-leaf stage were more easily killed than older plants, and increasing conidial concentration from 105 to 106 or 107 conidia ml-1 did not result in higher mortality levels. Among the adjuvants tested, polyalkyleneoxide-modified heptamethyltrisiloxane, algal polysaccharide, hyrdroxyethyl cellulose, and octylphenoxy polyethoxyethanol reduced conidial germination. Conidia applied with invert emulsion caused the highest plant mortality (74%) but invert emulsion alone caused 33% plant death due to phytotoxicity. Results indicate that P. amaranthicola can infect and kill Amaranthus spp. under a range of temperature, dew period, and inoculum levels and, therefore, has good potential as a bioherbicide agent.