The Bioherbicidal Potential of the Seed-Borne Pathogen 'Pyrenophora semeniperda' for Control of Annual Grass Weeds in Cereal Crops

Abstract

Annual grass weeds are a major limitation to cereal production throughout the world and reproduce and spread only by the production of seed. One strategy for the control of annual grass weeds may be the inundative application of indigenous seed-borne pathogens as mycoherbicides to curb seed production and therefore re-infestation. A feasibility study was undertaken to investigate the potential of the seed-borne pathogen 'Pyrenophora semeniperda' for bioherbicidal control of the grass weed 'Bromus diandrus'. The requirements for optimal growth and sporulation were defined. Growth and sporulation were optimal at 23.2 °C and 19.2 °C respectively when grown on modified alphacel medium with a pH of between 4.7 and 5.7. 'P. semeniperda' required mycelial wounding and an alternating light/dark sequence for best sporulation with light wavelengths shorter than 500 nm. The variation in growth and sporulation of isolates of 'P. semeniperda' was investigated after using single-spore and mass transfer isolation techniques and storage. Some variation occurred but was attributed to chance and not genetic make-up. Conidia stored in aqueous suspension retained viability and pathogenicity for three months. 'P. semeniperda' was shown to produce toxic metabolites under 'in vitro' conditions. Using a bioassay which used coleoptile and radicle length as an assay of toxicity, it was shown that filtrates of 'P. semeniperda' cultures had a severe impact on seedling growth of both wheat and 'B. diandrus'. Infiltration of leaves with culture filtrates resulted in symptoms similar to those produced by conidial inoculation. The infection process of 'P. semeniperda' on seedling and adult leaves of wheat and 'B. diandrus' and floral tissue of wheat was investigated. ... The infection of field grown and inoculated grass species was investigated in three trials in Armidale, NSW and one in Orange, NSW. In one trial 73% of 'B. diandrus' seeds were infected by the fungus. Several different inoculum types were trialed. Application of conidia resulted in the greatest level of infection and application of mycelium resulted in moderate levels of infection. 'P. semeniperda was able to infect a variety of annual grass species under field conditions.