The potential use of ascospores as the active ingredient for aSclerotinia sclerotiorum-based mycoherbicide

Abstract

A Sclerotinia sclerotiorum-based mycoherbicide has shown potential in previous studies for control of Ranunculus acris in New Zealand dairy pastures. Historically, mycelium has been used to inoculate wheat grains, which are then milled and broadcast onto the R. acris-infested pasture. Since ascospores are responsible for most infections under natural conditions, this inoculum type may be more pathogenic than mycelium and, therefore, better suited than mycelium as the active ingredient of a mycoherbicide. To test this hypothesis, two screening studies were conducted on detached R. acris leaves: one to determine an appropriate nutrient amendment for ascospore infection and another to determine the effect of leaf wounding on ascospore infection. The ascospores infected detached, wounded leaves when amended either with distilled water or inorganic phosphate (KH2PO4 or Pi)+5.5mM glucose but they infected intact leaves only when amended with Pi+5.5mM glucose. Subsequently, an experiment using detached leaves was conducted (1) to compare infection by mycelium and ascospores of intact leaf tissue and (2) to determine an ascospore concentration that produced equivalent infection to that of the mycelial inoculum. Only the highest ascospore concentration (2.5 ×106 ascospores/mL) gave similar infection to a 5-mm mycelial plug from 8 days after inoculation onwards and no infection was observed with ascospore concentrations below 2.5×103 ascospores/mL. Results fromthis research indicate that ascospores have potential as the active ingredient for the mycoherbicide and should be further investigated in whole plant experiments.