The potential of Sclerotinia sclerotiorum as a mycoherbicide for the perennial pasture weed Ranunculus acris subsp. acris L.

Abstract

Ranunculus acris subsp. acris L. is a vigorous, perennial weed, and a serious problem in dairying regions of New Zealand. Evolved resistance to the phenoxy herbicides (MCPA, MCPB and 2,4-D) prompted the search for an alternative method of control, based on the mycoherbicide strategy.

Nineteen isolates of Sclerotinia sclerotiorum and three isolates of Sclerotinia minor were inoculated onto excised tissues and intact plants of R. acris to evaluate their pathogenicity. All isolates proved pathogenic, with S. sclerotiorum more pathogenic than S. minor, on both excised tissues and intact plants. Seven of the S. sclerotiorum isolates were more pathogenic than the others on excised tissues, but no differences in pathogenicity were found between any of the isolates when inoculated onto intact plants. Many plants were able to recover from buds on the crown, the underground perennating organ of R. acris. The results from the two screening tests did not correlate because the capacity of an isolate to invade the living crown could not be determined using the excised tissue method. This indicated that the excised tissue method could not be used to predict whole plant mortality, nor therefore the mycoherbicide potential of isolates of S. sclerotiorum for this weed.

Aspects of the infection process in intact plants, and in the crown of R. acris, were investigated. The apical and axillary buds on the crown were identified as the specific targets for S. sclerotiorum inoculation since recovery occurred from these sites. The relative importance of crown resistance, and rapid rate of regrowth of R. acris, were ascertained by inoculating plants with and without pre-treatment with chlorsulfuron to suppress regenerative growth. Crown resistance prevented plant mortality, and all plants retained potentially viable axillary buds on remaining crown fragments. Crown resistance was identified as the key factor facilitating plant recovery.

An anatomical study of the crown of R. acris was combined with histopathological studies of S. sclerotiorum infection in the crown. Crown tissues varied in their susceptibility to S. Sclerotiorum. Resistance was largely related to morphological features of the crown, these being a thickened, possibly suberised peripheral cortex, apparent deposition of lignin and suberin at the margin of infected tissue, and the resistance of the crown's dense network of vascular tissues. In addition, flowering stems were not rapidly invaded by the pathogen. Together, these non-specific defence mechanisms limited infection within the crown of vegetative and flowering plants, and enabled recovery of the plant from the regenerative buds.

Factors influencing the regenerative potential of R. acris were investigated. The biomass and available carbohydrate status of the crown, and the regenerative potential of R. acris were studied at six phenological stages from pre- to post-flowering. There was no obvious period of reserve depletion in the crown during flowering to provide an opportunity for control with S. sclerotiorum. The crown biomass and available carbohydrate status were lowest at preflowering, and increased during the flowering period. Crown available carbohydrate reserves constituted 60% of crown dry weight at post-flowering. The regenerative potential of R. acris after S. sclerotiorum inoculation was generally lowest at pre-flowering, and greatest at late flowering.

The capacity of this weed for vegetative multiplication following S. sclerotiorum inoculation, particularly at late-flowering, was an important factor influencing regenerative potential, and hence plant survival.

The effect of repeated defoliation on regenerative potential, crown biomass, crown available carbohydrate, and susceptibility to S. sclerotiorum, was evaluated. Six month old plants of R. acris maintained regrowth during repeated defoliation without depletion of crown biomass nor crown available carbohydrate. This indicated that these plants utilised resources other than carbohydrate reserves to support regrowth. In contrast, the regenerative potential of two year old plants of R. acris declined during repeated defoliation, and the crown biomass, and available carbohydrate reserves were depleted. This provided strong evidence for utilisation of carbohydrate reserves to support regrowth in these plants. Six month old plants which had been repeatedly defoliated were more susceptible to S. sclerotiorum. In all plants, repeated defoliation reduced the recovery response following S. sclerotiorum inoculation.

The results of this study have demonstrated how the perennial nature of R. acris limits the efficacy of S. sclerotiorum as a mycoherbicide. An effective control strategy for R. acris may require an integrated approach which combines defoliation followed by inoculation with S. sclerotiorum during the pre-flowering phase, when regenerative potential may be lowest.