The biological control of giant buttercup (Ranunculus acris) with a plant pathogen (Sclerotinia sclerotiorum)

Abstract

Ranunculus acris, a perennial pasture weed, is a problem in many dairy pastures throughout New Zealand, particularly Takaka Valley, Golden Bay. It is toxic to cattle, reduces feed in pasture and is a substantial cost to farmers through inadequate control techniques and lost productivity. It is resistant to the phenoxy herbicides MCPA (2-methyl-4-chlorophenoxyacetic acid) and MCPB 4-(4-chloro-2-methylphenoxy butanoic acid) at many sites throughout New Zealand, making these once useful herbicides largely ineffective at acceptable application rates. These characteristics of R. acris make it a suitable candidate for an alternative method of control, based upon the mycoherbicide strategy.

Sclerotinia sclerotiorum (isolate G45) when applied as a mycelial-on-wheat formulation can kill the petioles and leaves of R. acris in a glasshouse but the crown can persist, allowing the plant to regenerate. The aim of this study was to improve control of R. acris with S. sclerotiorum by increasing the efficacy of the pathogen.

Glasshouse pathogenicity tests were conducted to expand the number of isolates previously screened and to find other isolates as pathogenic as G45, should its virulence change during culture and storage. Fifty-four isolates of Sclerotinia spp. from R. acris and other hosts were applied as mycelial infested kibbled wheat onto 6 month old R. acris plants. Most isolates (90%) did not differ (P<0.05) in their virulence on R. acris. Several isolates (G1, G14, G41, G56, G64 and G75) including G45 were selected for further experimental use based on their ability to cause severe disease and most importantly suppress regeneration of R. acris. The apparently narrow range of virulence exhibited by isolates of S. sclerotiorum may have been caused by the physiological variation between plants or because there is no intraspecific variation of S. sclerotiorum isolates within New Zealand.

A serial dilution technique was used in an attempt to quantify the inoculum. The technique was simple and reproducible; there was no significant difference between dilution replicates. Crude estimates only were obtained (4.7 x 102-4.5 x 103 colony forming units per gram of mycelial infested kibbled wheat) because of difficulties with mycelial quantification including lysis of mycelial cells and the fact that one mycelial fragment may give rise to several S. sclerotiorum colonies. This method can be used to ensure the dose applied to each plant is of a roughly similar concentration and to allow comparisons of inoculum concentration between experiments.

A field experiment was conducted to determine S. sclerotiorum efficacy against R. acris infested dairy pasture in Takaka. The experiment was a split-plot design of 5 farms (replicates) each with two paddocks (mainplots) one mid-rotation and the other just grazed at the time of treatment with S. sclerotiorum and there were six sub-plots with 4 treatments. S. sclerotiorum was applied at 20 or 40 ml per plant. There were two controls - no inoculum and a sterilised slurry of 20 ml. Isolate G45 was formulated by Crop Care Holdings (NZ) Ltd., as a wettable powder (slurry) for application. After 10 weeks, regeneration was evident on most plants so a second application of the four treatments was made - one subplot was not inoculated, 20 ml of the sterilised slurry was applied to another subplot. Two subplots had 20 ml and the remaining two had 40 ml of the slurry. The first application had a significant effect on R. acris. Plants inoculated with 40 ml of the S. sclerotiorum slurry significantly reduced the total dry weight up to 57% of the untreated plants. There was no significant difference between application rate or the timing of the application within the grazing regime. By contrast, the second application had no significant effect on R. acris. This was probably because environmental conditions were less conducive to S. sclerotiorum infection at this time of application. This study has confirmed that S. sclerotiorum can control R. acris under glasshouse and field conditions. The pathogen has the potential to reduce the density and cover of R. acris in infested pasture, thus reducing the amount of artificial inputs (herbicides) on our pastures to create more sustainable agricultural systems. It will be necessary to conduct further investigation of techniques which manipulate the host, pathogen and environment to enhance S. sclerotiorum efficacy against R. acris.