Biological control of Californian thistle (Cirsium arvense) with the rust fungus (Puccinia punctiformis)

Abstract

Cirsium arvense, commonly known as Californian thistle, is a problematic weed in New Zealand and around the world. In agricultural systems, C. arvense affects animal health and severely decreases plant yield. C. arvense is a highly invasive weed due to its creeping root system. This root system can be difficult for herbicides to control because it does not completely kill the roots, which overwinter in the soil, producing shoots in the spring. Puccinia punctiformis is a rust fungus that only infects C. arvense. P. punctiformis has the potential to be a successful biological control agent as it is already present in New Zealand and can systemically and locally infect C. arvense. The systemic infection by the rust fungus damages the roots and reduces shoot abundance in the weed. The aim of this research was to determine if the observed differences in the levels of infection in C. arvense populations within New Zealand was due to genetic differences between P. punctiformis isolates. A survey of 22 sites in the North and South Islands of New Zealand was undertaken to determine the percentage of P. punctiformis infected C. arvense shoots in a designated area. Puccinia punctiformis was found in all 22 populations surveyed and the amount of P. punctiformis infected shoots varied between all sites. The Ruakura site had the highest amount of P. punctiformis infected shoots (11.1%), which was significantly higher than 15 other sites. Genetic analysis of the P. punctiformis isolates from some of the survey sites was undertaken by PCR. Sequencing of the ITS region indicated only one isolate (Hurst) out of 11 sequenced differed genetically (6 bp substitution). Comparatively, polymorphisms were observed with RAPD amplification which indicated genetic diversity across the whole genome for samples within both the population and between populations. Quantification of P. punctiformis in planta by qPCR determined that rust levels were at higher concentrations at the top of the plant and decreased to the base.

A glasshouse study was carried out to facilitate infection in order to increase the amount of inoculum for further work. No discernible infection methods were obtained. Only three out of the 32 treated plants had any infection, thus, considerable future work needs to be undertaken for a better infection method to be obtained. The study has highlighted the difficulties in dealing with P. punctiformis which infects C. arvense. Infection in the field and the glasshouse is limited and variable. The study has indicated there is genetic diversity in P. punctiformis in New Zealand which could explain the observed differences in infection of C. arvense in the field. Keywords: Puccinia punctiformis, Cirsium arvense, infection, genetic, diversity