Location: Foreign Disease-weed Science ResearchTitle: Changes in Canada thistle (Cirsium arvense) shoot density following inoculation with Puccinia punctiformis and induction of symptomatic and asymptomatic systemic disease Author
|Mcmahon, Michael - Mike|
|Keller, Kelli - Kelli Thomas|
|Luster, Douglas - Doug|
Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2015
Publication Date: N/A
Citation: N/A Interpretive Summary: Canada thistle (Cirsium arvense, CT) is one of the worst weed problems to farmers and ranchers in temperate areas of the world, and is not easily controlled with conventional weed management strategies. The rust fungus Puccinia punctiformis was first proposed as a biological control agent for CT in 1893. The rust causes systemic disease that ultimately kills CT plants. We demonstrated in international trials that applying rust spores to CT rosettes in the fall can successfully initiate systemic rust disease. We tested the rust fungus at 11 sites in four countries, and found that a significant percent of the infected CT plants gave rise to a systemically diseased shoot the following spring. CT density declined over time following infection. Means of improving disease spread and thistle density decline are discussed. These results demonstrate that the fungus can be an effective biological control for CT, and could therefore provide farmers and ranchers with an effective tool to manage this invasive weed.
Technical Abstract: Canada thistle (Cirsium arvense, CT) is one of the worst weeds in temperate areas of the world. The rust fungus Puccinia punctiformis was first proposed as a biological control agent for CT in 1893. The rust causes systemic disease which ultimately kills CT plants. In 2013, it was demonstrated in four countries that inoculation of CT rosettes in the fall with ground telia-bearing leaves can successfully initiate epiphytotics of systemic rust disease. With this approach, an average of 28.38 percent of inoculated rosettes in 11 sites in these countries gave rise to a systemically diseased shoot the following spring. Other rosettes that emerged near inoculation points in the spring were stunted and appeared diseased in the absence of a systemically diseased shoot. To determine whether other rosettes were actually diseased, a chemiluminescence western slot blot test, applying polyclonal antibodies raised against P. punctiformis antigens, was developed to detect the fungus in root systems of asymptomatic rosettes, i.e., those not bearing a systemically diseased shoot. Rosettes were inoculated with telia-bearing leaves in the fall in Maryland, USA and Veroia, Greece, and roots of asymptomatic rosettes that emerged adjacent to inoculation points the following spring were tested for the presence of the fungus with the slot blot test. Rosettes that gave rise to systemically diseased shoots were recorded. Based on the slot blot tests, 20.0 to 50.0 percent of the asymptomatic rosettes adjacent to inoculation points were positive for presence of the rust. CT densities per sq m were sampled annually in 9 sites, in three countries, that were inoculated with telia-bearing leaves in the fall between 2009 and 2012. Changes in CT densities over time were analyzed with a non-linear mixed model and with Markov Chain Monte Carlo Bayesian analysis. Both analyses showed that the CT density data fit an exponential decline model indicating CT density was predicted to decline to 1 shoot per sq m by about 3.5 years after inoculation with telia-bearing leaves. Average reductions in CT density across the 9 sites were 36.8 + 8.7 percent at 18 months after inoculation, 59.3 + 7.4 percent at 30 months after inoculation, and 71.4 + 23.4 percent at 42 months after inoculation; the 42 month reduction was, however, based on only two sites. Means of improving disease spread and thistle density decline are discussed.