Proceedings of the Sixty-first Annual Meeting of the Northeastern ...

19
BIOLOGICAL CONTROL OF CANADA THISTLE IN PASTURES AND PARKS: A CALL
FOR COLLABORATORS ON SIMPLE FIELD TESTS. D.K. Berner, USDA, ARS,
FDWSRU, Ft. Detrick, MD and P.A. Backman, Dept of Plant Pathology, The Pennsylvania
State Univ., University Park.
ABSTRACT
Canada thistle (Cirsium arvense (L.) Scop.) is one of the most noxious invasive
weeds in North America. It occurs on millions of acres of pastures, agricultural land, and
natural areas. It is difficult to control because it has an extensive root system and
reproduces by seeds and root buds, the latter of which are multiplied and distributed by
plowing. It can be controlled by intensive herbicide applications, but in pastures and
natural areas this is economically prohibitive. Biological control may affordable and
effective. The obligate rust fungus, Puccinia punctiformis (F. Strauss) Rohl., is perhaps
the first plant pathogen proposed as a biological control agent for Canada thistle or any
other weed. In 1893 a NJ farmer noticed diseased thistle patches virtually disappeared
after a few years; he proposed that the rust should be widely disseminated for weed
control. Successful biological control of Canada thistle using P. punctiformis is hindered
due largely to a heterogeneous distribution of teliospores in the soil leading to low
incidence of systemically infected (SI) shoots. Homogeneous distribution of teliospores
over extended periods of time may overcome this problem.
In 2003-2005, we conducted experiments in two field sites naturally infested with
Canada thistle and the rust. SI thistle shoots in each replicate in each field site were
counted, and randomly selected plots were mowed while others remained unmowed. Our
hypotheses were that a) mowing could remove apical dominance and result in emergence
of more SI shoots within the current and subsequent growing seasons, and b) mowing
could re-distribute teliospores from SI shoots and result in more homogeneous
distribution and greater number of diseased shoots the following season. Mowing
increased numbers of SI shoots within and between seasons compared to unmowed
plots. Numbers of SI shoots in unmowed plots also increased, but not to the degree of the
mowed plots. Results were consistent with a 1923 study, which indicated that in unplowed
pastures, SI shoots increased exponentially. Predictions from both studies indicated that
100% disease incidence could be expected in mowed and unmowed plots within 2.7-5.2
yr of disease establishment. With greater initial disease (shoots), progression to 100%
disease incidence might proceed more rapidly.
We propose to field-test, in multiple pasture and natural area sites, the ability of the
rust to reduce healthy thistle density. We would produce SI seedlings in our facilities and
place them in Canada thistle patches. This should provide prolonged production of
teliospores and more homogeneous distribution of inoculum. We seek collaborators to
help establish test sites and monitor disease progression and healthy thistle density. Our
procedures for establishing disease and monitoring field sites will be presented.
1