Biology and control of centaurea diffusa lam. and centaurea maculosa lam
Creators: name: Watson, Alan Kemball
Publisher: name: University of British Columbia
Year: 2011

description: Centaurea diffusa was found to infest 64,079 acres in the semiarid interior of southern British Columbia. C. maculosa has infested an additional 8,420 acres. These weedy plant species are common along roadsides-and in waste places and are spreading rapidly over vast areas of semiarid rangeland. The density of the knapweed species could only be correlated with the degree of soil disturbance and was not correlated with any chemical property of the soil. The knapweeds are not generally grazed by domestic livestock and substantially reduce the forage yields in heavily infested areas. C. diffusa and C. maculosa prevent soil erosion on disturbed sites as these pioneer species are capable of rapid colonization of these sites. These species are utilized to some extent as honey plants. However, the losses attributed to knapweed infestations generally override any potential use of the knapweeds. Seeds of C. diffusa and C. maculosa germinate readily over a wide range of conditions. Continuous light was found to significantly retard the gemination of both species. The optimum temperature for germination of C. maculosa was found to be lower than that for C. diffusa seed. The optimum soil depth for emergence of both species was on the soil surface. C. diffusa seed did not emerge from soil depths below 3 cm. C. maculosa seed emerged from a soil depth of 5 cm. Phenological data were recorded for both species. C. maculosa begins to flower in early July with C. diffusa flowering approximately one or two weeks later. The annual reproductive capacity of C. diffusa was determined to be 665 with C. maculosa exhibiting an annual reproductive capacity of 298. Dual mechanisms of seed dispersal were observed for both species. Propagule dispersal is mainly by wind. Cultural methods of control were generally found not to be beneficial in controlling the knapweeds. The knapweeds can be adequately controlled by using the herbicide picloram, 4-.amino - 3,5,6 - trichloropicolinic acid, however, the high cost of this herbicide has greatly limited its use. Phytophagous insects, Urophora affinis and Metzneria paucipunctella, have been studied as potential biocontrol agents for the knapweeds. Initial releases of Urophora affinis indicate that the insect is capable of survival in the southern interior of British Columbia. The population has increased substantially since the initial release in 1970. It will take up to 10 years or more, however, before the releases of U. affinis are sufficiently well established to reduce the knapweed infestations. Two fungal organisms, Sclerotinia sclerotiorium and Microsphaeropsis centaureae, were isolated from diseased C. diffusa in the Vernon area. The potential of these organisms as biotic agents has, as yet, not been determined. Weed control methods must be associated with other appropriate management practices to produce increases in forage yields. Such an integrated approach to the control of diffuse and spotted knapweed will substantially reduce the extent of the knapweed infestation in the southern interior of. British Columbia.