Submitted to: Journal of Plant Diseases and Protection
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/1/2004
Publication Date: 3/4/2004
Citation: Widmer, T.L., Guermache, F. 2004. Filtrates of rhizosphere bacteria suppressive to Centaurea solstitialis seed germination. Journal of Plant Diseases and Protection. Special Issue XIX:497-502 Interpretive Summary: Yellow starthistle (YST) is considered a noxious weed in the United States, originating from the Mediterranean region. It is a serious pest of pastures, rangelands, croplands, natural areas, and recreational areas. It is not completely understood why this weed is problematic in the U.S. but not in its native range where it exists in a nonaggressive manner, but lack of natural enemies in the U.S. is the likely cause. In its natural habitat in France populations are low and not invasive. We conducted this research to discover if some organism is limiting the spread. Bacteria isolated from the soil around the roots of YST were found to produce chemicals that inhibit the seed germination of YST. Some of these compounds were found to be inhibitory against YST seeds collected in France but not the U.S. One bacterium produced chemicals that were inhibitory to both. The primary benefit of this research is that it demonstrates that natural bacteria found in the soil may play a role in limiting the spread of this weed. This could be important for agencies and farmers who are interested in a biological approach to control of this weed.
Technical Abstract: Centaurea solstitialis (yellow starthistle) is an invasive weed in the United States that infests millions of hectares of rangelands, roadsides, and recreation areas. It is not completely understood why weeds often become invasive when transported to a new ecosystem. Many factors may be involved including soil microorganisms that influence plant growth and seed germination. Yellow starthistle (YST) roots removed from four sites in southern France were washed in distilled water. Individual bacterial colonies were isolated from the root wash and plated on nutrient agar. Sixteen isolates were grown on two different media for 1 week at 25ºC at either 150 or 250 rpm. The resulting culture was centrifuged and the supernatant adjusted to pH 7.2. The effect of the cell-free filtrates on the germination of YST seeds from France and the United States were tested by placing seeds on filter paper moistened with the bacterial filtrate in Petri plates. Five isolates showed some inhibition dependent upon the shaker speed and media used. Isolates 2(8) and 3(11) shaking at 250 rpm and 1(10) and 4(15) shaking at 150 rpm reduced germination of only French YST seeds. Isolate 2(10), identified as Streptomyces griseus, grown in a medium recommended for Streptomyces spp. at 250 rpm and isolate 4(15), identified as an Actinomycete, grown in a basic nutrient broth medium at 250 rpm inhibited both French and American YST seeds. The other eleven isolates showed no or limited inhibition of germination. These results suggest soil bacteria produce metabolites that inhibit YST seed germination and there is some response difference in the French and American seeds.