A 3-yr field study was conducted to determine the effect of a biological control agent, Sclerotinia minor Jagger, and a common herbicide, Killex, on the population dynamics of dandelion and other broadleaf species and on the dandelion seed bank. Treatments were applied as one spring, one early autumn, or a spring plus an early autumn treatment per year. The response of the dandelion population to a spring herbicide treatment was similar to two applications per year (spring and early autumn). Significantly less dandelion control occurred after the first early autumn application of the herbicide. Two weeks after application, spring or early autumn treatments with S. minor were equally effective in suppressing dandelions. In the second year of the two applications per year of S. minor treatment, weed control was equivalent to the herbicide. By the third year of the one spring application of S. minor, weed control was equivalent to the herbicide. Generally over the study period, the early autumn application of S. minor was less effective than the spring or the spring and early autumn applications. The S. minor treatments significantly reduced the dandelion seed bank, and this effect was not significantly different from the Killex herbicide treatment. The rate, frequency, and seasonal timing of application had no effect on the dandelion seed-bank size, but terminating the application would gradually replenish the seed bank. Populations of white clover, broadleaf plantain, birdsfoot trefoil, and common ragweed were similarly suppressed by either the S. minor or the herbicide treatments. Yellow woodsorrel significantly increased after 1 yr of herbicide treatment compared with the S. minor and untreated control treatments, indicating a possible weed species shift. Turf quality was improved because of the herbicide and S. minor treatments, but grass injury and smooth crabgrass invasion were recorded in 17% of herbicide-treated plots.

Nomenclature: Killex (2,4-D, mecoprop, dicamba), birdsfoot trefoil, Lotus corniculatus L. LOTCO, broadleaf plantain, Plantago major L. PLAMA, common ragweed, Ambrosia artemisiifolia L. AMBEL, dandelion, Taraxacum officinale Weber ex Wiggers TAROF, smooth crabgrass, Digitaria ischaemum (Schreb.) Schreb. ex Muhl. DIGIS, white clover, Trifolium repens L. TRFRE, yellow woodsorrel, Oxalis stricta L. OXAST