Effects of Dew, Plant Age, and Leaf Position on the Susceptibility of Yellow Starthistle to Puccinia jaceae. A. R. Bennett, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Foreign Disease-Weed Science Research, Fort Detrick, Bldg. 1301, Frederick, MD 21702. W. L. Bruckart, and N. Shishkoff. Research Plant Pathologists, Agricultural Research Service, U.S. Department of Agriculture, Foreign Disease-Weed Science Research, Fort Detrick, Bldg. 1301, Frederick, MD 21702. Plant Dis. 75:499-501. Accepted for publication 3 November 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/PD-75-0499.

Potential for biocontrol of yellow starthistle (YST) (Centaurea solstitialis) by Puccinia jaceae was evaluated in the greenhouse over a wide range of dew temperatures and periods to determine optimum conditions for infection. Disease severity (pustules per square centimeter of leaf area) was greatest when inoculated plants were exposed to dew for 12 or 16 hr at 20 C. Pustule development occurred 15 days after inoculation at 15 C and 10 days after inoculation at either 20 or 25 C. Inoculation of YST plants four times per week beginning 4 wk after planting resulted in reductions of the dry weights of root by 40% and shoots by 50% compared with the uninoculated control. Plants inoculated more than once beginning 4 wk after planting also had significantly lower root biomass than controls. YST rosettes were most susceptible 4–6 wk after planting, based on pustule counts. Nearly all leaves on 6-wk-old rosettes were uniformly susceptible to infection. Based on results of this study, P. jaceae has the potential to become established in and damage YST in North America.