1USDA-ARS-FDWSRU, 1301 Ditto Ave., Ft Detrick, Maryland 21702, USA; 2Consultant, Gaithersburg, Maryland, USA

Background and objectives
Yellow starthistle (Centaurea solstitialis), YST, infests over 3 million ha in the state of California and is a major weed pest of ranges and pastures in the western USA. Because this introduced plant infests large areas and is difficult to control using conventional herbicide or cultural approaches, several natural enemies have been evaluated for the biological control of YST, including foreign isolates of a rust fungus, Puccinia jaceae. One objective of this research is to learn if the candidate pathogen is safe to use around valuable crop and native US plants before introduction is allowed. A risk assessment is made in a containment greenhouse facility to identify potential hazards. Comparative studies with indigenous US pathogens clarify and substantiate judgments about risk based on containment greenhouse data [1]. Hazards deemed reasonable (low risk) are further characterized until final judgment can be made regarding the safety of the organism for introduction and biological control. P. jaceae was identified in earlier studies as low risk to safflower, because urediniospore inoculations caused only limited foliar infections, and no damage, to safflower (Carthamus tinctorius), compared to disease from P. carthami, the safflower rust pathogen. But P. carthami can limit safflower production in the USA as a seedling disease from teliospore-infested seeds or soil. Data on safflower seedling susceptibility to P. jaceae teliospores are presented as part of the risk assessment of this candidate pathogen for biological control.

Materials and methods
Teliospores of P. jaceae from Iran and Bulgaria were collected from pustules on dry leaves of YST, surface sterilized, rinsed, and re-suspended in 0.1% water agar with a wetting agent (WAWA). The teliospores were primed at 4C on water agar for 4 weeks and then re-suspended for inoculation. Teliospores of P. carthami were stored dry at 4C before suspending in WAWA for inoculation. Spore concentrations were adjusted before inoculation. 2-day-old safflower seedlings of nine US cultivars were inoculated with a 1 l drop of teliospore suspension on the hypocotyl at the base of the cotyledonary leaves. Seedlings were set on the surface of a potting mix and covered with 2.5 cm sand. Plants were grown at 13-15C in a greenhouse and were misted with 2.5 cm of water twice weekly. Rating for disease and record of top fresh weights was made after 6 weeks. Teliospore viability was determined on water agar in a petri dish that included a germinated safflower seedling. The average number of viable teliospores in each treatment was calculated to determine the amount of viable inoculum per treatment.

Results and conclusions
No macroscopic symptoms occurred on safflower in the controls or P. jaceae treatments, but the average disease rating in the P. carthami treatment was 1.14 (0-2, dead/dying). Amounts of inoculum did not differ statistically (P=0.05) between P. jaceae and P. carthami, and the amounts used were sufficient to cause significant damage by P. carthami. Mean fresh top weights of nine cultivars were 10.0, 9.5 and 7.3 g in the control, the Iranian P. jaceae (at 38.8 teliospores/seedling) and the P. carthami (at 30.0 teliospores) treatments, respectively. Similar results occurred in a second study comparing the Bulgarian isolate of P. jaceae with P. carthami. In each experiment, the fresh weight of safflower inoculated with P. carthami was at least 20% less than controls (significant, P=0.05). These and other data [2] in the risk assessment of P. jaceae suggest it will be safe to introduce in the USA for biological control of YST.

1. Bruckart WL, Supkoff DM, Yang SM, 1996. In Proceedings of the IX Symposium on Biological Control of Weeds, University of Cape Town, South Africa, pp. 71-77.
2. Bruckart WL, 1989. Plant Disease 73, 155-160.