Supplemental risk evaluations of Puccinia jaceae var. solstitialis for biological control of yellow starthistle

Additional tests of native North American Cirsium species, Saussurea americana, and modern safflower cultivars (Carthamus tinctorius) were requested by regulators and specific interest groups during the risk assessment of foreign isolates of Puccinia jaceae var. solstitialis for biological control of yellow starthistle (YST, Centaurea solstitialis) in the United States. These tests supplement an earlier, extensive host range determination that established P. jaceae from YST as generally host specific and potentially useful for biological control. The additional research was in response to potential hazards identified in an earlier study, changes in safflower cultivars, and concern that P. jaceae might cause a safflower seedling disease similar to hypocotyl infections from infestation by Puccinia carthami teliospores. S. americana, a close relative of yellow starthistle, had not been tested previously. All tests were conducted in a containment greenhouse. Foliage of 19 Cirsium species, 11 safflower cultivars, and S. americana was inoculated with urediniospores and subjected to a 16-h dew period at 18–20 °C. Neither the Cirsium species nor S. americana became infected after foliar inoculations. Compared to foliar infections by P. carthami from safflower in California, only minor infections developed from inoculations with P. jaceae. These were similar to infections observed in earlier studies, and it was not possible to maintain P. jaceae under optimal greenhouse conditions on safflower foliage. Quantitative teliospore inoculations with P. jaceae did not cause infection on safflower hypocotyls, even though large cankers occurred on plants inoculated with P. carthami teliospores. Clear microscopic evidence of infection also was observed in hypocotyls inoculated with P. carthami. These data suggest that native (including rare, threatened, or endangered) Cirsium spp., modern safflower cultivars, and S. americana are not likely to be adversely affected by the use of P. jaceae for biological control of YST. Results from these studies substantiate previous findings and were incorporated in a proposal for permission to use P. jaceae for YST control in California.