Population dynamics and spread of Puccinia carduorum in the eastern United States

Puccinia carduorum was first introduced into western Virginia in 1987 for biological control of musk thistle (Carduus thoermeri). The pathogen's distribution was surveyed in 1992; it had spread to South Carolina, Georgia, Tennessee, Kentucky, Ohio, southeast Indiana, Maryland, and Delaware, some points more than 500 km from the release site. In 1994, it was found west of the Mississippi River in north central Missouri. Population development of the rust was monitored in several natural musk thistle stands in 1991 and 1992. Average pustule numbers per leaf were 0 to 0.7 in early May during early stem elongation, 0 to 17 (1991) and 8 to 52 (1992) at seed ripening in late June and early July as old plants died, 0.2 to 2.5 on young rosettes in September and October, and declining to near 0 by early December. Germinability of urediniospores from green tissue ranged from 10 to 88% (mean 51%) from May to October, with no significant seasonal trend. Teliospores were present on dead plants in late July and August but did not become prevalent on young rosettes until October and November. Latent periods (days from inoculation to first open pustule) on plants in the field were 13 to 14 days for inoculations in late April and early May, 8 days in June, 17 days in early October, and about 25 days in late October.