Latent period and viability of Puccinia jaceae var. solstitialis urediniospores: Implications for biological control of yellow starthistle

Since the introduction of Puccinia jaceae var. solstitialis into California in 2003, multiple urediniospore generations have been observed, along with teliospores and pycnia, on yellow starthistle (Centaurea solstitialis). A field and laboratory study of urediniospore viability was conducted to determine the potential of using infected plant material for redistribution of inoculum into new areas. To better understand the life history of P. j. solstitialis, urediniospore viability was measured during the summer and fall when the host plant is usually senescent. Latent period (time from inoculation to pustule eruption) after field and laboratory inoculations was measured in order to determine the potential number of urediniospore generations that occur per year. Viability of urediniospores stored in the laboratory gradually declined over a period of 10 weeks; spore hydration increased germination. Based on the results obtained in this study, introductions of harvested infected plant material would have to be done quickly to offset losses in urediniospore viability. Urediniospore viability in the field decreased exponentially 86% near the coast and 97% in the Central Valley three weeks after production, and was negligible (0–0.2%) thereafter. Latent period ranged from four to five weeks at cool winter temperatures to two to three weeks at warm summer temperatures in both the laboratory and field. Results of this study suggest that urediniospores are not likely to remain viable through the summer and fall dry season. Teliospores, therefore, serve as the inoculum source when YST seedlings germinate in the winter. Approximately six urediniospore generations can be expected to occur each year in central-northern California under suitable conditions for disease.