VIEW ARTICLE

Ecology and Epidemiology

Evaluation of Infection of Target and Nontarget Hosts by Isolates of the Potential Biocontrol Agent Puccinia jaceae that Infect Centaurea spp.. N. Shishkoff, Research affiliate, U.S. Department of Agriculture, Agricultural Research Service, Foreign Disease-Weed Science Research, Bldg. 1301, Ft. Detrick, Frederick, MD 21702, Present address: Dynamac Corp., 2275 Research Blvd., Rockville, MD 20850-3268; W. Bruckart, research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Foreign Disease-Weed Science Research, Bldg. 1301, Ft. Detrick, Frederick, MD 21702. Phytopathology 83:894-898. Accepted for publication 3 May 1993. 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, 1993. DOI: 10.1094/Phyto-83-894.

In order to evaluate damage caused by the rust Puccinia jaceae to target and nontarget plants, number of pustules per leaf, dry root weight, and rate of leaf senescence were assessed. Each of five rust isolates was inoculated onto the host from which it was collected (yellow starthistle, purple starthistle, or diffuse knapweed) and also onto a nontarget host (cornflower) that is susceptible to all five isolates. Skeleton weed inoculated with P. chondrillina was included in the study as a comparable successful biocontrol system. On target hosts, there was no significant correlation between number of pustules and root biomass, which suggests that disease severity assessments based on pustule counts alone are a weak measure of biocontrol potential. When whole plants were inoculated, isolate YST71 caused a significant reduction in the root biomass of yellow starthistle, but other isolates of P. jaceae had little effect on their target hosts. The root biomass of yellow starthistle and that of diffuse knapweed were reduced significantly with inoculation of up to eight leaves, whereas no significant reduction of root biomass was seen in purple starthistle. When P. jaceae was inoculated onto cornflower, infections caused by the five isolates did not differ in severity when measured as pustules per leaf and caused no reduction of root biomass. The differences in infection-induced leaf senescence among target hosts and cornflower appeared host-related, which suggests that characteristics of plants, such as the number of leaves per rosette and a healthy leaf lifespan, may influence their responses to infection. The response of yellow starthistle to P. jaceae in greenhouse tests was comparable to that of skeleton weed to P. chondrillina.