| Chrysanthemums are perennial flowering plants that were illustrated in Chinese literature as early as the 15th Century B.C. and have been cultivated as an herb for more than 3000 years. Chrysanthemums arrived in Japan during AD386 where highly prized cultivars were developed within a short period of time. Since the earliest recognition of chrysanthemums in China, popularity of this flower has continued to grow throughout the world.;Chrysanthemum White Rust (CWR), caused by Puccinia horiana Henn., is an autoecious, microcyclic rust that is pathogenic on many chrysanthemum species (Chrysanthemum spp.) and close relatives within the Asteraceae family. Chrysanthemum white rust is economically important due to its ability to infect florist chrysanthemum (Chrysanthemum x morifolium) cultivars used as cut flowers and potted plants throughout the world. Puccinia horiana can cause serious damage to chrysanthemums in commercial greenhouses as well as at homeowner sites. Many countries, including the United States, have established Phytosanitary Quarantines against CWR, due to financial loss from CWR and difficulty of eradicating the disease. Currently P. horiana is not considered indigenous in the United States.;The objectives of this study include determining the ability of P. horiana to overwinter in Pennsylvania, and to illustrate morphological details of P. horiana within infected chrysanthemum leaves. Although we previously reported the presence of P. horiana in the roots and stems of asymptomatic chrysanthemum plants, we had not provided detailed descriptions of the morphology, intercellular colonization, and intracellular growth of P. horiana in chrysanthemums until this paper. Images produced using compound microscopy and SEM in this paper provide additional, detailed illustrations regarding P. horiana colonization of chrysanthemum stems and leaves. Also, anastomosis between two adjacent strands of P. horiana hyphae is illustrated for the first time.;Another objective was to determine the economic impact of CWR based on incidence and severity of P. horiana on selected varieties at one location in central Pennsylvania, and to develop a molecular screening technique sensitive enough to detect latent, hidden infections of CWR. Federal regulations state that, for a chrysanthemum plant to be considered positive for P. horiana, it must exhibit visible teliospores on or in the infected plant. Thus, chrysanthemum plants that are determined to be positive for P. horiana via molecular screening do not legally have be eradicated, if teliospores or telial sori are not visible. This need for visualization of morphological symptoms may cause a serious time delay in recognition of diseased plants. This delay in identification of CWR increases the financial costs, since later eradication may involve digging and removing larger established plants. Sensitive and accurate molecular screening of chrysanthemum plants by using molecular methods, as demonstrated by our research, employed prior to dispersal of asymptomatic, yet P. horiana-positive, chrysanthemum plants could greatly reduce cost of production for wholesale and retail horticultural businesses, as well as homeowners.;The final objective of this study was to genotype P. horiana isolates collected throughout the United States, and to compare the genotypes of the United States isolates with previously collected worldwide isolates. This phase of the study considered 101 isolates, including 61 collected within the United States. The 61 United States isolates were collected from seven states. Six multilocus genotypes were identified within the United States, as compared to 28 for the worldwide collection. We determined there are four main genotypes in the United States, with one newly discovered in this study. This study suggests that five or six separate introductions into the United States have occurred. Supported by confirmation of P. horiana overwintering in the United States, this study indicates that P. horiana is now endemic within the United States. |
