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Title: Evidence for systemic infection by Puccinia horiana, causal agent of Chrysanthemum White Rust, in Chrysanthemum

Author
item Bonde, Morris
item Murphy, Charles - Charlie
item Bauchan, Gary
item Luster, Douglas - Doug
item PALMER, CRISTI - Rutgers University
item Nester, Susan
item REVELL, JASON - Rutgers University
item Berner, Dana

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2014
Publication Date: 8/14/2014
Citation: Bonde, M.R., Murphy, C.A., Bauchan, G.R., Luster, D.G., Palmer, C.L., Nester, S.E., Revell, J., Berner, D.K. 2014. Evidence for systemic infection by Puccinia horiana, causal agent of Chrysanthemum White Rust, in Chrysanthemum. Phytopathology. http://dx.doi.org/10.1094/PHYTO-09-13-0266-R.

Interpretive Summary: Chrysanthemum white rust, caused by PUCCINIA HORIANA Henn., is a significant fungal pathogen first discovered in the U.S. in 1977. The disease was eradicated and for many years successfully controlled by fungicides and strict regulatory measures, but more recently Chrysanthemum white rust has reappeared at an increasing frequency in the U.S., causing significant economic losses to chrysanthemum producers. We initiated the present study to identify how the disease develops and progresses within the chrysanthemum plant after infection by the fungus. Electron microscopy was used to follow the development of the fungus on the surface of leaves, as well as inside leaves, stems, and crowns of inoculated chrysanthemum plants artificially exposed to simulated northeastern U.S. winter conditions. We found the fungus in conductive tissue within leaves, stems and the crown, often nearly replacing entire cell walls. Individual fungal cells in mature cells of the crown and stems arising from infected crowns suggested the pathogen might have been moving at least partially by means of the water transpiration stream. The demonstration that chrysanthemum plants can be systemically infected by P. horiana suggests that additional disease control measures are required to effectively control Chrysanthemum white rust. The results will lead to improved prediction of disease development, as well practical methods for control of the disease by growers.

Technical Abstract: PUCCINIA HORIANA Henn., causal agent of the disease commonly known as Chrysanthemum white rust, is a quarantine-significant fungal pathogen of chrysanthemum in the United States (U.S.) and indigenous to Asia. The pathogen was believed to have been eradicated in the U.S. but recently re-appeared on several occasions in northeastern U.S. The objective of the study presented here was to determine if P. horiana could systemically infect chrysanthemum plants, thus providing a means to survive through winters. Scanning and transmission electron microscopy were used to follow the development of P. horiana on the surface of leaves, and in leaves, stems, and crowns of inoculated chrysanthemum plants artificially exposed to northeastern U.S. winter conditions. P. horiana penetrated leaves directly through the cuticle, then colonized the mesophyll tissue both inter- and intra-cellularly. An electron dense material formed at the interface between fungal and host mesophyll cells suggesting the pathogen adhered to the plant cells. P. horiana penetrated mesophyll cell walls by enzymatic digestion, as indicated by the absence of deformation lines in host cell walls at sites of penetration. The fungus was common in vascular tissue within the infected crown, often nearly replacing the entire contents of tracheid cell walls. P. horiana frequently passed from one tracheid cell to an adjacent tracheid cell by penetration either through pit pairs or non-pitted areas of the cell walls. Individual fungal cells in mature tracheid cells of the crown and stems arising from infected crowns suggested the pathogen might have been moving at least partially by means of the transpiration stream. The demonstration that chrysanthemum plants can be systemically infected by P. horiana suggests that additional disease control measures are required to effectively control Chrysanthemum white rust.