Systemic infection in chrysanthemum plants by Puccinia horiana, causal agent of chrysanthemum white rust
M. R. BONDE (1), C. A. Murphy (2), G. R. Bauchan (2), D. G. Luster (1), C. L. Palmer (3), S. E. Nester (1), D. K. Berner (1). (1) USDA ARS, Frederick, MD, U.S.A.; (2) USDA ARS, Beltsville, MD, U.S.A.; (3) Rutgers University, Princeton, NJ, U.S.A.
<i>Puccinia horiana</i> Henn. is a quarantine-significant fungal pathogen and causal agent of chrysanthemum white rust (CWR), first discovered in the U.S. in 1977. Our purpose was to determine if <i>P. horiana</i> infects chrysanthemum systemically, acting as a means of over-wintering. Plants, cv. Vicki, were inoculated in a mist tent and placed in a greenhouse. Up to 36 days later, leaves were collected and fixed in 3% glutaraldehyde. Other plants were placed in a growth chamber simulating fall, winter, and spring temperatures in northeastern U.S. After simulated winter, the crown, root, and newly formed stem tissues displaying CWR symptoms were collected and fixed as above. Tissues were post fixed in 2% buffered OsO<sub>4</sub>, dehydrated in ETOH, and infiltrated with Spurrs. Images were taken with an HT-7700 Hitachi microscope at 80kV. Observations showed <i>P. horiana</i> entered through the leaf cuticle and colonized inter- and intra-cellularly. The fungus adhered to the exterior of palisade and mesophyll cells. The pathogen became common in tracheid cells of the crown, roots, and newly developed stems. Penetration between tracheid cells was through pitted and non-pitted areas of walls and appeared enzymatic. <i>P. horiana</i> had an affinity for xylem cell walls, often replacing most of the wall. D-haustoria were not observed, indicating the fungus was monokaryotic. The study showed <i>P. horiana</i> can systemically infect chrysanthemum plants.
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