Tillettia indica Look-alikes and the Risk of Karnal Bunt False Positives in Northern Wheat States |
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Karnal Bunt Symposium |
Robert W. Stack. Plant Pathologist at North Dakota
State University, Fargo, North Dakota.
Emphasis is in mycology in both his research and teaching. His interests include studies in the ecology of fungal pathogens, mycorrhizae, soil borne diseases, and forest pathology. His current research effort is directed towards identifying sources of resistance to head blight (scab) of wheat. I am writing in reference to the potential for Karnal Bunt False Positives due to Tilletia indica (TI) look-alike fungi. The Karnal bunt pathogen, T. indica (TI) is limited to wheat and has a rather unique teliospore ornamentation shared by only a few other species, most notably the rice smut, T. barclayana (TB). This similarity between TB and TI has made identification of possible TI in grain samples more difficult than would otherwise be the case. Because rice is absent from cropping and grain handling operations in the northern wheat states, it might have been assumed that complications of identification between TB and TI would not need to be addressed in testing for KB in these areas. It turns out, however, that this same smut (TB), or a species indistinguishable from it (T. pulcherrima) (TP) causes smut of several widespread native grasses. Because of this, there is a potential for KB false positives from smut on these grasses. The purpose of this communication is to alert other scientists about this risk. |
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The genus Tilletia occurs worldwide with 76
described species, mostly on various gramineous hosts
(Duran and Fischer 1961). Some 25 (Farr et al. 1989) to
31 (Fischer 1953) of those are found in North America. Tilletia
species are identified by morphological characters
related to teliospore size and ornamentation, to the
sorus, and to characters of the sterile cells. Most of
the Tilletia spp on native grasses in North
America are readily distinguished from the KB fungus, TI,
by size and ornamentation of the teliospores. At least
one species, however, has a spore wall very similar to,
and a size range overlapping TI. That species was long
known as T. pulcherrima (TP), occurring on grasses
in the tribe Paniceae (Fischer 1953). The grass genus Panicum
is large, variously described as having 32 (Rydberg
1932), 65 (Gleason & Cronquist 1963) to 170 species
(Hitchcock and Chase 1950). TP has been reported from
several of these grasses but most often from Panicum
virgatum, switchgrass. P. virgatum is one of
the most widespread Panicum species, occurring
throughout North America east of the Rocky Mountains
including the entire spring wheat belt ( Gleason and
Cronquist 1963, Hitchcock and Chase 1950).
I am not hopeful that past collections in herbaria will give us anything very useful on distribution of this fungus. Classical mycology (i.e. collecting and identifying material for herbarium preservation) has been nonexistent or out of fashion at most United States universities for decades, so it's highly unlikely that there are enough recent records or specimens available to get any inkling of how widespread the grass smut is. A search of the holdings of the U.S. National Fungus Collection (BPI) Herbarium at Beltsville, MD., (Telnet: fungi.ars-grin.gov) revealed 64 specimens of Tilletia on Panicum spp from North America, including 43 of T. pulcherrima, mostly (27) on Panicum virgatum. There were collections from AZ (4), MS(5), OK(3), IL(9), PA(2), KS(2), NE (1), IA(1). There were no collections from MN, MT, ND, or SD. That might sound like a contradiction to my previous point, but note that the most recent(!) collections (two) were from the 1940's and the majority were from the decade just before and that following the turn of the century! In the 1961 monograph of the genus Tilletia, Duran synonymized TP into the rice smut T. barclayana.(TB) (Duran & Fischer 1961). Whether TP should or should not be included in the TB species concept may be a valid point of debate but that is of no concern here. What does matter is that the teliospores of Panicum smut closely resemble those of rice smut and could potentially be misidentified as TI by an untrained observer. I believe that a premature report of KB, even if later shown to be a false positive, will have serious consequences for our world markets. I believe it would be very advantageous to have the capability for genetic testing on a quick turnaround basis. I have already shared these thoughts with some colleagues in other wheat states. I believe some work is already being done to address the points I've raised (Blair Goates, USDA-ARS, pers. comm.) REFERENCES: Duran, R. and G. W. Fischer. 1961. The Genus Tilletia. Washington State Univ. Pullman, WA. 138p. Farr, D. F. et al. 1989. Fungi on Plants and Plant Products in the United States. APS Press, St. Paul, MN. 1252p. Fischer, G. W. 1953. Manual of the North American Smut Fungi. Ronald Press, New York. 343p. Gleason, H.A. and A. Cronquist. 1963. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. D. VanNostrand, NY. Hitchcock, A..S. and A. Chase. 1950. Manual of the Grasses of the United States, 2ed. Dover Publ. Reprint 1971. Rydberg, P.A. 1932. Flora of the Prairies and Plains of Central North America. Dover Publ. Reprint 1971. |
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