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United States Department of Agriculture

Agricultural Research Service

Title: An Allee Effect Limits the Invasive Potential of Tilletia Indica

Authors
item Garrett, Karen - KANSAS STATE UNIV
item Bowden, Robert

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 2002
Publication Date: November 15, 2002
Citation: GARRETT, K.A., BOWDEN, R.L. AN ALLEE EFFECT LIMITS THE INVASIVE POTENTIAL OF TILLETIA INDICA. PHYTOPATHOLOGY. 2002. 92:1152-1159.

Interpretive Summary: Karnal bunt is a minor disease of wheat that is economically important due to numerous international quarantines on infected grain. The Karnal bunt pathogen, Tilletia indica, is an obligate outcrosser and depends on encounters on wheat spikes between airborne spores of different mating types for successful infection and reproduction. This life history characteristic results in reduced reproductive success for lower population densities. Such density-dependence at low population levels has been described for a range of animals and plants and is often termed an Allee effect. Our objective was to understand how the Allee effect may affect the epidemic potential of this economically important pathogen. We developed a simple population model of T. indica that incorporates an Allee effect by calculating the probability of infection for different numbers of spores in the infection court. An Allee effect is predicted to be important at the frontier of an invasion, for establishment of new disease foci by a small population of resting spores, and when the environment is nonconducive for the production of new infective spores. Using the model estimates, we demonstrated a theoretical Allee threshold population size below which populations of T. indica were predicted to decline rather than increase. The threshold depended on the reproductive rate of the pathogen. Deployment of partial resistance and use of fungicides may be more useful if they push population levels below the Allee threshold.

Technical Abstract: The Karnal bunt pathogen, Tilletia indica, is heterothallic and depends on encounters on wheat spikes between airborne secondary sporidia of different mating types for successful infection and reproduction. This life history characteristic results in reduced reproductive success for lower population densities. Such destabilizing density-dependence at low population levels has been described for a range of animals and plants and is often termed an Allee effect. Our objective was to understand how the Allee effect may affect the epidemic potential of this economically important pathogen. We developed a simple population model of T. indica that incorporates an Allee effect by calculating the probability of infection for different numbers of secondary sporidia in the infection court. An Allee effect is predicted to be important at the frontier of an invasion, for establishment of new foci by a small population of teliospores, and when the environment is nonconducive for the production of secondary sporidia. Using estimated model parameter values, we demonstrated a theoretical Allee threshold population size below which populations of T. indica were predicted to decline rather than increase. Deployment of partial resistance and use of fungicides may be more useful if they push population levels below the Allee threshold. T. indica can compensate for the Allee effect to some extent when years are conducive through multiple cycles of increase of sporidia. Its ability to compensate in conducive years is also enhanced through a bank of dormant teliospores.

Last Modified: 9/2/2014
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