Control of emerging infectious diseases often hinges on identifying a pathogen reservoir, the source of disease transmission. The potential to function as a pathogen reservoir can be influenced by host lifespan, geographic provenance and phylogeny. Yet, no study has identified factors that causally determine the reservoir potential of diverse host species. We propose the host physiological phenotype hypothesis, which predicts that hosts with short-lived, poorly defended, nutrient rich and high metabolism tissue have greater values for three epidemiological parameters that determine reservoir potential: host susceptibility to infection, competence to infect vectors and ability to support vector populations. We experimentally tested these predictions using a generalist vectored virus and six wild grass species. Host physiological phenotype explained why hosts differed in all three epidemiological parameters while host lifespan, provenance and phylogeny could not explain host competence. Thus, a single, general axis describing variation in host physiological phenotype may explain reservoir potential.

In February 1997, a few trees in a grove of Valencia sweet orange on sour orange rootstock were found to be declining due to Citrus tristeza virus (CTV). This was the first grove in the Bog Walk Valley, Jamaica showing decline due to CTV. In the grove which originally had about 1,000 trees, 13 were dead and another 53 were in the final stages of decline. By August 1997, about 11% of the trees were dead and 5% were in the final stages of decline. By November 1997, an additional 25% of the trees had died, 43% were in the final stage of decline, and 25% were in early stages of decline. Only 38 trees in the original planting were rated as healthy; upon observation most of these trees had been unknowingly planted on CTV-tolerant rootstocks. By November 1997, isolated sweet orange trees on sour orange rootstock in neighboring groves were showing decline symptoms. In February 2000, the entire Bog Walk Valley was undergoing an epidemic of CTV decline on sour orange rootstock. Samples were collected from several declining trees in groves within 2.0 km of the original grove affected with CTV decline. The samples from the declining trees were tested by strain group specific probes (SGSP) and broad spectrum-, MCA13-, and orange stem pitting (OSP) -ELISA tests. All samples were broad spectrum and MCA13-ELISA positive, and several samples also reacted in the OSP-ELISA tests. In the SGSP tests, all isolates reacted with the universal probe for CTV, as well as several other probes indicating that the field isolates were composed of mixtures of CTV strains. Some of the SGSP probes which hybridized to the Jamaican isolates commonly cause stem pitting on grapefruit and/or sweet orange. OSP-ELISA tests also indicated the presence of CTV strains which commonly cause stem pitting in sweet orange indicator plants. In planta isolates are now established in the Collection of Exotic Citrus Pathogens, Beltsville, MD where further characterization will be done.

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