Factors associated with a rust infection (Sphenosphora saphena) in an epiphytic orchid (Tolumnia variegata)

The interaction of a rust infection (Sphenosphora saphena) with the orchid Tolumnia (Oncidium) variegata was studied in northern Puerto Rico to determine which intrinsic and extrinsic factors are associated with the development of the infection. The relationship between anatomical and morphological characters, life-history stage, density, herbivory, horizontal and vertical location, and frequencies of allo- and autoinfection were investigated. Stomatal densities and plant age were not associated with the incidence of disease. However, larger plants were more frequently infected than small ones. Censuses showed that seedlings were affected less frequently than adults, but inoculations demonstrated that both stages were equally susceptible to infection. Infection frequencies varied considerably over time and space. Population density was positively correlated with incidence of infection in two out of five censuses within a 3-year period. Vertical distribution of the orchid was not related to the incidence of infection. Infection frequencies were related to site location, but this varied with time. Infections were highly localized, and new shoots were more likely to become infected if the previous shoot was diseased. Herbivory was more frequent among noninfected plants and may impart some degree of resistance. Our results suggest that the infection frequencies are associated more strongly with infection histories, plant size, and ecological conditions rather than with age, life histories, or anatomical traits of the orchid.