FACTORS ASSOCIATED WITH A RUST INFECTION (SPHENOSPHORA-SAPHENA) IN ANEPIPHYTIC ORCHID (TOLUMNIA-VARIEGATA)

The interaction of a rust infection (Sphenosphora saphena) with the or chid Tolumnia (Oncidium) variegata was studied in northern Puerto Rico to determine which intrinsic and extrinsic factors are associated wit h the development of the infection. The relationship between anatomica l and morphological characters, life-history stage, density, herbivory , horizontal and vertical location, and frequencies of allo- and autoi nfection 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 seedlin gs were affected less frequently than adults, but inoculations demonst rated that both stages were equally susceptible to infection. Infectio n frequencies varied considerably over time and space. Population dens ity was positively correlated with incidence of infection in two out o f five censuses within a S-year period. Vertical distribution of the o rchid was not related to the incidence of infection. Infection frequen cies were related to site location, but this varied with time. Infecti ons were highly localized, and new shoots were more likely to become i nfected if the previous shoot was diseased. Herbivory was more frequen t among noninfected plants and may impart some degree of resistance. O ur results suggest that the infection frequencies are associated more strongly with infection histories, plant size, and ecological conditio ns rather than with age, life histories, or anatomical traits of the o rchid.