THE EFFECTS OF A RUST INFECTION ON FITNESS COMPONENTS IN A NATURAL-POPULATION OF TOLUMNIA-VARIEGATA (ORCHIDACEAE)

The effects of a rust infection (Sphenosphora saphena, Basidiomycetes) on several fitness components of the orchid Tolumnia (Oncidium) varie gata were quantified in a subtropical moist forest of Puerto Rico. Inf ected and uninfected plants of two size categories were observed and m anipulated by increasing natural levels of fruit production to determi ne the effects of infection on short and long-term sexual reproduction , subsequent vegetative and reproductive growth, and mortality. Under artificially high levels of fruit production, infection had no effect on short or long-term reproduction through male or female function exc ept for a modest decline in seed viability in small plants. Under natu ral levels of fruit production, infection was only related to a reduct ion in the number of leaves per shoot. At artificially high levels of fruit production, infection reduced leaf length, leaf width, and the n umber of live shoots. Survival was not associated with rust occurrence . The rust appears to have minor ecological impact on this orchid popu lation.