<p>Myrtle rust, caused by <i>Austropuccinia psidii</i>, is the most severe fungal disease in guava. This polycyclic disease can cause polyetic (i.e., cumulative) damage in perennial plants, reducing plant growth and yield in successive crop seasons. Since the evaluation of polyetic damage in the field requires many years, a possible alternative to estimate it would be through multiple inoculations of the pathogen in a controlled environment. The objective of this study was to estimate the impact of successive inoculations of <i>A. psidii</i> on damage of guava plants. Young shoots of guava plants were inoculated three times with <i>A. psidii</i> with intervals of 14 days between inoculations. Disease severity and vegetative growth of host plants were evaluated throughout the experiment. Leaf gas exchange, Rubisco activity, carbon mass isotopic signature and concentrations of ascorbate and glutathione were evaluated 14 days after each inoculation. High disease severity, defoliation and shoot dieback were observed in inoculated plants by the end of the experiment. Net CO₂ assimilation, stomatal conductance and transpiration showed average reductions of 47%, 31% and 19% respectively in inoculated plants, compared to healthy plants. Rubisco activity and carbon mass isotopic signature decreased in inoculated plants, especially after the third inoculation. Myrtle rust induced oxidative stress as indicated by the increased concentration of oxidized glutathione. The reductions in dry mass of leaves, stems and roots in inoculated plants ranged from 30% to 62%. Our controlled experiments, representing the rust polycycle in a single growing season of guava plants, revealed the mechanisms behind the damage caused by this disease. The reductions in photosynthesis and vegetative growth caused by the repeated inoculations of <i>A. psidii</i> indicate that rust can cause severe damage to guava trees, and serves as a validated proxy for predicting long-term polyetic damage.</p>