Linking <scp> <i>Hemileia vastatrix</i> </scp> Aggressiveness to Coffee Plant Physiology in Peru's Central Jungle

ABSTRACT Coffee provides a livelihood for thousands of small‐scale farmers in the Peruvian tropics. However, this crop is threatened by the biotrophic fungus Hemileia vastatrix , the causal agent of coffee leaf rust (CLR), a disease that substantially reduces yields and has challenged Peruvian producers since 2013. Despite its national importance, few studies have examined how this disease affects the physiological status of coffee plants under Peruvian conditions. Therefore, this study evaluated differences in pathogen aggressiveness—measured by infection frequency (IF), incubation period (IP) and latency period (LP)—and assessed the physiological response of coffee to CLR, quantifying Soil Plant Analysis Development (SPAD) index as well as maximum photochemical activity (F v /F m ). A total of 18 rust isolates (1RC–18RC) collected from 12 coffee farms in the central Peruvian jungle were analysed, along with their effects on the photosynthetic capacity of susceptible Bourbon and Caturra cultivars. Overall, significant differences were observed in the incidence of isolates on coffee plants, largely associated with their site of origin. Chlorophyll and fluorescence measurements varied in response to isolate aggressiveness, showing a moderate negative correlation. Isolates from San Ramón (12RC–18RC) exhibited higher levels of aggressiveness components, such as infection frequency of 7.5–9, and poorer physiological plant performance, reflected in reductions of up to 28% in SPAD readings and 42% in F v /F m . These findings contribute to the characterisation of rust samples in the Peruvian tropics and highlight the impact CLR can have on the physiological performance of coffee crops.