Nondestructive Detection of White Root Rot Disease in Avocado Rootstocks by Leaf Chlorophyll Fluorescence

White root rot (WRR) disease caused by Rosellinia necatrix is one of the most important threats affecting avocado orchards in temperate regions. In this study, we monitored the progression of WRR disease at the leaf and root levels by the combination of nondestructive chlorophyll fluorescence measurements and confocal laser-scanning microscopy on avocado genotypes susceptible to R. necatrix. Leaf photochemistry was affected at early stages of disease development prior to the appearance of aboveground symptoms, made evident as significant decreases in the trapping efficiency of photosystem-II (F_v'/F_m') and in the steady-state of chlorophyll fluorescence yield (F_s) normalized to the minimal fluorescence yield (F₀) (F_s/F₀). Decreases in F_v'/F_m' and F_s/F₀ were associated with different degrees of fungal penetration, primarily in the lateral roots but not in areas next to the main root collar. Aboveground symptoms were observed only when the fungus reached the root collar. Leaf physiology was also tracked in a tolerant genotype where no changes were observed during disease progression despite the presence of the fungus in the root system. These results highlight the usefulness of this technique for the early detection of fungal infection and the rapid removal of highly susceptible genotypes in rootstock avocado-breeding programs.