Epidemiological aspects of Phyllosticta citricarpa colonization and viability in Citrus sinensis

Citrus black spot (CBS) is a disease caused by the fungus Phyllosticta citricarpa that is associated with significant yield losses. The pathogen forms airborne ascospores that are collected in dead fallen leaves as well as waterborne conidia that are common in lesions on mature fruits and occasionally in leaves and dead twigs. In Brazil, cultural management combined with the application of systemic, protective fungicides is a practice adopted to reduce both inoculum sources. Nevertheless, the removal of mature infected fruits, dead twigs, and dead fallen leaves has not been shown to reduce the extent of disease. Moreover, the increasing use of fungicides has led to the selection of resistant strains, as well as human health concerns. To better understand this pathosystem and how the pathogen overcomes control measures currently in use, we evaluated branches at different growth stages in the field, the viability of the pathogen in detached branches, and the possibility of plant colonization through leaves. We observed that living branches are a reservoir for this pathogen and that once the branches senesce, the pathogen can persist up to 45 days, able to contaminate other plant parts. On inoculated leaves, we observed only a one-point latent infection, without signs of colonization. We also developed a faster protocol to produce pycnidia of the pathogen using autoclaved branches as a substrate, which proved useful for testing alternative compounds to control P. citricarpa. These data may be useful in the search for new effective control measures and their targets.