Relative importance of inoculum sources of Guignardia citricarpa on the citrus black spot epidemic in Brazil

Guignardia citricarpa, the causal agent of citrus black spot (CBS), forms ascospores and conidia in citrus leaf litter and only conidia on fruit and twigs. The objective of this work was to determine the relative importance of inoculum sources of ascospores and conidia in the progress of CBS under natural conditions in Brazil. A first experiment was conducted in an orchard subdivided into two areas. In one area, fallen leaves were removed and in the other area fallen leaves were maintained. In each area, two treatments were established: 1) CBS symptomatic mature fruit remained on the trees after the new fruit set, and 2) CBS symptomatic mature fruit were harvested before the new fruit set. A second experiment was carried out in a CBS-free experimental orchard where detached symptomatic fruit and leaf litter on the orchard floor and detached symptomatic fruit and dead twigs on the tree canopy were distributed to investigate the spread of disease. In the first experiment disease incidence was similar for all treatments in all years. However, in 2003, in the area without leaf litter (absence of the ascospores source), disease intensity and the rate of progress of disease incidence were higher when there was overlap of mature and young fruit. In the second experiment, only detached symptomatic fruit and dead twigs on the tree canopy were able to spread CBS in two years of the experiment. The distance of disease spread was less than 80 cm from these inoculum sources. The removal of fallen leaves was not sufficient to completely suppress the disease because of the presence of conidia in fruit and dead twigs. Therefore the reduction of conidia sources should be considered in CBS management in Brazil.