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

Abstract

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.

Scientific Opinion on the risk of Phyllosticta citricarpa (Guignardia citricarpa) for the EU territory with identification and evaluation of risk reduction options

“…Pycnidiospores produced in fruit lesions were indicated as a potential source of inoculum only where out-of-season fruit or late-hanging fruit with lesions remain on the trees after blossoming and fruit set (Kotzé, 1981). Further experiments demonstrated that CBS-affected fruit and dead twigs on the tree were able to spread the disease in the canopy, supporting the importance of rain-dispersed pycnidiospores in field epidemics (Spósito et al, 2011). However, epidemiological studies conducted in Sao Paulo, Brazil, found an aggregated spatial pattern of CBS-affected trees in the orchard as well as diseased fruit in the canopy, suggesting that splash-dispersed conidia have an important role in this region (Spósito et al, 2007.…”

“…Pycnidia and pycnidiospores are produced mostly in dead twigs (Whitesite, 1967;Spósito et al, 2011) and on fruit lesions during the latest stages of fruit development (Kotzé 1981). Pycnidia and pycnidiospores are produced mostly in dead twigs (Whitesite, 1967;Spósito et al, 2011) and on fruit lesions during the latest stages of fruit development (Kotzé 1981).…”

“…About 8 % (70) of interceptions were made on shipments of lemon ( Figure 11), the citrus species most susceptible to P. citricarpa, of which more than half (43) originated from South Africa. However, in Brazil, cultural practices such as the early harvest of symptomatic citrus fruit and the removal of leaf litter from the orchard floor have been shown to be incapable of reducing CBS incidence and severity to satisfactory levels (Spósito et al, 2011). However, in Brazil, cultural practices such as the early harvest of symptomatic citrus fruit and the removal of leaf litter from the orchard floor have been shown to be incapable of reducing CBS incidence and severity to satisfactory levels (Spósito et al, 2011).…”

“…However, susceptible live tissues (leaves, twigs and fruit on the canopy) of citrus trees in commercial orchards in the risk assessment area are normally very close to the soil, and leaves and fruit very often directly touch the orchard floor (see Figure 16 below and section 3.3.3.3), depending on the cultivation technique and the season. Spósito et al (2011) evaluated the spread of symptoms from different inoculum sources. In addition, during the last few years, there has been a trend towards the cultivation of shorter citrus trees by grafting onto dwarfing rootstocks.…”

“…In addition, during the last few years, there has been a trend towards the cultivation of shorter citrus trees by grafting onto dwarfing rootstocks. in the risk assessment area (Figure 16), the mature pycnidiospores exuded from pycnidia under wet conditions could be splash dispersed by rain (Whiteside, 1967;Spósito et al, 2011) onto the lower parts of the canopy, infecting leaves, twigs and fruit at a susceptible stage. No symptoms of the disease were observed in the tree canopy when five sweet orange fruits with more than 10 lesions were placed on the orchard floor 30 cm under the fruits on the canopy.…”