Citrus black spot (CBS), caused by the fungus
Phyllosticta citricarpa, is a critical disease of citrus in SA because of its phytosanitary status. By Providence Moyo, Elma Carstens, MC Pretorius, Andre Combrink, Jan van Niekerk, Paul Fourie and Vaughan Hattingh (CRI)
The primary inoculum (ascospores) produced in leaf litter on the orchard floor, after the leaves have been exposed to suitable environmental conditions, is discharged during wet periods. These ascospores then land on susceptible leaves and fruit. If, during the fruit susceptibility period, the leaves or fruit are wet long enough and the temperature is conducive to ascospore germination, primary infection of fruit and leaves occurs. Lesions later develop on the leaves and fruit (Figure 1), which then produce pycnidiospores (conidia). Under conducive conditions, the pycnidiospores can be washed down within trees and can contribute to the build-up of the disease.
CBS epidemics build up gradually, and in the case of orchards with their annual harvesting, infected leaves that remain up to three years on trees cause disease build-up. Therefore, CBS is hardly a problem in younger orchards.