Failure of Phyllosticta citricarpa pycnidiospores to infect Eureka lemon leaf litter
Pycnidiospores of Phyllosticta citricarpa from pure cultures, symptomatic citrus black spot Valencia orange fruit and peelings were evaluated for their potential to infect and colonise citrus black spot-free Eureka lemon leaf litter in a controlled environment and in the field in different production regions of South Africa. Leaf litter, consisting of freshly detached green and old brown leaves that were exposed to viable pycnidiospores under controlled conditions or in the field underneath citrus trees, were not infected and colonised by P. citricarpa. Ascospores, conforming to Guignardia citricarpa, the pathogen, or G. mangiferae, a cosmopolitan endophyte, were collected with a Kotzé Inoculum Monitor from leaves placed in the field only at Tzaneen and Burgersfort. Distinguishing between these two species on ascospore morphology alone is not possible. A diagnostic polymerase chain reaction conducted on representative leaf material from all the treatments revealed the presence of only G. mangiferae on 12.5% of the treatments. This study demonstrated the failure of P. citricarpa pycnidiospores to infect citrus leaf litter under controlled and field conditions. Symptomatic citrus black spot fruit or peel lying on the ground underneath citrus trees, therefore, cannot lead to infection and colonisation of freshly detached leaves or natural leaf litter or represent a source of inoculum in citrus orchards for these leaves.