Two distinct Guignardia species associated with citrus in South Africa.
Meyer L., Slippers B., Korsten L., Kotzé J. M., Wingfield M. J.
Author Affiliation: Department of Microbiology and Plant Pathology, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Pretoria 0002, South Africa.
South African Journal of Science 97 : 191-194
Abstract : The fungus Guignardia citricarpa causes a phytosanitary restrictive disease called citrus black spot (CBS). Some researchers have suggested the existence of two strains, but these cannot be distinguished by mere microscopy. South African Guignardia isolates from different lesion types, as well as from symptomless fruit, were compared by means of ribosomal DNA internal transcribed spacer sequence analysis, proving the existence of two Guignardia species on citrus. Restriction enzyme (Cfol) digestion fingerprints of the PCR products clearly distinguished the two species, providing a quick yet reliable identification tool. Growth rate in culture also corresponded with the two species. The first species, G. citricarpa, is confirmed as the causal organism of CBS and is restricted to citrus. This fungus occurs in all major citrus-producing areas of South Africa, except the Western Cape. The pathogen can be isolated from hard spots, virulent spots, freckled spots and false melanose lesions from sweet orange, grapefruit, lemon and tangerine. The second species, which is harmless to fruit, can be isolated from symptomless citrus products, but also from avocado, mango, banana (Musa acuminata), cabbage tree (Cussonia) and kumquat (Fortunella), which occur in various geographical areas. Phytosanitary measures may be used against the export of citrus fruit suspected of being infected with CBS. The DNA tests we have devised are able, for the first time, to distinguish the pathogenic from the harmless endophyte of citrus and other plants.