Two distinct Guignardia species associated with citrus in South Africa

The fungus Guignardia citricarpa causes a phytosanitary restrictive disease called citrus black spot (CBS). Some researchers have suggested the existe nce of two strains, but these cannot be distinguished by mere microscopy. S outh African Guignardia isolates from different lesion types, as well as fr om symptomless fruit, were compared by means of ribosomal DNA internal tran scribed spacer sequence analysis, proving the existence of two Guignardia s pecies on citrus. Restriction enzyme (Cfol) digestion fingerprints of the P CR products clearly distinguished the two species, providing a quick yet re liable identification tool. Growth rate in culture also corresponded with t he two species. The first species, G. citricarpa, is confirmed as the causa l organism of CBS and is restricted to citrus. This fungus occurs in all ma jor citrus-producing areas of South Africa, except the Western Cape. The pa thogen 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 sympto mless citrus products, but also from avocado, mango, banana, cabbage tree a nd kumquat, which occur in various geographical areas. Phytosanitary measur es may be used against the export of citrus fruit suspected of being infect ed 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.