Phytophtho citophthro trunk and branch canker on Clementine mandarins in the Western Cape province of South Africa

Abstract

Bark and soil samples collected from trunks of Clementine mandarin cv. `Nules’ orchards in the Knysna area of South Africa were used for isolations to determine the cause of a canker on tree trunks. Phytophthora citrophthora was the predominant species isolated from trunks and was identified by its cultural, morphological, physiological and molecular profiles. A PCR study confirmed that the isolates were P. citrophthora. Using the BLASTn algorithm, all strains showed a high percentage sequence similarity (>98%) with significant alignments of E values = 0.0 and bit scores, aligning all South African strains with P. citrophthora ITS sequence data accessiblefrom Genbank and the Phytophthora database. P. citrophthora isolates showed that colonies on PDA had a petaloid growth habit. P. citrophthora was isolated from the bark and soil and was used in stem-inoculation studies. Under cool greenhouse conditions (<25°C), nursery trees inoculated with P. citrophthora, resulted in extensive gumming and eventual death of the nursery trees within two weeks whilst the rootstocks were not affected. P. citrophthora was also successfully re-isolated from the inoculated nursery trees, thereby confirming Koch’s postulates that P. citrophthora is the cause of trunk and branch cankers on Clementine mandarins in South Africa.