MOLECULAR CHARACTERIZATION OF SLOW-GROWING ORANGE AND KEY LIME ANTHRACNOSE STRAINS OF COLLETOTRICHUM FROM CITRUS AS C-ACUTATUM

Three forms of Colletotrichum are recognized on citrus: the fast-growi ng gray (FGG) form, a common saprophyte and postharvest pathogen that is morphologically distinct from the other two; the slow-growing orang e (SGO) form, the causal agent of postbloom fruit drop; and the Key li me anthracnose (KLA) form, the cause of lime anthracnose. Pathogenicit y tests confirmed that the SGO isolates infected sweet orange flowers only, the KLA isolates infected both sweet orange flowers and Key lime foliage, and the FGG isolates did not infect either of these host tis sues. Polymerase chain reaction (PCR) with the C. acutatum-specific pr imer CaInt2 and the internal transcribed spacer (ITS)4 primer (from a conserved sequence of ribosomal DNA [rDNA]) amplified a 490-bp fragmen t from the SGO and KLA isolates but not from the FGG isolates. PCR wit h the C, gloeosporioides-specific primer CgInt and the ITS4 primer amp lified a 450-bp fragment only from the FGG isolates. The nucleotide se quence of the rDNA ITS1 of four SGO and four KLA isolates indicated ei ther complete homology or a I-base difference from a C. acutatum refer ence isolate but only 80 to 81% homology to a C. gloeosporioides refer ence isolate. The ITS1 sequence of the FGG isolates had 97% homology t o the C. gloeosporioides reference isolate but only 80% homology to th e C. acutatum reference isolate. These data confirmed the classificati on of the SGO and KLA isolates as C. acutatum and the FGG isolates as C. gloeosporioides. Cluster analysis of rDNA and mitochondrial DNA res triction fragments divided the SGO and KLA isolates into two recogniza ble groups with genetic similarity of 87%. These two groups were more closely related to each other than to any of the reference isolates us ed (<80% similarity). It is suggested that the SGO and KLA isolates ha d a recent common ancestor.