First, second, and fifth authors: Department of Plant Pathology, and third author, Horticulture Department, University of Arkansas, Fayetteville 72701; and fourth author: U.S. Department of Agriculture--Agricultural Research Service, Beltsville, MD 20705.
Based on spore morphology, appressorium development, sequence similarities of the rDNA, and similarities in amplified restriction fragment length polymorphism (AFLP), it has been proposed that Colletotrichum orbiculare, C. trifolii, C. lindemuthianum, and C. malvarum represent a single phylogenetic species, C. orbiculare. In the current study, the phylogenetic relationship among isolates in the C. orbiculare species complex was reassessed. In all, 72 isolates of C. orbiculare from cultivated cucurbit or weed hosts, C. trifolii from alfalfa, C. lindemuthianum from green bean, and C. malvarum from prickly sida (Sida spinosa) were examined for mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs), RFLPs and sequence variation of a 900-bp intron of the glutamine synthetase gene and a 200-bp intron of the glyceraldehyde-3-phosphate dehydrogenase gene, and vegetative compatibility. In addition, host specificity was examined in foliar inoculations on cucurbit, bean, and alfalfa hosts. Inoculations also were conducted on cucumber fruit. Genetically distinct isolates, based on vegetative compatibility, within the species complex (C. orbiculare, C. trifolii, and C. malvarum) had an identical mtDNA haplotype (haplotype A) when examined with each of three different restriction enzymes. Isolates of C. lindemuthianum had a very similar mtDNA haplotype to haplotype A, with a single polymorphism detected with the enzyme HaeIII. The four species represent a phylogenetically closely related group based on a statistical analysis of the 900- and 200-bp intron sequences. However, distinct RFLPs in the 900-bp intron were consistently associated with each species and could be used to qualitatively and quantitatively distinguish each species. Furthermore, each of the species showed distinct host specificity, with isolates of C. orbiculare (from cucurbits), C. lindemuthianum, and C. trifolii being pathogenic only on cucurbits, green bean, and alfalfa, respectively. Consequently, distinct and fixed nucleotide, or genotypic (intron sequences and RFLPs) and phenotypic (host specificity) characteristics can be used to distinguish C. orbiculare, C. lindemuthianum, and C. trifolii from one another; therefore, they should be recognized as distinct species. This species delineation is consistent with the most current species concepts in fungi. More isolates and further characterization is needed to determine whether C. orbiculare from cocklebur and C. malvarum represent distinct species. RFLPs of the 900-bp intron may represent a relatively inexpensive, reliable, and useful diagnostic tool for general species differentiation in the genus Colletotrichum.