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Mitochondrial DNA (mtDNA)—Relatedness among Formae Speciales of Fusarium oxysporum in the Cucurbitaceae. D. H. Kim,Graduate research assistant, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843; R. D. Martyn, and C. W. Magill. associate professor and professor, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843. Phytopathology 83:91-97. Accepted for publication 17 August 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-91.

Thirty-nine isolates of Fusarium oxysporum encompassing five formae speciales causing vascular wilt in cucurbits were examined for genetic similarity by restriction fragment length polymorphisms (RFLP) analysis of mitochondrial DNA (mtDNA). Total DNA was digested with three enzymes (PstI, HindIII, and EcoRI), Southern blotted, and hybridized with a F. o. niveum mtDNA polyprobe (pFON2a–pFON8b). The presence or absence of mtDNA fragments was examined by the unweighted-paired-group method using averages (UPGMA) and parsimony analysis. A total of 14 mtDNA RFLP groups were detected. Within each forma specialis there were unique RFLPs; however, one pattern generally occurred most frequently for each forma specialis. Two RFLP groups (RFLPG-fspI and IX) occurred in four and two formae speciales, respectively. The RFLP haplotype most common in F. o. niveum (RFLPG-fspI) also occurred in one or more isolates from every other formae speciales except F. o. luffae and was present in isolates from North America, Europe, and Asia. Genetic distances generated by UPGMA suggest that F. o. niveum was the least diverse forma specialis, while F. o. cucumerinum was the most diverse. However, both cluster analysis and parsimony analysis indicated that all of F. oxysporum formae speciales in the cucurbits are closely related and, in some cases, isolates of different formae speciales were genetically more similar than isolates of the same forma specialis.