CHARACTERIZATION OF THE FORMAE SPECIALES OF FUSARIUM-OXYSPORUM CAUSING WILTS OF CUCURBITS BY DNA-FINGERPRINTING WITH NUCLEAR REPETITIVE DNA-SEQUENCES

The genetic relatedness of five formae speciales of Fusarium oxysporum causing wilts of cucurbit plants was determined by DNA fingerprinting with the moderately repetitive DNA sequences FOLR1 to FOLR4. The four FOLR clones were chosen from a genomic library made from F. oxysporum f. sp. lagenariae 03-05118. Total DNAs from 50 strains representing f ive cucurbit-infecting formae speciales, cucumerinum, melonis, lagenar iae, niveum, and momordicae, and 6 strains of formae speciales pathoge nic to other plants were digested with EcoRV and hybridized with P-32- labeled FOLR probes. The strains were clearly distinguishable at the f ormae specialis level on the basis of FOLR DNA fingerprints. Fifty-two fingerprint types were detected among the 56 strains by using all FOL R probes. These probes were used to infer phylogenetic relationships a mong the DNA fingerprint types by the unweighted pair group method usi ng averages and parsimony analysis. The fingerprint types detected in each of the formae speciales cucumerinum, lagenariae, niveum, and momo rdicae were grouped into a single cluster. However, two different gene tic groups occurred in the formae specialis melonis. The two groups al so differed in pathogenicity: one group caused wilts of muskmelon and oriental melon, while the second was pathogenic only to muskmelon. The fingerprint types of different formae speciales pathogenic to plants other than cucurbits were distinguishable from one another and from th e fingerprints of the cucurbit-infecting strains. These results sugges t that the cucurbit-infecting formae speciales are intraspecific varia nts distinguishable at the DNA level and in their host range.