Genetic structure and variation in aggressiveness in Fusarium virguliforme in the Midwest United States

Sudden death syndrome of soybean, caused by Fusarium virguliforme, is one of the most damaging diseases affecting soybean production in the Midwestern region of the USA. To date, there has been very low genetic variation detected in F. virguliforme populations, although isolates can exhibit differing aggressiveness on soybean. To further investigate the genetics behind this variation in aggressiveness, multiple fingerprint analyses were conducted on a collection of F. virguliforme isolates from Iowa and neighbouring states, and their aggressiveness was tested using greenhouse and culture filtrate assays. Twelve RAPD primers identified 13 genotypes, while hybridization of the (CAT)5 probe to Pst I-restricted genomic DNA yielded eight genotypes. Fingerprint patterns obtained by RFLP analysis of mitochondrial DNA with the enzyme HaeIII were identical for all F. virguliforme isolates. Combined analysis of the RAPD and (CAT)5 binary data identified 25 genotypes within F. virguliforme, indicating a greater amount of genetic diversity than was previously known. Disease severity and plant growth varied significantly among isolates, but isolate aggressiveness was not associated with genetic variation. This study provides evidence for the existence of genetic variation in F. virguliforme but suggests that minor quantitative traits and environmental interactions are primarily responsible for the variation in aggressiveness found among isolates within the species.