|Kistler, H - Corby|
Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/2/2003
Publication Date: 12/1/2003
Citation: Cai, G., Gale, L.R., Schneider, R.W., Kistler, H.C., Davis, R.M., Elias, K.S., Miyao, E.M. 2003. Origin of race 3 of Fusarium oxysporum f. sp. lycopersici at a single site in California. Phytopathology. 93:1014-1022. Interpretive Summary: Fusarium wilt of tomato, caused by Fusarium oxyporum f. sp. lycopersici (FOL) is a major disease in nearly all major tomato growing regions worldwide. The disease has been reported in at least 32 countries. Three races of FOL are known to exist, distinguished by their virulence to tomato cultivars that contain particular genes for disease resistance. FOL race 3 is especially problematic as few commercial tomato cultivars with race 3 resistance are available. This study demonstrates that FOL race 3 may arise spontaneously in farmers fields from a resident race 2 population in California. Therefore, quarantine efforts in order to exclude or to limit the occurrence of race 3 in California ultimately may fail due to the ability of this pathogen to change race. This publication therefore would benefit regulatory agencies involved in quarantine decision making as well as other researchers who study how fungi change to over time to become more virulent pathogens. Ultimately this knowledge will aid tomato growers by allowing information-based decisions on the types of tomato varieties they may plant.
Technical Abstract: Thirty nine isolates, presumed to be Fusarium oxysporum f. sp. lycopersici (FOL), were collected from tomato plants displaying wilt symptoms in California in 1989, one year after race 3 was first observed in the field. These isolates and isolates of FOL from other locations were characterized using vegetative compatibility grouping (VCG) and pathogenicity. Six California isolates, all race 2, belonged to the new VCG 0035; three nonpathogenic isolates grouped in VCG 0031. A bridging isolate was vegetatively compatible with some members of VCG 0030 as well as with VCG 0032. Restriction fragment length polymorphisms (RFLPs) and sequencing of the intergenic spacer (IGS) region of rDNA identified five IGS/RFLP haplotypes. Analysis of 1 kb of the IGS region showed that VCG0035 was more closely related to isolates of F. oxysporum f. sp. radicis lycopersici than to other VCGs of FOL. RFLPs of genomic DNA indicated that two race 3 isolates in VCG0030 from Florida were more closely related to VCG0032 than to other isolates in VCG0030. The nonpathogenic isolates in VCG0031 could not be differentiated from pathogenic isolates in this VCG. RFLP and DNA sequence data are consistent with the hypothesis that race 3 in California originated from the local race 2 population.