Fusarium Wilt

Fusarium wilt causes major crop losses worldwide. Currently, there are three races (races 1, 2, and 3) of the causal organism Fusarium oxysporum f. sp. lycop-ersici. Yellow older leaves are the first symptoms of the disease, this is followed by entire branches turning yellow, but often only one side or part of a leaf or branch will turn yellow. When the outer part of the vascular tissue is examined a characteristic red brown discoloration can be found which extends up into the plant. This disease can also be seed-borne and survive in the soil for long periods of time. Thus, care must be made to reduce contaminated machinery, infected plant debris and water as the fungus can be easily transported. Bohn and Tucker (1939) reported the first vertical resistance gene, I, found on chromosome 11 near TG523 (Scott and Gardner 2007). A gene for resistance to the second race, I - 2, has also been mapped to chromosome 11 near TG105 (Later-rot 1976; Sarfatti et al. 1989). The I - 2 gene has been cloned and is a complex locus (Ori et al. 1997; Simons et al. 1998). A gene for resistance to the third race was linked to the isozyme Got-2 on chromosome 7 (Bour-nival et al. 1989). A fourth gene, I - 1, has also been placed on chromosome 7, but is not an allele of I - 3 (Sarfatti et al. 1991; Scott et al. 2004). Of these genes, two (I and I - 2) have been used most often in breeding cultivars (Scott and Gardner 2007), however the I - 3 gene is now being used in developing breeding lines and cultivars (Scott and Jones 1995).

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