It is generally accepted that both races of Fusarium oxysporum f. sp. lycopersici are controlled by gene I-2. Only Cirulli and Alexander (Phytopathology, 1966, 56(11):1301-1304) presented findings which attempt to show that the resistance to each of the two races is governed by a different gene. In the course of our selection work, we encountered a case which leads us to the same hypothesis as Cirulli and Alexander.
The anomaly leading us to this hypothesis appeared in the descendants of a cross between a breeding line susceptible to both races of Fusarium, and "Florida MH1" resistant to both races.
One of the F4 lines from this cross was found to be susceptible to race 0 and in segregation for the resistance of race 1 (we use here the nomenclature proposed by Gabe in Rev. of Plant Pathol., 1975, 54(7):555-556 (2971)). At level F8, several lines appeared to be at one and the same time f ixed as susceptible to race 0 and as resistant to race 1. Susceptibility to race 0 was observed with several different isolates.
The lines studied appear to have the genotype I^+^/I^+^ I-2/I-2 following crossing-over between the genes I and I-2 present in "Florida MH1." It is to be noted that the gene I-2 by itself gives, in addition to resistance to race 1, a reduced susceptibility to race 0 (after inoculation with race 0, a non-negligible percentage of plants are found to be healthy).
We do not know of any other lines carrying I-2 with I. We have named such lines "Ideucenzi."
As gene I-2 controls only resistance to race 1 in the absence of gene I, it is important, when selecting to test lines resistant to race 1 for reactions to race 0, so as to keep gene I and thus have varieties resistant to both races.
Literature cited:
Laterrot, R. and J. Philouze. Recombination between resistance to pathotype I (I-2 allele) and susceptibility to pathotype 0 (I^+ allele) of Fusarium Oxysporum f. sp. lycopersici in tomato (Lycopersicon esculentum Mill.). Eucarpia, Tomato Working Group. Synopses IXth Meeting, May 22-24, 1984, Wageningen, The Netherlands, 70-74.