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Description
In 2002, a strain of Fusarium oxysporum f. sp. vasinfectum was found in California cotton fields and identified as race 4. Stem inoculations with isolates of the California strain (CA Fov-4) do not elicit symptoms in controlled-environmental chamber experiments, while stem inoculations with Fov race 1 (Fov-1) cause foliar wilt symptoms. In contrast, soil inoculation with CA Fov-4 results in extensive root rotting, which is not true for soil inoculation with Fov-1.
Stem inoculation with a vascular competent strain of Verticillium dahliae initiates a series of biological events in the cotton plant including an increase in the biosynthesis of protective compounds called phytoalexins. When the resistant cotton Seabrook Sea Island (SBSI) (Gossypium barbadense) is stem inoculated with V. dahliae, the concentrations of these terpenoids increase more rapidly than they do in a susceptible cotton.
Soil inoculation with CA Fov-4 in environmental chambers established that SBSI has resistance similar to that of PhytoGen 800. G. barbadense Pima S-6 also shows good resistance to CA Fov-4. It is not known if the resistance in Pima S-6 and that in SBSI are due to the same genes; the phytoalexin response of Pima S-6 to pathogen inoculation has not been reported. To determine if both SBSI and Pima S-6 resistance to Fov-4 is due to rapid response to the infection, SBSI, Pima S-6 and susceptible Pima S-7 were stem inoculated with CA Fov-4 and the levels of phytoalexin sesquiterpenoids were determined. The concentration of individual phytoalexins and total phytoalexins in SBSI plants were significantly higher after 24 and 48 hr than in either resistant Pima S-6 or susceptible Pima S-7.
These preliminary results support the hypothesis that the early response to the pathogen and the resulting induced phytoalexin biosynthesis is different in SBSI than that in Pima S-6 and Pima S-7.