Evaluation of Four Upland Cotton Genotypes for a Rate-Limiting Resistance to Phymatotrichum Root Rot

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Abstract
Resistance to Phymatotrichum root rot was evaluated in 4 genotypes of upland cotton reported to possess varying degrees of resistance. Under conditions of controlled soil temperature, inoculum density, inoculum distribution, and host population density, no significant resistance was observed. The 4 host genotypes had no effect upon the rate of growth of Phymatotrichum omnivorum through the soil. Under a regime of increasing soil temperatures, the pathogen grew at a rate best fit by a Gompertz growth model. Wilt symptom progression from plant to plant within rows of the 4 genotypes did not differ significantly. All symptom progression rates were highly linear. The symptom-expression interval, or the time between fungal contact with the host root system and wilt symptom expression, was highly variable within genotypes but not significantly different between genotypes. The time of initial symptom development did vary among the 4 genetic stocks and appeared to be influenced by the growth phase and maturity of the hosts. Under the conditions of the investigation, no evidence of host tolerance of rate-limiting resistance was observed.