Genetic Gain through Breeding for Resistance to Rhizoctonia Solani and Pythium Ultimum in Cotton Seedlings

Two Cotton Seed Disease Complex (CSDC) pathogens, Rhizoctonia solani and Pythium ultimum, are the most significant soilborne pathogens of cotton in the United States. Symptoms of the disease include poor plant stands due to pre- and post-emergence damping off and seed rot, seedling root rot, hypocotyl lesions and stunting that delays growth. Previous studies indicate that breeding programs utilizing single plant selections have been successful in achieving resistance and partial resistance to some CSDC pathogens for other crop species and cotton respectively. This study aims to evaluate genetic gain achieved through individual plant selections and to produce cotton germplasm that is resistant to R. solani and P. ultimum. Six potentially resistant, high-tannin lines (originally bred for insect resistance) and two high-yielding checks were screened for R. solani and P. ultimum resistance under greenhouse conditions conducive to disease presence. Single plant selections have been made from C₀ (original lines or Cycle 0) to produce C₁ and C₂ (putative resistant lines after one and two generations of selection, respectively) test lines. These test lines show a range of resistance from complete susceptibility to fully resistant. All lines from both cycles of selection, plus the parent populations and the susceptible cultivars, were challenged with R. solani or P. ultimum, as appropriate, in a bulk seedling test to determine genetic gain. Each family tested was replicated four times and challenged with three inoculation levels (0, original selection level, and 2x original selection level).