Genetic Gain in Resistance to Rhizoctonia Solani and Pythium Ultimum in Upland 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 States¹. 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 evaluated genetic gain in resistance achieved through individual plant selections 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 were made from C₀ (original lines or Cycle 0) to produce C₁ (putative resistant lines after one generation of selection) test lines. These test lines show a range of resistance from complete susceptibility (phenotypic rating of 6) to fully resistant (phenotypic rating of 0). Future work entails another round of selection (to produce C₂ lines), resulting in 2 generations of single plant selections. A final experiment will be conducted to evaluate the progress of single plant selections for resistance by comparing the C₀, C₁, and C₂ generations for both R. solani- and P. ultimum-resistant test lines.