Jones, Whitney M (2014-05). Single Plant Selection as a Screening Method for Resistance to Rhizoctonia solani and Pythium ultimum in Cotton. Master's Thesis.
Upland cotton, Gossypium hirsutum L., is grown extensively in the southern United States with an annual farmgate value of $6 billion and an annual national economic impact of over $120 billion. Damage due to biotic pests, including what is known as the cotton seedling disease complex (CSDC), contribute to these losses. Two particular CSDC pathogens, Rhizoctonia solani and Pythium ultimum, are the most significant soilborne pathogens of cotton in the United States. A program for R. solani and P. ultimum resistant cotton germplasm was established at Texas A&M University AgriLife Research. Five germplasm families selected for elevated levels of condensed tannins were evaluated for resistance to R. solani and P. ultimum. Two generations of single plant selections resulted in three generations, C_(0) (original families or Cycle 0) C_(1), selected from the C_(0) family, and C_(2), selected from the C_(1) generation. C_(1) and C_(2) were putative resistant families after one or two generation(s) of selection, respectively. Individual plants from the three generations within five families were challenged with either or both R. solani or P. ultimum to evaluate the progress of single plant selection for resistance. A susceptible cultivar for R. solani- and P. ultimum-resistance respectively, were included. Different R. solani and P. ultimum families from each generation of selection were evaluated at three inoculation levels with four replications per family. Differences in level of resistance between each generation were evaluated by comparing disease level in a randomized complete block. Cross-resistance was evaluated, i.e., C_(2) families originally screened under R. solani pressure were inoculated and screened for P. ultimum resistance and vice versa. Individual plant selection (IPS) in an artificial environment may be a useful and important tool in developing seedling disease-resistant cotton germplasm. Furthermore, it can be concluded that the family evaluated is of importance to determine the amount of progress made in terms of disease resistance with IPS. Individual plant selection when challenged with appropriate levels R. solani and P. ultimum appears to be an effective tool for selection of germplasm resistant to these seedling disease causing pathogens.