The Evaluation of High Tannin Cotton Lines and Their Use in Breeding for Resistance to Xanthomonas axonopodis pv. malvacearum, Pythium aphanidermatum and Rhizoctonia solani

Xanthomonas axonopodis pv. malvacearum(Smith), Pythium aphanidermatum(Edson) and Rhizoctonia solani(Kuhn) have all been shown to cause significant yield losses in cotton. Previous work has demonstrated that a set of high tannin cotton germplasm lines developed and released in 1989 by Texas A&M AgriLife Research may possess resistance to these three diseases. In this research, the usefulness of these high tannin lines in breeding for resistance to these pathogens as well as the role of tannin in conferring this resistance were examined. The high tannin lines were screened for their resistance to Xanthomonas axonopodis pv. malvacearum and five highly resistant lines were subjected to generation means and diallel analysis in order to determine the relative importance of different types of gene action in conferring resistance as well as which lines would be most useful in breeding for resistance. The effect of selection for R. solani resistance was measured and selected lines were subjected to diallel analysis. Lines showing elevated resistant to P. aphanidermatum were subjected to a single cycle of selection and the effect of selection was measured. Lines possessing high degrees of P. aphanidermatum resistance were subjected to generation means and diallel analysis. The importance of tannin content in conferring resistance to X. axonopodis and R. solani was also measured. Four high tannin lines were found to be not different from Tamcot Sphinx in their resistance to Xam. Generation means analysis for Xam resistance showed that in the five high tannin lines tested, additive gene effects were of the greatest importance. No correlation between tannin content and Xam resistance was detected. Under the conditions of this study tannin content was also shown to be unimportant in R. solani resistance. Two rounds of recurrent phenotypic selection were effective in increasing the resistance of selected HT lines to R. solani. One cycle of phenotypic selection for P. aphanidermatum resistance was found to produce significant improvements in seven of the most resistant HT lines. Generation means analysis for P. aphanidermatum resistance showed that in the HT lines tested, additive genetic effects were by far the most important.