AN EVALUATION OF WATERMELON (Citrullus spp.) GERMPLASM FOR ADDITIONAL SOURCES OF RESISTANCE TO THE TWOSPOTTED SPIDER MITE (Tetranychus urticae Koch)

Fourteen U.S plant introduction (PI) accessions of C. lanatus var. lanatus (4), C. lanatus var. citroides (5) C. colocynthis (5) and a known susceptible commercial cultivar 'Sugar Baby' were evaluated for resistance to the twospotted spider mite, Tetranychus urticae Koch, in a limited free-choice and free-choice bioassay under laboratory conditions. The limited free choice bioassay, involved nine Petri dish cages that held five randomly assigned leaves individually inoculated with two adult females and one adult male. Eggs, larva, and adults were counted over a nine day period. The free choice bioassay involved the even distribution of three mite infested pinto bean pots among the 15 accessions per tier under evaluation. Four tiers (syn. replications) consisting of fifteen randomized accessions were evaluated over a three week period. In both bioassays the two-spotted spider mite strongly preferred feeding and completing its life cycle on C. lanatus var. lanatus and C. lanatus var. citroides compared to C. colocynthis. Among the C. colocynthis accessions evaluated, PI 388770, PI 525080, and PI 537300 had consistently lower injury ratings and total mite populations (eggs, adults, larva) when compared to the other PIs and the susceptible cultivar 'Sugar Baby'. Preliminary research indicated that feeding tolerance was also found to be significantly different by changing the way we did mite counts (i.e., uncut mite counts; excised leaf counts), but a more thorough study is needed. Ultimately, this study has identified two more possible sources of two-spotted spider mite resistance in PI 525080, and PI 537300 and adds further support for the already identified resistant PI 388770. iv ACKNOWLEDGEMENTS I would like to thank Dr. Tom Hoegemeyer and Dr. P. Stephen Baenziger for stepping in and co-advising my research efforts. I couldn't have completed this thesis without the continued encouragement of Dr. Hoegemeyer and the excellent advice given by Dr. Baenziger. Given the obstacles I faced at a distance, they never lost faith in me and continued to research options to make this project possible. A special thanks to Dr. Deana Covert who played a crucial role in organizing my new co-advisement and for taking a genuine interest in helping me achieve this accomplishment. I would also like to thank Dr. Gary Hein for being a part of my graduate committee. This research would not have been possible if it wasn't for the collaboration and support of Dr. Paul T. Smith at California State University Bakersfield. Out of 20 …