Genetics of hornworm resistance in Maryland tobacco
During outbreak years, the two species complex of tobacco hornworms, Manduca sexta (L.) and tomato hornworms, M. quinquemaculata (Haworth), may cause considerable economic loss to Maryland tobacco, Nicotiana tabacum (L.). This study was designed to aid breeding efforts by providing basic quantitative genetic information on the inheritance of host-plant resistance to hornworm damage in Maryland tobacco and to evaluate methods of rating this resistance in a breeding program. A two-year study was conducted to evaluate "MD 341" and "ND 872" (two Maryland tobacco cultivars),"I-35"(source of hornworm resistance), and the F1, F2, and both backcross populations (BC1 and BC2) derived from each cross of MD 341 x I-35 and HD 872 x I-35. Measures of resistance evaluated included the number of eggs and larvae found on each plant and visual damage ratings. Low egg and larval counts and low visual damage ratings appear to be related to hornworm resistance. These measures of resistance indicate that host-plant resistance is a partially dominant trait in Maryland tobacco. Significant additive effects were observed for the egg and larval counts. This indicates that there are several genes acting in an additive fashion for resistance as measured by egg and larval counts and that accumulation of desirable factors through selection is possible. Visual damage ratings as utilized in this study would not be as useful as quantitative measures of insect activity. However, better separation into more distinct classes and ratings taken on pre-flowering plants could aid in increasing the precision of this method of rating resistance. A modified breeding program utilizing recurrent selection and/or backcross breeding would be most suitable to incorporate the resistance present in I-35 into the Maryland tobacco breeding population for development of improved cultivars.