Horizontal resistance insolanum tuberosum to colorado potato beetle (leptinotarsa decemlineata say)

Horizontal resistance (HR) was assessed as a strategy to reduce potato defoliation by the Colorado potato beetle (CPB). Hybrids amongSolanum tuberosum varieties grown in Iowa were first evaluated with a detached leaflet feeding bioassay. Beginning in 1998, resistance was increased annually over three years by intermating and selecting the most resistant l%–2% of each generation of 300–400 plants using the bioassay. When C (= mg leaf tissue consumed* neonate larva-1 over 48 h) was measured for the 1% most-resistant genotypes of all generations grown side by side in 2000, it ranged from 6.1 mg* larva-1 in the parents to 2.3 for progeny of the F3 generation compared to 2.7 in ND2858-1, a resistant check. Field trials in Wisconsin showed less defoliation in the F1 and F2 selected genotypes than in the parents. A second breeding program using moreS. tuberosum parents with greater genetic diversity, and much larger breeding populations, resulted in significantly faster accumulation of resistance, further suggesting that recurrent mass selection is effective. Foliar glycoalkaloid analyses revealed low levels of solanine and chaconine in the F3 selections; no leptines or leptinines were detected. Because the parents wereS. tuberosum varieties in which vertical resistance to CPB is unknown, and because the resistance to feeding from parents to the F3 resistant plants was linear and gradual while ranging from initially very low to considerably higher in the third year, horizontal resistance is strongly implicated. The development of a significant degree of resistance after three rounds of breeding using relatively small breeding populations suggests that HR can be developed from varieties ofSolanum tuberosum lacking initial high levels of resistance to CPB.