Phylloxera has been an important pest in California since its discovery in 1880 within Sonoma and Napa counties. Phylloxera-resistant rootstocks and germplasm were selected from American grape species native to the eastern United States. Breeding programs to develop improved phylloxera-resistant rootstocks were started in the late nineteenth century. Resistance to phylloxera has been reported to be controlled by several genes. To focus on one aspect of resistance to phylloxera, a greenhouse screening method was used to observe absence/presence of root nodosities produced by phylloxera. Only one source of phylloxera biotype A from Fresno County was used to reduce the complication of various biotypes. Grape rootstocks with known field reaction to phylloxera were evaluated to test the reliability of the greenhouse test. A design II mating factorial cross was made between male and female rootstocks with a range of resistance to susceptibility. The reaction of their progeny to phylloxera was observed in the greenhouse. All populations segregated for resistance/susceptibility with a few exceptions. Dog Ridge crossed with two susceptible genotypes gave all susceptible offspring. Kober 5BB crossed with susceptible or resistant genotypes gave all resistant offspring. The segregation of resistance to nodosity development could be explained by two complementary dominant genes in most families.
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