Use of Trifoliate Orange as a Comparative Standard for Assessing the Resistance of Citrus Rootstocks to Citrus Nematode. R. K. Niles, Department of Botany and Plant Sciences, University of California, Riverside 92521. D. W. Freckman, Department of Nematology, and M. L. Roose, Department of Botany and Plant Sciences, University of California, Riverside 92521. Plant Dis. 79:813-818. Accepted for publication 24 April 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0813.

Twenty-one citrus rootstocks were screened in a greenhouse for resistance to Tylenchulus semipenetrans. Parasitism by T. semipenetrans was confirmed by verifying the host suitabilities of rootstocks expected under California conditions to be resistant (C-35 citrange, Swingle citrumelo, and Poncirus trifoliata cv. Rubidoux) or susceptible (Citrus sinensis cv. Argentina and Carrizo citrange) to T. semipenetrans. P. trifoliata cv. Rubidoux was the resistant standard for comparing the host suitabilities of individual rootstocks. Five previously unscreened root-stocks expressed greater resistance than P. trifoliata: 1452 citrumelo, Siamese pummelo x P. trifoliata, and three selections from crosses of C. sunki and P. trifoliata (C-54, C-57, and C-146). Rank correlation showed the relative host suitabilities of individual rootstocks were consistent between two experiments.