SELECTING DISEASE-RESISTANT CITRUS ROOTSTOCKS

Hybrid rootstocks from a United States Department of Agriculture citru s breeding program were compared for their resistance to phytophthora collar rot (caused by Phytophthora citrophthora) and citrus tristeza v irus (CTV), With 6 rootstocks used commercially in Australia. Rootstoc ks most resistant to collar rot were clone 22 of Poncirus trifoliata, F-1 Cleopatra mandarin x English P. trifoliata, and Benton citrange. M ean lesion lengths for F-1 Sunki mandarin x Mars P. trifoliata, Troyer citrange, (Rangpur lime x Swingle P. trifoliata) open pollinated, and F-1 Cleopatra mandarin x Christiansen P. trifoliata were not signific antly different from clone 22 of P. trifoliata. Some P. trifoliata hyb rids were more susceptible to collar rot than sweet orange, Cleopatra mandarin, or rough lemon. High titres of CTV in ELISA tests were evide nt in some hybrids, but only 1 F-1 Rangpur lime x Carrizo citrange dev eloped stem pitting symptoms. CTV titres in late spring (November) wer e higher in bark than in leaves.