The genetics of tolerance to tristeza disease in citrus rootstocks

Controlled pollinations between four elite citrus rootstocks, Citrus limonia -'Limeira' rangpur lime (Cravo), C. sunki-'Sunki' mandarin (Sunki), C. aurantium -'Sao Paulo' sour orange (Azeda) and Poncirus trifoliata -'Davis A' trifoliate orange (Trifoliata), resulted in 1614 nucelar and 1938 hybrid plants identified by the isozyme loci Pgi-1, Pgm-1, Got-1, Got-2, Aps-1, Me-1, Prxa-1 and or by the morphological markers broadness of leaf petiole wing or trifoliolate leaves. Tolerance to the citrus tristeza virus (CTV) was evaluated under nursery and field conditions for several years by the reaction of Valencia orange infected with a severe strain of CTV and grafted onto the hybrids and nucellar clones. Genetic analyses indicated that tolerance was controlled by at least two loci designated here as Az and t interacting in dominant-recessive epistasis. Genotypes Az_ _ _ and _ _ tt were tolerant while azaz T_ was intolerant. The intolerant Azeda was azaz TT, the tolerant rootstocks Sunki and Cravo were Azaz tt and the Trifoliata was Azaz TT The different degrees of intolerance seen in some hybrids may reflect the inability of segregating modifiers from parental clones to overcome the epistatic interaction that controls the major tolerance reaction.