Somatic hybridization of high yield, cold-hardy and disease resistant parents for citrus rootstock improvement

Increasing losses of trees to diseases, expansion of plantings into more marginal production areas and the need to control tree size to reduce harvesting costs have elevated the demand for new improved citrus rootstocks in Florida. A major strategy of the CREC rootstock improvement programme has been to use protoplast fusion to produce allotetraploid somatic hybrids that combine complementary rootstock germplasm. Tetraploid citrus rootstocks have been shown to have a built-in tree size control component due to polyploidy. This report focuses on the incorporation of newly identified superior rootstock germplasm into the somatic hybridization programme. Poncirus trifoliata 50-7, a selection of trifoliate orange selected for superior resistance to Phytophthora nicotianae, was hybridized with sour orange, Changsha mandarin, Navel orange, and a seedy white 'Duncan' type grapefruit. High-yielding Benton citrange (Citrus sinensis X Poncirus trifoliata) was hybridized with Changsha mandarin and sour orange. More than 200 plants of each of these six new somatic hybrids were propagated via tissue culture and rooted cuttings. To assess their horticultural performance, all the somatic hybrids were budded with commercially important scions and planted in replicated trials representing the two most important soil types in Florida. Seed trees of each somatic hybrid have also been planted in the fit ld to determine fruiting potential and level of polyembryony.