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 ma rginal production areas and the need to control tree size to reduce harvest ing costs have elevated the demand for new improved citrus rootstocks in Fl orida. A major strategy of the CREC rootstock improvement programme has bee n to use protoplast fusion to produce allotetraploid somatic hybrids that c ombine complementary rootstock germplasm. Tetraploid citrus rootstocks have been shown to have a built-in tree size control component due to polyploid y. This report focuses on the incorporation of newly identified superior ro otstock germplasm into the somatic hybridization programme. Poncirus trifol iata 50-7, a selection of trifoliate orange selected for superior resistanc e to Phytophthora nicotianae, was hybridized with sour orange, Changsha man darin, Navel orange, and a seedy white 'Duncan' type grapefruit. High-yield ing Benton citrange (Citrus sinensis X Poncirus trifoliata) was hybridized with Changsha mandarin and sour orange. More than 200 plants of each of the se six new somatic hybrids were propagated via tissue culture and rooted cu ttings. To assess their horticultural performance, all the somatic hybrids were budded with commercially important scions and planted in replicated tr ials 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 fr uiting potential and level of polyembryony.