Breeding Improved Forage Leucaenas for Cold and Psyllid Tolerance.

The 22 species of the mimosoid American genus Leucaena range from Texas to Peru and embrace many tropical and subtropical ecosystems. The species L. leucocephala (2n=104) is widely grown globally as a multipurpose tree for fuelwood, fiber, lumber, shade, green manure, and fodder. Its most significant commercial uses have been as a forage legume supplementing grass on >150,000 ha in Australia, and as an arboreal source of fuelwood and paper in India. Principal targets for forage research have been tolerances of cold temperatures and of the psyllid insect, Heteropsylla cubana. Resistance to the psyllid was discovered in 1984 among several species of the genus, and validated by international trials. We report progress in development of species hybrids incorporating this resistance, together with cold tolerance, primarily from the self-sterile highland species L. pallida (also 2n=104). One self-sterile cultivar, KX2-Hawaii, has been released after five cycles of recurrent selection from the hybrid of L. pallida and L. leucocephala, and a second is being developed in Australia. The species hybrid KX3 of L. leucocephala x L. diversifolia (also 2n = 104) has shown promise internationally as a cold-tolerant arboreal type, but lacks psyllid tolerance. Since 18 of the 22 L. species are diploids (2n = 52 or 56), many of the 80 interspecific hybrids that have been produced in Hawaii are triploids, often sterile. Among these KX4 (L. leucocephala x L. esculenta) and KX5 (L. diversifolia x L. trichandra) show promise as seedless hybrids for shade or high-value hardwood (growth to 15m in 5 years). Chromosome-doubling offers promise for transfer to L. leucocephala of genes for tolerance both to colder temperatures (including frost, in L. retusa and L. greggii) and to psyllids. Promising diploid parents are L. collinsii, L. lanceolata and L. multicapitula.