Selection of Psyllid-Resistant Forage Varieties from an Inter-Specific Breeding Program of \u3cem\u3eLeucaena leucocephala\u3c/em\u3e with \u3cem\u3eL. pallida\u3c/em\u3e

Leucaena (Leucaena leucocephala) pastures for beef cattle production are productive and sustainable; however, susceptibility to the psyllid insect (Heteropsylla cubana) has limited expansion of current commercial cultivars into more humid areas (\u3e 800 mm/yr) (Shelton and Dalzell 2007). Psyllids can also cause intermittent damage in lower rainfall regions during humid periods. The psyllid, which arrived in Australia in 1986, is a leaf-sucking insect specific to the Leucaena genus, feeding on the growing tips of susceptible cultivars (Bray 1994). Psyllid damage can reduce production by as much as 50-70% in humid regions and 20-50% in subhumid environments (Bray 1994; Mullen and Shelton 2003). Work on psyllid resistance in the Leucaena genus through the 1990s showed that several Leucaena species, including the tetraploid L. pallida, had good levels of resistance (Mullen et al. 2003). A breeding program to develop psyllid-resistant varieties began in 2002 at The University of Queensland (UQ) based on the F1 inter-specific hybrids between L. leucocephala and L. pallida (known as ‘KX2’), developed at the University of Hawaii (Brewbaker 2008). Between 2002 and 2005, UQ initiated a program of recurrent selection in an attempt to produce stable outcrossed KX2-derived lines but inbreeding depression for yield and poor forage quality led to a change in the breeding strategy, and a backcrossing program was implemented between 2005 and 2008. Two cycles of backcrossing to elite L. leucocephala ssp. glabrata material were completed followed by 2 cycles of progeny testing and selection for self-compatibility to achieve stability and uniformity (2009 - 2012). Forty elite psyllid-resistant lines were then evaluated to identify the most suitable lines for release to industry. This paper describes the results of these trials

Breeding improved Leucaenas for forage and wood with psyllid and cold tolerance

The 22 species of the woody mimosoid genus Leucaena range from Texas to Peru and from sea level to 2500 m elevation. Genetic improvement of these multipurpose trees has focused on the lowland tropical species L. leucocephala ssp. glabrata (2n=104). This species is widely used for forage and now supplements >150,000 ha of grass pastures in Queensland. Internationally it is respected for fuelwood, lumber, paper, charcoal, shade and green manure. Interspecific hybridization can facilitate its genetic improvement as a pasture legume and high-value hardwood by overcoming its susceptibility to cold temperatures and to the leucaena psyllid (Heteropsylla cubana). More than 80 interspecific hybrids have been produced in the genus. Psyllid resistance was identified in several species and has been transferred to KX2 hybrids with L. leucocephala from cold-tolerant L. pallida (2n=104). The University of Hawaii recently released the largely self-sterile “KX2-Hawaii” after six cycles of recurrent selection. The University of Queensland has a backcrossed KX2 variety in advanced stages of breeding. Genetic improvement involves only the arboreal subspecies glabrata, not the shrubby subspecies leucocephala. Arboreal KX3 hybrids of L. leucocephala with the highland Mexican L. diversifolia (2n=104) have excelled in wood production in cooler tropics. As 18 of the 22 species are diploids (2n=52 or 56), many hybrids are sterile triploids. Among these KX4 (L. leucocephala × L. esculenta) and KX5 (L. diversifolia × L. trichandra) are attractive as high-value hardwoods grown on 10 to 12 year coppiced rotations. Chromosome doubling offers promise for inducing self- and cross-fertility of species like L. collinsii ssp. collinsii, L. lanceolata var. sousae and L. multicapitula and additional gene transfer of genes for cold and even frost tolerance from L. trichandra, L. pulverulenta, L. retusa and L. greggii. Collaboration between Hawaii and Queensland on leucaena improvement began in 1961, and continues to provide the best source of quality seeds. Expanded utilization of improved cultivars is notable in Australia, India, Paraguay, Thailand and SE Asia