Psyllid resistance in Leucaena. Part 1. Genetic resistance in subtropical Australia and humid-tropical Philippines

One hundred and sixteen accessions of Leucaena were assessed for resistance to the leucaena psyllid that has devastated stands of L. leucocephala pan-tropically since the mid-1980s. Damage to young leaf was assessed using a ratings scale, developed at the University of Hawaii, in replicated field trials at Brisbane, Australia and Los Baños, Philippines. The study identified a continuum of resistance among the accessions, from highly resistant to highly susceptible. Resistance levels were consistent between the two sites with few exceptions. L. collinsii subsp. collinsii, L. confertiflora, L. esculenta, L. pueblana, L. retusa, L. greggii and L. matudae were highly resistant at both sites, while L. leucocephala, L. lempirana, L. involucrata and L. multicapitula were highly susceptible at both sites. Other species were intermediate and were allocated to either resistant or susceptible categories. There was considerable variation in psyllid resistance within some species especially, L. trichandra, L. diversifolia and L. collinsii. Regression analysis was used to determine the influence of environmental variables on psyllid populations. Psyllid population outbreaks were restricted to a temperature range of 10?°C minimum temperature and 33?°C maximum temperature, but additional, unidentified factors influenced populations within this range. Leaf chemical characteristics were examined for their potential to confer psyllid resistance, but none were strongly correlated with accession damage ratings.