2018 ASHS Annual Conference

‘UCB-1’ is currently the most commonly planted pistachio rootstock in California, home to the majority of pistachio orchards in the USA. UCB-1 is grown from F₁ hybrid seed created by crossing Pistachia atlantica and P. integerrima. However, UCB-1 varies as is expected from a cross between two heterozygous outbreeding species. Some rootstocks in the population have low vigor resulting in low-yielding trees. In order to study this variability, we planted 960 ‘UCB-1’ seedlings in 2013 that have been phenotyped annually thereafter. Data have been collected on tree height, trunk caliper, and tree architecture. Early growth was a poor predictor of subsequent growth of the non-grafted trees supporting the grower experience that early rouging is ineffective. As the trees aged, the population showed greater variation and there are now six standard deviations in trunk caliper. We have generated draft genome assemblies for the parental P. atlantica and integerrima, as well as sequenced a total of nearly two thousand UCB-1 F₁ seedlings from our experimental and four commercial orchards. To generate these data, we used 10x Chromium, Dovetail Hi-C, and reduced representation sequencing. Genome-wide association studies identified three major loci controlling trunk caliper in our experimental orchard of ungrafted trees. Two of these loci were also highly significant in determining the trunk caliper of grafted trees in commercial orchards. Development of predictive markers for UCB-1 rootstock vigour will be highly beneficial to the industry, allowing removal of seedlings with likely poor performance prior to planting in orchards.