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Husk spot, a fungal disease of macadamia pericarps (Pseudocercospora macadamiae), induces premature abscission in several major commercial cultivars. Breeding for resistance to husk spot is a priority of the Australian macadamia industry. Due to the large tree size of macadamia and high numbers of progeny in breeding populations, inoculating for resistance screening is laborious and time consuming. Previously utilized methods included direct applications of P. macadamiae suspensions and the hanging of bags of diseased husks above developing fruit in tree canopies. In this study, both methods were modified to allow for efficient application in large-scale breeding populations, and their efficacy was evaluated. Two quantities of diseased husk per bag, ‘large’ (75 g) and ‘small’ (30 g), and two concentrations of sprayed P. macadamiae suspensions, ‘stock’ (5 × 105 propagules/ml) and ‘dilute’ (2.5 × 105 propagules/ml), were tested across two fruiting seasons. Treatments were compared against a control (sterile water) in commercial cultivars A38 and A4. Husk spot incidence and severity produced by small bags were significantly affected by season. A significant season effect was less common for other treatments. All four treatments infected over 50% of target fruit in each season, but the highest husk spot incidence across both seasons (≥85%) was produced from large bags. Overall, the large bags were the most reliable method for infection of target fruit. Results also demonstrate the importance of considering the effect of season when selecting husk spot inoculation methods.

Husk spot, a fungal (Pseudocercospora macadamiae) disease of macadamia pericarps, induces premature abscission in several major commercial cultivars. Breeding for resistance to husk spot is a priority of the Australian macadamia industry. Due to the large tree size of macadamia and high numbers of progeny in breeding populations, inoculating for resistance screening is laborious and time consuming. Previously utilised methods included direct applications of P. macadamiae suspensions and hanging of bags of diseased husks above developing fruit in tree canopies. In this study, both methods were modified to allow for efficient application in large-scale breeding populations and their efficacy was evaluated. Two quantities of diseased husk per bag: 'large' (75 g) and 'small' (30 g) and two concentrations of sprayed P. macadamiae suspensions: 5 x 105 propagules/mL 'stock' and 'dilute' 2.5 x 105 propagules/mL were tested across two fruiting seasons. Treatments were compared against a control (sterile water) in commercial cultivars 'A38' and 'A4'. Husk spot incidence and severity produced by small bags were significantly affected by season. A significant season effect was less common for other treatments. All four treatments infected over 50% of target fruit in each season, but the highest husk spot incidence across both seasons (≥85%) was produced from large bags. Overall, the large bags were the most reliable method for infection of target fruit. Results also demonstrate the importance of considering the effect of season when selecting husk spot inoculation methods.

This paper evaluates a novel approach in horticulture of using quantitative survey methodology to inform horticultural tree crop breeding objectives. Using the Australian macadamia breeding program as a case study, the aim was to elicit Australian macadamia industry stakeholder preferences for economical equivalent trait improvements, estimate preference heterogeneity, and identify associations with stakeholder characteristics. A total of 19 respondents participated in the survey, representing 20% of the Australian macadamia industry. Respondents displayed a high willingness to use pairwise comparison implemented in the 1000minds software. Respondents exhibited a wide variability in preferences, however, variation in average preference among trait improvement levels was not large, suggesting on average preferences were consistent with economic values of unit of trait change derived from an industry developed bio-economic model. Observed heterogeneity in preferences were not associated with respondents’ role in industry, orchard size, or growing region.

Macadamia pericarps that fail to abscise (‘stick-tights’) are an important trait to select against in breeding as they can harbour pests and diseases. Traditional macadamia breeding cycles are lengthy and expensive due to long juvenilities and large tree sizes. Thus, genome-wide association studies (GWAS) are an important investigative tool to identify candidate trait-linked markers to enable potential reductions in evaluation and selection cycles via marker-assisted selection (MAS) in young seedlings. This study assessed 199 wild macadamia germplasm accessions for stick-tight prevalence across two years. As the number of stick-tights per tree is limited by the number of nuts per tree, we conducted association analyses to identify SNPs linked with the number of stick-tights per tree, and examined whether such SNPs were also associated with, and thus confounded with, the number of nuts per tree. We also assessed associations with the proportion of stick-tights per total number of nuts. Thirty-two SNPs were associated with at least one of the stick-tight traits in one year (p < 0.001). Of all such SNPs, only one was associated with the number of nuts per tree (p < 0.001), indicating that most associations were not confounded with yield.

The four Macadamia species (M. integrifolia, M. tetraphylla, M. ternifolia and M. jansenii) occur naturally in the wild in fragmented habitats in Australia and there is limited information on their vulnerability to pathogens including species of the genus Phytophthora. Macadamias in commercial orchards are affected by P. cinnamomi and P. multivora causing stem canker and root rot. Wild germplasm is often regarded as sources of resistance in macadamia breeding program. We assessed the performance of 152 trees of wild macadamia genotypes in the field using a Phytophthora disease severity rating scale and used in vivo leaf assay to examine their susceptibility to P. cinnamomi and P. multivora. Macadamia ternifolia trees showed the highest Phytophthora disease severity compared with the other species. In the in vivo trial, there were significant variations in disease severity among the genotypes within each Macadamia species. Comparison of the mean leaf lesion area of the Macadamia spp. showed that M. tetraphylla and M. jansenii were the most resistant to P. cinnamomi, whereas, M. ternifolia and M. jansenii followed by M. tetraphylla had the least disease severity to P. multivora. The quantitative variations among the genotypes with strong differential effects resulted in demarcation of the wild genotypes into three resistance groups. Overall, a total of 14 M. tetraphylla and two M. integrifolia genotypes were identified as resistant to both P. cinnamomi and P. multivora in the in vivo assay. This article is protected by copyright. All rights reserved.

Improving yield prediction and selection efficiency is critical for tree breeding. This is vital for macadamia trees with the time from crossing to production of new cultivars being almost a quarter of a century. Genomic selection (GS) is a useful tool in plant breeding, particularly with perennial trees, contributing to an increased rate of genetic gain and reducing the length of the breeding cycle. We investigated the potential of using GS methods to increase genetic gain and accelerate selection efficiency in the Australian macadamia breeding program with comparison to traditional breeding methods. This study evaluated the prediction accuracy of GS in a macadamia breeding population of 295 full-sib progeny from 32 families (29 parents, reciprocals combined), along with a subset of parents. Historical yield data for tree ages 5 to 8 years were used in the study, along with a set of 4113 SNP markers. The traits of focus were average nut yield from tree ages 5 to 8 years and yield stability, measured as the standard deviation of yield over these 4 years. GBLUP GS models were used to obtain genomic estimated breeding values for each genotype, with a five-fold cross-validation method and two techniques: prediction across related populations and prediction across unrelated populations.

Breeding for new macadamia cultivars with high nut yield is expensive in terms of time, labour and cost. Most trees set nuts after four to five years, and candidate varieties for breeding are evaluated for at least eight years for various traits. Genome-wide association studies (GWAS) are promising methods to reduce evaluation and selection cycles by identifying genetic markers linked with key traits, potentially enabling early selection through marker-assisted selection. This study used 295 progeny from 32 full-sib families and 29 parents (18 phenotyped) which were planted across four sites, with each tree genotyped for 4113 SNPs. ASReml-R was used to perform association analyses with linear mixed models including a genomic relationship matrix to account for population structure. Traits investigated were: nut weight (NW), kernel weight (KW), kernel recovery (KR), percentage of whole kernels (WK), tree trunk circumference (TC), percentage of racemes that survived from flowering through to nut set, and number of nuts per raceme.

Breeding for new macadamia cultivars with high nut yield is expensive in terms of time, labour and cost. Most trees set nuts after four to five years, and candidate varieties for breeding are evaluated for at least eight years for various traits. Genome-wide association studies (GWAS) are promising methods to reduce evaluation and selection cycles by identifying genetic markers linked with key traits, potentially enabling early selection through marker-assisted selection. This study used 295 progeny from 32 full-sib families and 29 parents (18 phenotyped) which were planted across four sites, with each tree genotyped for 4113 SNPs. ASReml-R was used to perform association analyses with linear mixed models including a genomic relationship matrix to account for population structure. Traits investigated were: nut weight (NW), kernel weight (KW), kernel recovery (KR), percentage of whole kernels (WK), tree trunk circumference (TC), percentage of racemes that survived from flowering through to nut set, and number of nuts per raceme.

In Australia, Phytophthora cinnamomi is the only species reported as the causal agent of stem canker and root rot in macadamia. In other countries, five Phytophthora species have been reported to cause disease in macadamia, which led us to question if more than one Phytophthora species is responsible for poor tree health, in macadamia orchards in Australia. To investigate this, samples were collected from the rhizosphere, stem and root tissues of symptomatic and asymptomatic trees, nursery and water sources from 70 commercial macadamia orchards in Australia. Phytophthora isolates were identified based on morphological characteristics and DNA sequencing. Phytophthora cinnamomi was the most predominant, widely distributed and was obtained from the different types of samples including asymptomatic root tissues. In addition to P. cinnamomi, only P. multivora was isolated from diseased tissue (stem canker) samples. Six other Phytophthora species were obtained from the rhizosphere samples including P. pseudocryptogea, P. citrophthora, P. nicotianae, P. gondwanense, P. sojae and a new Phytophthora taxon. Only P. cinnamomi was obtained from macadamia nursery samples while five Phytophthora species were obtained from water sources. Of the heterothallic Phytophthora species, mating type A2 isolates were dominant in P. cinnamomi isolates, whereas only mating type A1 isolates were obtained for P. nicotianae, P. pseudocryptogea and P. citrophthora. Pathogenicity assays revealed that P. cinnamomi and P. multivora caused significantly larger stem and leaf lesions than P. citrophthora, P. nicotianae and P. pseudocryptogea. Phytophthora sp. and P. sojae were non-pathogenic towards leaf and stems.

Two of Australia’s endemic rainforest species, Macadamia integrifolia and M. tetraphylla, produce edible nuts with high-value kernel. The majority of commercial varieties are based on the limited gene pool of M. integrifolia and hybrids between M. integrifolia and M. tetraphylla. An understanding of the existing genetic diversity within the species can be utilized for future genetic improvement. Aiming to identify the genetic diversity, we conducted phylogenetic analyses of 98 wild accessions of M. integrifolia collected from 23 population sites, and 94 accessions of M. tetraphylla from 21 sites using 2,872 high-quality DArTseq-based SNP markers. The phylogenetic tree clearly distinguished clades of the two species. Within species, most of the accessions from the same population clustered together. Genetic and geographic distances among M. integrifolia populations were positively correlated (r = 0.26, p = 0.01), whereas the correlation was poor and not significant among M. tetraphylla populations (r = 0.08, p = 0.13). The relationship between genetic and geographic distance allows evaluating the relative influences of gene flow and drift on population structure within and between regions.

Macadamia nut kernels are high in oil, of which the majority is mono-unsaturated and considered to be beneficial for cardiovascular disease (CVD). In particular, macadamias are unusually high in the omega-7 fatty acid, palmitoleic acid, which has been linked to the amelioration of both CVD and additional metabolic disorders. At the same time, macadamia oil has a significant proportion of saturated fat, which is considered to be detrimental to cardiovascular health. Although this level is relatively low compared to the 'good' mono-unsaturated fats present, its concentration is marginally high enough to affect US and Australian label health claims. In order to remove this restriction, a slight reduction of saturated fat concentration would be required in macadamia nuts. A potential way to achieve this is to identify macadamia lines with increased capability of desaturating palmitic acid (saturated) to palmitoleic acid (mono-unsaturated). The current paper summarizes an exploration of fatty acid profiles in existing commercial cultivars of macadamias, wild macadamia accessions and related Macadamia species, and factors that may influence the fatty acid profile.