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The annual sale of agricultural pesticides in Australia doubled from 2010 to 11 to 2021-22. While an analysis found that 73% of Australian crop production in 2015-16 relied on herbicides, fungicides and insecticides, a large proportion of pesticide applications miss the right targets and become “spray drift”. For horticulture, according to previous studies, about 50 to 70% of the applied chemicals can either land directly on the ground or drift into the air. The off-target spray brings no gain to growers but instead is unnecessary waste, unnecessary pollution, and unnecessary risks in potentially damaging neighbouring farms. Many growers hope to maximise the use of their existing sprayers with minimal cost before considering retrofitting or replacing them. To facilitate this objective, a digital-twin-based solution has been developed to simulate orchard characteristics and spray processes that closely mirror reality. It enables high simulation fidelity and aims to serve as a decision-support tool for practical operations. This approach is powered by high performance computing to ensure both accuracy and complexity, without compromising one for the other. It had been field-tested in a tropical research orchard with a strong correlation to real-world measurement. A subsequent digital-twin-based investigation has demonstrated that the spray drift in the same orchard environment can be reduced by 36% through simple modifications of sprayer settings. Such outputs cannot only be used to optimise the adjustment of traditional equipment, but can also be loaded to robotic/automatic sprayers to make them more precise and responsive, yet simpler and cheaper. In addition to reviewing the pain points in reduction of spray drift, introducing the digital twin solution and reporting insights that have been gained from it, the research investigates potential development pathways for similar digital twins in a broader range of orchard and vineyard systems.

Increasing the planting density of mango orchards appears promising for obtaining higher yields, particularly during the first productive years. However, the challenge is to maintain a good balance between vegetative growth and fruit production in the longer term. The objective of this study was to decipher the effects of planting density, training system and cultivar on young mango trees' growth and production. The experiment, conducted in North Queensland, consisted of five combinations of planting density and training system applied to the cultivars Keitt, Calypso and NMBP-1243. The planting densities were low (208 tree ha−1), medium (416 tree ha−1) and high (1250 tree ha−1). The closed vase conventional training system was applied at each density. Single leader and espalier on trellis training systems were applied at medium and high densities, respectively. The tree canopy dimensions were measured every 6 months from planting, and tree production was recorded from the third to the fifth years after planting. Vegetative growth and fruit production were the results of complex interactions between planting density, training system, cultivar and/or time. The expected increase in orchard yield with higher planting density was observed from the first productive year, despite lower individual tree production at high planting density. Lower vegetative growth and fruit production at high planting density were probably caused by competition between trees. NMBP-1243 and Keitt showed more rapid vegetative growth. Keitt was the most productive cultivar during the first three productive years. The detailed results of this study provide avenues to further explore the behaviour of mango trees at high planting densities.

Mango flowering is interannually variable and temperature dependent. Flowering intensity declines with increasing pre-flowering temperatures associated with global warming, potentially limiting yields. Additionally, floral density that does not contribute to greater yield represents an unnecessary drain on plant resources such as nutrients, water, and carbohydrates. This can lead to reduced fruit bearing capacity in later stages of cropping or contribute to interannual yield variation. The objective of this work was to identify the floral density compensation point (FDCP), above which additional flowering becomes redundant, and below which surplus carbohydrates are partitioned to existing flowers, fruit, vegetative growth, or stored. Over two growing seasons, the number of inflorescences on 32 Calypso® mango trees in Dimbulah, Australia were thinned by up to 95%, resulting in floral densities between 0.09 and 4.14 inflorescences per trunk cross sectional area (cm2). Fruit yield, number, composition, vegetative growth, and starch concentration in terminal growth units were measured for each of the trees. Calypso® mango trees have excess floral density, beyond the maximum fruit biomass the tree can support to harvest. Below the FDCP, the tree is unable to compensate for low numbers of inflorescences, and floral density was strongly positively correlated with fruit number and yield, and strongly negatively correlated with fruit per panicle and total soluble solids concentration of ripe fruit. Above the FDCP fruit yield, number and total soluble solids concentration were only weakly related to floral density as they appear limited by carbohydrate availability rather than flowering. Fruit weight, vegetative regrowth of fruiting and non-fruiting terminals, and the starch concentration in terminal growth units were less dependent upon the FDCP and changed continuously across the floral densities evaluated. This information will be valuable for managing mango flowering unde

Mango productivity and fruit quality in Australia can be improved through transforming low-density plantings to high-density plantings and intensive training systems. Several planting density and training systems were established in Australia to investigate optimizing light interception and distribution, yield and fruit quality, and to reduce tree vigor and biennial bearing through the manipulation of canopy architecture. In this research, we studied light relations and yield in conventional low-density commercial orchards of different ages and investigated light relations and yield of ‘National Mango Breeding Program NMBP1243’, ‘Keitt’ and ‘Calypso’ grafted to ‘Kensington Pride (KP)’ rootstock in a replicated experiment. Trees were trained to five planting systems: high-density espalier and conventional, medium-density single leader and conventional, and low-density conventional. Our study in commercial orchards showed that maximum yield was ~16,000 kg/ha when light interception reached ~49% and declined at higher levels of light interception. In the high-density intensive training systems, we found that light interception increased with canopy volume, with high-density espalier training systems intercepting more available light compared to medium-density training systems and low-density conventional trees. Yield/ha increased to ~50,000 kg/ha in espalier training systems when light interception was ~40%. Light interception, canopy volume and yield/ha varied between varieties.

New orchard systems of the future feature unconventional layouts and narrow canopies with fruit walls under intensive pruning and training practices. The development of such systems is not only time- and - labour consuming, but also faces other challenges in potential overuse of chemicals, in maintenance of fruit/nut quality and in response to climate change. Our DigiHort solution is to address those challenges via development of Digital Twins powered by High Performance Computing and Artificial Intelligence. It includes two components: LIGHT and SPRAY. The LIGHT component allows its users to have 3D digital twins of their orchard designs and pre-evaluate how different factors such as row orientation, tree spacing, canopy size and terrain could affect the potential orchard light environment. This can help decision makers optimise their orchard designs in silico before creating/reconfiguring real orchards in situ. Using the SPRAY component, the growers can operate their sprayer digital twin within their orchard digital twin, configure the virtual sprayer with different nozzle types and directions, adjust vehicle speed and/or flight height, simulate spray operations with explicit droplets, evaluate wastage and coverage, and then figure out the optimal instructions to guide their real-world practices. The ultimate purpose is to develop these two technologies into adoptable, affordable and easy-to-use decision-support tools for tree crop growers and horticulturalists to optimise orchard light quality, minimise spray wastage, narrow the range of investigations, and accelerate the innovation of future horticultural systems with better yield, lower cost and less pollution.

An intensification experiment in Far North Queensland, Australia, has shown that planting Mango (Mangifera indica) orchards at high density (>1250 trees ha-1) can increase orchard productivity early in an orchard’s development. Despite this, there is little information available on the best training systems (TS) to apply in high density orchards. The aim of this research was to review three different TS (palmette, cordon and espalier systems on trellis), planted at high density (1,250 trees ha-1) to understand how TS influences canopy characteristics and yield of three different mango cultivars (‘Keitt’,’Calypso’ and ‘National Mango Breeding Program (NMBP) accession 1243’ and ‘4069’) between 2 and 5 years of age. Light interception, canopy characteristics (volume and canopy dimensions), vegetative terminals and flowering rate, tree productivity, orchard yield, and yield efficiency were compared. HD TS were compared to low-density (LD) (208 trees ha-1) and high-density (HD) (1,250 trees ha-1) trees trained using a conventional, closed vase TS of a similar age. While the alternate TS at high density led to taller and thinner canopies by 5 year-of-age, they were also characterized by lower individual tree productivity. Despite this, the increased tree density led to an increased light interception and improved orchard yields compared to the conventional LD orchards of the same age. There were also some interactions between cultivar and the alternate TS. Future research should aim to understand the mechanisms behind these results better. © 2022 International Society for Horticultural Science. All rights reserved.

In Australia, commercial tree size management in mango orchards involves annual machine hedging and heavy hand pruning. In tropical growing regions, heavy pruning often stimulates strong vegetative regrowth that is less likely to flower due to insufficient terminal growth-unit dormancy required for flowering. A few mango rootstocks have been shown to reduce scion vigour and maintain regular, high yields; however, there is a lack of rigorous field-testing of these and other rootstocks under Australian scions growing in Queensland. Apart from some scion vigour control, the effects of mango rootstocks on scion architecture have not been studied in Australia and reports in the international literature are scarce. In this study, 90 genetically diverse rootstocks have been evaluated for their ability to reduce vigour in two Australian mango scion varieties from the National Mango Breeding Program (NMBP); ‘NMBP-1243’ and ‘NMBP-4069’. Tree height, canopy volume and shape, rootstock and scion trunk cross-sectional area were measured in young trees 30 months after planting to evaluate tree growth and vigour. Branch angle, length and diameter, number of growth units, number of leaves, and leaf size were also measured to characterize scion architecture. Ten rootstocks were identified as reducing vigour while maintaining the percentage of flowering terminals per canopy volume. Four rootstocks were also found to influence scion architecture by altering secondary branch angles to be closer to horizontal, potentially making them more suited to single-leader training used in high-density orchards systems. Canopy architecture parameters measured in this study suggest that rootstocks may be a useful technique for reducing tree vigour and altering the architecture of mango scion canopies to make them more suited to intensive orchard systems. © 2022 International Society for Horticultural Science. All rights reserved.

Mangoes are grown in over 100 countries throughout the tropical and subtropical world with a total production of over 42×106 t of fruit sold in both local and export markets. In tropical regions, mangos are typically large, vigorous, low yielding trees with very low production and harvest efficiencies. In Queensland, Australia, a collaborative research project between the Department of Agriculture and Fisheries, the University of Queensland and Hort Innovation Australia has been developing high density, intensive production orchard systems in tropical and subtropical tree fruits. The project is investigating and documenting the relationships between the key drivers of productivity orchards. In mango, the aim of this research is to radically redesign orchard systems, increasing their production efficiency and profitability. Experimental intensification using higher planting densities and single leader training of canopies has led to early gains in productivity (~47 t ha‑1 in year 4). This paper outlines some of the key relationships that are underpinning these early productivity gains in areas such as tree architecture and canopy volume, canopy training, canopy light interception and light distribution.

The effect of preharvest applications of nitrogen (N) fertiliser to mango trees during the late fruit maturation stage was investigated over 3 years (2012-2014) to determine the effects on canopy growth in 8 year-old 'Kensington Pride' (KP) mango trees grown in Queensland. The experiment consisted of six treatments where 156 g N tree-1 was applied as a single treatment, as 340 g of urea or split between three different times. The N treatments were 1) 100% of N 2 weeks postharvest (control), 2) 50% of N 2 weeks preharvest, plus 50% 2 weeks postharvest, 3) 65% 2 weeks preharvest, plus 35% 2 weeks postharvest, 4) 35% 4 weeks preharvest plus 65% 2 weeks postharvest, and 5) 65% 4 weeks preharvest, plus 35% 2 weeks postharvest. Vegetative growth was investigated at the branch, sub-branch (shoots), growth unit (GU) and leaf scales. Branches from different aspects (north, south, east and west), in the upper and lower canopy were assessed, and for 2 branch types (apical or lateral) was considered. Fruit counts were also considered at the branch level. Results showed that when 50-65% of the N was applied 2 and 4 weeks before harvest with the balance 2 weeks postharvest, the number of branches, branch length and fruit count increased. When 35% N was applied 4 weeks preharvest with the balance 2 weeks postharvest, only stem biomass was increased. Generally, branches in the upper canopy had longer sub-branches, longer and thicker, first level growth units, shorter leaf petioles and shorter and thinner leaves compared to those in the lower canopy. The SPAD N index was lowest in the northern canopy aspect. Differences were also observed between lateral and apical branches. Lateral branches had longer leaves than apical branches while apical branches had a trend for higher leaf area, stem biomass and leaf counts. There were also negative relationships between fruit count and measurements for stem biomass, leaf count and leaf area for apical branches however for lateral branches the

The amount of light intercepted by a tree and its distribution within the canopy is critical in optimizing tree photosynthesis efficiency, carbon partitioning and productivity. Here we compare light relations in experimental high-density mango orchard systems with current commercial orchards. A baseline study of current commercial mango orchards showed a maximum light interception of approximately 67% was reached in trees aged between 26-31 years old, with canopy volume of approximately 15,187 m3/ha. Light interception did not significantly increase beyond 67% regardless of increase in canopy volume per hectare or the increase in orchard age. In these conventional orchards, maximum yields were reached approximately 16 t/ha when light interception was around 49% and as light interception increased above 49%, yields declined. In the high density orchard systems, light interception increased with canopy volume, with high density systems intercepting more available light than medium and low density orchard systems. In 5 year old high-density, espalier trained trees with approximately 5000 m3/ha canopy volume, yields reached close to 50 t/ha at 40% light interception. Trees trained as single leader or espalier had more evenly distributed light in the inner canopy compared to conventionally closed vase trained trees. in current commercial orchards, increasing the canopy volume and light interception above the maximums noted above had no benefit to increase economic returns whereas in high density mango orchards with alternate tree training, yields increases more than four times early in the orchards development. Further productivity gains are likely as the experimental high density have yet to fill their allocated space in the orchard.

Vigour reduction in many tree crops is an essential element of highly productive, high density systems that is often achieved through rootstocks. Mangoes are large vigorous tropical trees that are traditionally grown at low density as suitable vigour reducing technologies are not commercially available. The aim of this work was to identify rootstock cultivars for mango that reduced scion vigour while maintaining or boosting yields. Ninety rootstocks are being evaluated for their ability to reduce vigour in two Australian mango scion cultivars from the National Mango Breeding Program (NMBP); ‘NMBP-1243’ and ‘NMBP-4069’. The evaluated rootstocks were sourced from a wide genetic range within Mangifera indica and related Mangifera spp. from the Australian National Mango Gene Bank and the Australian Mango Breeding Program. Rootstock-scion combinations were field planted at Walkamin, Queensland, Australia over three years, from May 2014 in randomised incomplete blocks. Tree height, canopy depth, canopy length (along the row), canopy width (across the row), rootstock trunk diameter (10 cm above the ground and 10 cm below the graft point) and scion diameter (10 cm above the graft) were measured every six months as indicators of tree growth and vigour. This is a report on the progress of 29 rootstocks from the May 2014 planting. There was a significant (p<0.05) effect of rootstock on tree height, canopy size and trunk diameter at 24 months. Seven rootstocks were consistently among the 13 lowest vigour trees for tree height, canopy length, canopy width and scion/rootstock trunk diameters, when ranked from lowest to highest vigour. These rootstocks show promise for high-density mango orchard systems. Evaluation is ongoing and will evaluate effect of rootstock on scion flowering and cropping.

This project has identified a relationship between foliar δ13C and foliar δ18O suggesting that Avocado water use efficiency (WUE) (as indicated by foliar δ13C), grown in nursey conditions, varies with transpiration. This result has not been previously been identified in avocado in the literature and may be a useful to explore in avocado and other horticultural crops breeding programs in Australia.  We have also shown that management influences long-term water use efficiency where the avocado variety Shepard had very different foliar δ13C with the two different irrigation treatments (Sprinkler and drip irrigation). Although not conclusive, the foliar δ13C was higher under the sprinkler irrigation which was likely in response to increased growth or water stress at this site.  The relationship between foliar N, chlorophyll and the chlorophyll SPAD meter indicates that the N could be monitored with the SPAD meter in avocado orchards. This may be used to supplement dry matter analysis and help improve N use efficiency in North Queensland catchments draining into the Great Barrier Reef, increasing sustainable horticultural outcomes.  The relationship between foliar N and δ13C (less negative δ13C), indicates that the north facing canopy had a greater variation in N and WUE compared to the south facing and the shoot function site.  There was variation in nitrogen (N) within the canopies particularly when the shoot function and canopy sampling position samples were pooled, indicating a potential response in the canopy to N cycling at the different sites. This is suggested as a response to Nitrogen management at the different sites. Further work would be required to better understand the mechanisms behind this relationship.  Finally, the correlation between δ15N and δ13C at the pooled sites indicates how increasing foliar δ15N was related to increasing WUE (less negative δ13C), showing potentially how N availability across sampled orchards influences growth. Again furthe

Full Text, Thesis (PhD Doctorate), Doctor of Philosophy (PhD), Griffith School of Environment, Science, Environment, Engineering and Technology, Post-planting silviculture in the exotic pine plantations of Southeast Queensland focuses on fertilisation and weed control at early plantation establishment. Early establishment silviculture in pine plantations aims to reduce the competition for light, nutrients and water in the short term, while maximising resource conversion for growth in the long term. However, silviculture can be applied in a systematic way without necessarily considering the limitations to maximum tree growth at each site. Accordingly, silvicultural treatments could be applied in a site specific manner to better reflect the limitations to tree growth at each site (e.g. nutrition and/or water), or for maximum effectiveness based on seasonal limitations (e.g. weed competition, water availability). This research aimed to investigate the effects of early establishment weed control and fertilisation practices on soil carbon (C) and nitrogen (N) cycling (as an indicator of soil fertility), tree nutrition (particularly N nutrition), growth and eco-physiological responses, in the F1 hybrid exotic pine plantations (Pinus elliottii Engelm var. elliottii x Pinus caribaea Morelet var. hondurensis (Sènècl.) W.H.G. Barett & Golfari.) in the subtropics of Southeast Queensland.