The Small Tree High Productivity Initiative - mango update for 2016

- Dr Ian Bally and Dr Paula Ibell, Department of Agriculture and Fisheries, Queensland

This is an update on the Department of Agriculture and Fisheries, Queensland’s (QDAF’s) ambitious Small Tree High Productivity Initiative, introduced to readers in the Winter 2015 edition of Mango Matters. The research aims to increase the productivity of several tree crops, including mango, by improving our understanding of major factors influencing productivity and development of high density and high productivity orchard systems.

Planting Systems Trial

A planting systems trial was set up in December 2013 to investigate how Keitt, Calypso® and NMBP-1243 respond to planting densities of 208, 450 and 1250 trees per hectare and with canopies trained conventionally or with a single leader tree, on and off trellis (photos 1, 2 and 3). The trees are now two and a half years old and canopies are beginning to take their designed shape.

Photo 1. Keitt planted at low density
with conventional training

Photo 2. Keitt planted at medium
density with single leader training

Photo 3. Keitt planted at high density and trainied as a single leader
on trellis

Light Interception

Regularly measuring light interception by the tree canopies under the different plant spacing and training regimes will identify which systems capture the most light at various growth stages and therefore, have the highest yield potential. Canopy volume was highest in Keitt followed by NMBP-1243 and Calypso® and the total light intercepted per hectare also followed this order. Early light interception measurements (photo 4) have shown more light was intercepted per hectare in higher density plantings than at lower densities.

Photo 4. Dr Paula Ibell measuring light interception in the low density, conventionally-pruned trees at Walkamin Research Station. This equipment, developed by Small Tree High Productivity Initiative researchers, uses an array of light meters to allow simultaneous measurement of light both directly under the canopy and
across the inter-row

The light intercepted by two-year-old high density orchards (9 to 15%) is greater than the light intercepted by medium and low density conventional orchards (<5%), (figure 1) and equivalent to that measured (in other trials) in seven-yearold trees in conventional Kensington Pride (KP) orchards. These early results confirm that higher density, trained trees can intercept and take advantage of more sunlight earlier in the orchard’s life than conventionally trained and planted trees.

Figure 1. Comparison of the light intercepted by the canopy of two-year-old trees in high, medium and low density plantings and trained as single leaders on trellis (SLT), single leaders (SL) or conventionally in the mango planting systems trial at Walkamin Research Station

Crop Load

This research aims to investigate the effects of flowering and crop load on yield, canopy growth and subsequent flowering to maximise yields and reduce irregular and biennial bearing.

Over two seasons this experiment has demonstrated that reducing the number of flowering panicles on Calypso® trees had minimal effect on productivity because the tree had the ability to regulate the numbers of fruit through natural fruit shedding. Trees with 5% flowering terminals held on to an average of 2.8 fruit per panicle, compared to trees with 100% flowering (no flowering panicles removed) that held on to 0.96 fruit per panicle. This compensation by higher fruit retention meant that total tree yields were similar across a range of flowering percentages and only dropped when 95% of flowering panicles were removed leaving 5% of terminals flowering (figure 2 and 3). In trees where low flowering percentages resulted in reduced numbers of fruit, the average fruit size increased and in one season, increased fruit total soluble sugars (°brix).

In another crop load experiment using KP, flowering percentages and subsequent fruit loads were maintained at 25%, 50% and 100% throughout fruit development. Tree yields were similar across crop load treatments, but the average fruit weight increased in trees with 25% and 50% crop loads.

The mango tree’s capacity to compensate for poor flowering by reducing fruit shedding throughout fruit development and increasing fruit size, demonstrates a capacity to self-regulate crop load. Factors other than the percentage of flowering terminals may have a greater influence on crop yields. This crop load research is pointing to resource limitations or available carbohydrate being a major factor in tree productivity. The next step is to investigate the effects of high density and training on crop load and regularity of bearing.

Figure 2. Calypso® crop load experiment. Trees with 100%, 60% and 5% flowering (top row)
and their corresponding yields at harvest (bottom row)

Figure 3. Effect of flowering density on yield (kg/tree) in Calypso®, yield efficiency falls only after more than 95% of the flowers have been removed (to the left of the red line)

Figure 3. Effect of flowering density on yield (kg/tree) in Calypso®, yield efficiency falls only after more than 95% of the flowers have been removed (to the left of the red line)

ROOTSTOCK

Ninety rootstocks selected across the genetic diversity of the Australian Mango Germplasm collection and the Mango Breeding Program are being tested with two scions (NMBP-1243 and NMBP-4069) for their ability to reduce scion vigour while maintaining heavy and regular crops. Low vigour trees are an integral part of high density, high productive tree cropping systems as they reduce tree size, enabling higher density plantings that do not rely on heavy annual pruning.

The oldest 30 rootstocks, planted two years ago, are beginning to influence scion canopy vigour with several rootstocks already inducing lower canopy height, width and stem diameter (figure 4 and photo 5). These results are from juvenile trees that have not yet cropped and may change when trees begin cropping. Further tree vigour and canopy architecture evaluations will continue as trees start cropping and as younger trial plantings mature.

Figure 4. Average tree height of scions NMBP-1243 and NMBP-4069 at two years old, grown on a range of rootstocks being evaluated on Walkamin Research Station

Figure 4. Average tree height of scions NMBP-1243 and NMBP-4069 at two years old, grown on a range of rootstocks being evaluated on Walkamin Research Station

Photo 5. PhD Student, Anahita Mizani, demonstrating the effect of rootstock on two-year-old trees (scion NMBP 4069), left rootstock = KP and right rootstock = WR100

These promising early results are encouraging and indicate that rootstocks may be a useful way of managing tree vigour in Australian mangoes.

In summary, research conducted as part of the Small Tree High Productivity Initiative has demonstrated that:

  • Increasing planting density can increase canopy volumes and light interception in developing orchards, providing a potential way of improving returns earlier in an orchard’s life than with conventional management strategies.
  • Mango has a large capacity to compensate for decreasing floral density through increasing fruit set per panicle and average fruit size (future work may use this knowledge to develop more efficient production methods).
  • In young trees, rootstocks are having an effect on reducing scion vigour. If further results confirm the lower vigour scions are productive, this may provide rootstocks that significantly reduce pruning and harvesting costs for growers.

Acknowledgements

The Small Tree High Productivity Initiative is an initiative run by the Queensland Government. Major partners include QDAF, Queensland Alliance for Agriculture and Food Innovation (QAAFI) – an institute of the University of Queensland supported by the Queensland Government and the NSW Department of Primary Industries.

A key element of this initiative has been co-funded by Horticulture Innovation Australia Limited (Hort Innovation) using the across horticulture levy, voluntary contributions from QDAF and matching funds from the Australian Government through the Hort Innovation project Transforming Tropical/Subtropical Tree Crop Productivity. We are especially grateful to Hort Innovation and the various associated industries and horticultural businesses for their support of this initiative.

We would also like to acknowledge the mango growers of the Mareeba-Dimbulah region including Raymond Bin, Edward Balzarolo and David and Raymond Courtice for allowing us to undertake trial work in their orchards.