Marcelo H. Amaral, Cameron McConchie, Geoffrey Dickinson and Kerry B. Walsh*

Based on a multi-region, cultivar and season, an improved method of calculating heat units (HU) or Growing Degree Days is proposed. This method includes a penalty for high temperatures above 32°C as well as low temperatures below 12°C.

The HUs required to progress from the asparagus stage of flowering to harvest maturity, as defined by cultivar specific flesh colour and dry matter content, was documented as 2185, 1728, and 1740 HU for the cultivars Keitt, Calypso and Honey Gold, respectively.

A national set of colour cards suitable for on-tree maturity assessment of all cultivars is also proposed. A flesh colour harvest maturity card specification of 9 was proposed for the cultivar Honey Gold and 13 for the cultivar Keitt. Similar work is proposed for other cultivars.

The aim of the work

A forward estimate of harvest date is required several months before harvest for organisation of harvest resourcing, including for hire of harvest labour, and for market planning. A forecast of harvest date from a given date of flowering can be done using heat units (HU). HU are estimated using a historical record of daily minimum and maximum temperatures, updated with current season temperature data as the season progresses.

This study was undertaken to improve harvest maturity prediction, with the aim of developing a method for assessing heat unit requirements for fruit maturation, and an all-cultivar colour card set for determination of harvest maturity by flesh colour.

Why do this work?

In past research to establish HU targets there was no agreement on the base threshold temperature nor the starting flower development stage. Moreover, the assessed flower development stage was an ‘eyeball’ average across the orchard and harvest maturity was often taken as the date of commercial harvest.

The use of whole tree or orchard assessments of flowering and commercial harvests is convenient, but a more accurate estimate of the heat unit target can be achieved by tracking of individual fruit from flowering to harvest maturity.

Several groups have produced colour cards to assist growers in judging flesh colour. The Calypso colour cards were a product of the work of Whiley and Hofman. The NT Farmers Association has produced a ‘business card’ colour swatch for Kensington Pride, and the US National Mango Board has released a set of colour swatches for Keitt and other Floridian cultivars. However, colour differences exist between printing runs, and no ‘all cultivar’ colour swatch series exists.

Calculating heat units

The heat unit calculation involves summation across days of the average of daily minimum and maximum temperature minus a ‘minimum base temperature’ (Tb), which assumes fruit development halts below this temperature, and can involve penalty for temperatures above a ‘maximum base temperature’ (TB), which assumes fruit development slows at high temperatures.

An alternative calculation uses a function that penalises high temperatures as well as low temperatures. This method produced later maturity date estimations in hotter climates, e.g., in Darwin, but had no impact in cooler climates, e.g., Rockhampton.

We also investigated the optimal value of the minimum and maximum base temperature. This work requires data from multiple sites. From this work, we recommend use of 12o and 32oC as the minimum and maximum base temperatures, respectively.

Using the method detailed below, we recommend the heat unit values shown in Table 1. Note that different flowering stages can be used as a start point. This work should be confirmed and extended to other cultivars.

Colour cards

We propose a new maturity colour card series to service all cultivars (Table 2), with use of a colour quality control on printing.

Recommended method for HU estimation

Temperatures can vary across a farm and in context of tree canopies, shed walls, etc. We found that placing sensors inside dense tree canopies resulted in lower daily maximum temperatures such that, in hot climates, HU calculated without use of a high temperature penalty from the inside-canopy sensor was similar to HU calculated with a high temperature penalty, from the outside-canopy sensor. We recommend that a sensor for temperature measurement should be mounted in a shade screen, 1.2m above mulched or grassed ground (not bare earth) and located close to the orchard.

As the HU calculation using an upper base temperature somewhat complicated, an online calculator (http://fruitronics.com/) is recommended. This resource accesses a remotely logged temperature logger in each growing region or farm specific sensors, to calculate cumulative HU for the season, with forecast onwards from the current date based on 10-year historical average temperatures.

At a given site, tag 10 panicles at asparagus stage on each of 20 trees, with panicles selected from around the tree canopy. Panicles should be tagged at a reproductive stage that has a short duration, commonly asparagus stage (Figure 1). If time allows, visit the site as flowers mature and record the time when ‘elongation’ and ‘two thirds of flowers open’ stages are reached. The 200 panicles typically result in >20 fruit, but in some cases fewer fruit will retained to harvest maturity.

Fruit from these panicles can be destructively harvested in the weeks before and after the date of anticipated harvest, as estimated from existing HU recommendations. Fruit flesh colour (card value) and NIR or oven-DM are then used to establish the date of optimal harvest maturity.

Fruit should be cut lengthwise about 1cm from the stone (Fig. 2). Flesh colour should be assessed indoors under good light.

Recommended use of HU

Visit each orchard block weekly and record the week at which a significant number of panicles, e.g., more than 20% of terminals, are at elongation phase. This defines the flowering events of the orchard. Enter these dates into the on-line calculator along with the target HU for the cultivar.

The time between harvest maturity dates will be shorter than between flowering events as fruit of later flowering event ‘catch up’ as weather warms. You can then decide on how many actual harvests are warranted for the block.

See online article: ‘Growing Degree Day Targets for Fruit Development of Australian Mango Cultivars’ for more information.

*AUTHORS: Marcelo H. Amaral1 , Cameron McConchie1, Geoffrey Dickinson2 and Kerry B. Walsh1, are Queensland based researchers from the following institutions:

1 Institute for Future Farming Systems, Central Queensland University, Rockhampton

2 Department of Agriculture and Fisheries, Mareeba. Contact: geoff.dickinson@daf.qld.gov.au

Article supplied by Kerry Walsh, a Professor at CQUniversity Rockhampton who has been involved in technology development for the mango industry over several decades. Contact: k.walsh@cqu.edu.au