Mangoes - Talk by Terry Campbell, DPI Bundaberg

I am a little bit out of my depth tonight. Firstly I am not used to speaking to such a fantastic roll up of people so thank you all for coming out on a stormy night like this to talk mangoes.  I normally now work in the post harvest system for mangoes and why they go green and not ripen properly sometimes, but I did spend 17 years in Mareeba, the home of tropical mango production in Queensland.  I have spent 6 years in Bundaberg up until now, which is still home to some quite large mango plantations so I have a pretty big interest in mangoes.

Our contribution to being a member sometimes is to get up and speak about some topics that might be interesting to people.  So I looked at a few of our training packages and tried to pull out all the things that were interesting at this time of the year in mangoes. I find this the most interesting part of your newsletter. If there is enough discussion we will all learn something about mangoes.

I recognize that mango growing in this area is one of the greater challenges in life. It is all because of what happens during flowering and fruit set.  I think you will realize why it is so difficult to grow mangoes in SE Queensland after you hear some parts of my talk because the flowering is difficult, pollination and fruit set is quite difficult and maintaining disease and insect control at times in this environment is quite difficult - not impossible, so don’t give up!

We will go back to a little bit of science to explain that mangoes do follow that yearly pattern of vegetative growth after harvest, where we get 1 or 2 flushes. We also get a bit of a root flush. Then we move into some sort of vegetative dormancy and that might happen from about May onwards when we get set up for flowering.  It is the environmental conditions that happen in winter - June and July that really determines what sort of level of flowering you get. After flowering it is the environmental conditions that we get in August-September-October that determines what sort of crop we get.

I reckon there are 5 IFs in mangoes:

IF they flower;
IF they set;
IF we get them through to maturity;
IF they ripen;
IF there is any money around on the market.
If those 5 IFs fall into place, it is a pretty profitable crop.

That is the basic phenological cycle that mangoes go through. As I hear people talk about manipulating flowering, they are doing it by trying to manipulate that phenological cycle: when the rest phase is and when the growth phase is. Here are a couple of facts:  For mangoes to flower they have to have cold weather.
There is a way around it but it is quite difficult to manage but the traditional one is that you need cold weather to come in. 10 - 15°C to come in at the very early stage of flower development as long as those terminals are at least 7 weeks old. So we need dormant terminals that have built up carbohydrates, some cold and then action to happen, which is what induces flowering.

We can manipulate this in a couple of ways. We can manipulate dormancy like they do in the Philippines where they use Cultar® to actually put the tree into a false dormancy and then break dormancy with potassium or calcium nitrate sprays.

Sheryl:  Can you go back a bit and discuss about the terminals being 7 weeks old?
Terry:  That is the suggestion that the flush must be a hardened flush and building up carbohydrates otherwise it just doesn’t flower.  We know young red flushes can flower and I have occasionally seen this in Mareeba, but a much better flowering occurs on buds dormant since end of May. We then need the cold as a trigger but not too cold or we get a male flowering.

Of course during fruit set it can also be too cold and if you start getting night-time temperatures below 13° during flower set you get quite poor set in Kensington Pride.  13°C is the benchmark figure but we know it is a little lower for R2E2. Calypso is certainly lower but for Kensington Pride we have a figure of 13°C that has worked out in the growth chambers.  In Bundaberg up to 20th Sept we normally get overnight temperatures of 10, 11 or 12°C yet we can still set fruit, so when we get warm days, I think we get a bit of an offset from that.  So now you can see that to get mangoes to work in an environment can be a bit tricky.  You need cold weather to get initiation but not too cold. Then it has to warm up during flowering to actually get fruit set.

In some cultivars we can prune off early flowers to get re-flowering at a better time. In Nam Doc Mai which gets really bad powdery mildew, all the flowers can fall off but they will flower again.  Some varieties are very good at re-flowering.  Keitt mangoes are very good at re-flowering. Most of the Florida varieties are ok but Kensington Pride is pretty lousy. Ian Bally in Mareeba may have a list of cultivars which re-flower after flower pruning.

The other thing about flowering is the use of potassium nitrate to induce flowering. It only works on some varieties. It does not work at all on Kensington Pride. It works marginally on R2E2 but it works quite well on Carabao.  The Filipinos and Thais use it twice to induce flowering each year.  You have to have dormancy but calcium nitrate works just as well and is a little bit cheaper.

Watering 

There is a lot of conflicting information about how you water mangoes during flowering particularly about putting the water off until fruit set. The water usage is quite low through winter but they do need to be kept actively “alive”, and need to be watered to push flowering at the right time. Start early September to get those buds going.

Fertiliser

It is very essential to fertilize mangoes to get regular cropping. You need to get new flushes every year to get flowering to get fruit set.  Of course it is essential not to over-fertilize.  As a guide I like leaves to 1.2% nitrogen coming into flowering. 1.2% nitrogen is certainly not dark green - you would have to say light green and when you go to the sunny side of the tree you think you have under fertilized it. It looks a little bit on the pale side. That is about 1.2%.

We know boron is important because of fruit set, and to get pollen tube growth you have to bring them [the trees] it at 50ppm leaf boron. Boron you can overdo it. Mangoes are very intolerant of too much boron but you have to have it.  I am one of the people that advocates putting it on the ground because it is very important for sap flow and calcium nutrition in the tree. I notice your recommendations are giving boron sprays right through until October. Our recommendation is 0.9% soluble spray Solubor® at  stage B because that will give us enough boron for what we do, but we should actually have it on the ground.  Stage B is when the buds will be about 1-2cm long ie., early bud break. 1% is 10ml per litre.  Some varieties like R2E2 need a lot of boron and if you don’t have enough boron you get the bent flower tip and you end up with some internal issues.

Sheryl:  So you are saying spray the flowers with boron as well as putting a bit of boron on the ground.
Terry:  Yes, in very low doses on the soil. 1gm per square metre.  Mix with water and spray the ground. On our sandy soils we don’t exceed 10kg/hectacre treated area that is our top rate. Even at that rate in our R2E2 mangoes we get toxicity at times when the soil is not so deep.
Boron deficiency symptoms are typically holes in the leaves and splits and trunk weeping.
Toxicity is indicated by a very irregular burn on the margin of older leaves.

Calcium  If you don’t have enough calcium in Keitt mangoes you end up with internal troubles – what’s called Keitt Disorder. To overcome the problem we try to improve fruit load, lower nitrogen and increase calcium supply.  You will see occasionally that 1 or 2 kg gypsum per tree is recommended and that is the ball-park, any more than that would saturate the soil with calcium. If your pH is below 5.5 use lime, and above use gypsum. 

Sheryl:  What size tree would that be in?
Terry:  Probably a 3 metre tree.
Sheryl:  You mentioned to stop jelly seed we need the calcium. Getting calcium into the tree is a bit difficult.
George:  With calcium nitrate spray, does that actually burn the leaves?
Terry:  We have to be very careful of the rates we use. I think 1% is fine but higher than that we start getting little bits of tip burn and if it is fruit that is getting a little bit of burn then that ruins the fruit.  Depends on the pH, we mainly suggest using gypsum.
Sheryl:  So how much per square metre?
Terry:  The golden rule has always been about 10 tonne per treated hectare.
Sheryl:  I think Peter says a closed handful.
Terry:  That sounds a lot more sensible.  I am saying that the commercial trees are using 2 kg on a 3m tree per year of gypsum.
Sheryl:  What periods would you put them on? What growth of the tree?
Terry: It is very important to go for those major growth phases, it has to be on prior to flowering, again in November and then for the flush, but we do try to avoid the wet season particularly in the very sandy soils that we grow mangoes on you just can’t control and it all just goes through the system and you can’t control it.

Potassium and Magnesium They do like magnesium. When everything is set and we know we have a crop we can spend some money on the trees then we top up with potassium and magnesium but in a ratio of 2 to 1 potassium to magnesium. We tend not to put Potassium and Magnesium on until the fruit is about egg size. If you don’t have it you end up with a lot of internal troubles.

Diseases 

The other thing that has to happen at flowering time is the control of diseases. This is to protect flowers and leaves. If anthracnose gets on older leaves it remains as inoculum for later on.  Anthracnose is a problem in this area because we get rain at flowering and we have some inoculum in the tree and we get it spreading down to our flowers and young fruit and we get no fruit set.
With the way the disease spores infect the fruit we have to use a protectant fungicide like Mancozeb or a curative like Octave ®.

Sheryl:  Octave is quite expensive isn’t it?
Terry:  It is very expensive, but during flowering it is a best choice. If you are relying on protectants you need to ensure good coverage. So how do we control mango pests particularly without relying on inorganic chemicals? The first thing is never let a plant come onto your property that has diseases such as bacterial black spot.  (Always known by the angular pattern that is slightly raised and slightly greasy appearance of the leaf spot on the leaf).  It is such a difficult disease to try and cure - almost incurable on some varieties like Keitt.  The next trick that we have in controlling diseases is controlling the inoculum level in the tree. We have been doing some work of actually going through and removing all dead material in mango trees. Now this can cost about $10 or $12 on a commercial orchard to get in, get material out and sweep it out from under the tree and get it broken down to mulch.  But in some orchards it is the only way we have been able to control the disease, as whenever we get some dead material in a tree we end up with some quite nasty diseases like dendritic spot coming in on our fruit, so controlling the inoculum is really important.  What does all this mean to those of you who have only a few trees?  Well it means that before flowering, I like to have my trees really clean ie., when I go in and do that end of May prune to take the odd branch out and remove the dead wood.

George:  Does the sunlight getting into the tree help?
Terry:  It dries the tree out and lets a bit of wind go through it.  If you actually target the dead branches, stuff that has been hit by scale that is good.  The other thing is to have a uniform flush come away after, particularly to stay ahead of stem end rot. Do this in Feb.

Pruning 

Give your Mangoes a light prune straight after harvest to get all of those dead sticks and any branches that have gone a bit vigorous. I try to get some shape back into the tree and get my flush coming away and then I come back about May-June to take out some more dead wood and some vertical branches that are giving me too much height. Leaves older than 2 years are actually parasitic on the tree which means they no longer contribute to the carbohydrates of the tree so all pruning should be designed to remove those leaves so leave new leaves on and strip all the old leaves off. Do this in May.

Pests 

The other issue during flowering is the Flowering Caterpillar - not normally a problem but something we check for. We have some new chemicals which we use like Success [Yates® Success® Naturalyte® Insect Control] – active ingredient is Spinosad which is derived from naturally occurring beneficial soil bacteria and this is put on during early fruiting.  If you are growing trees in mid-suburbia where everyone has lots of ornamental trees you would know about fruit spotting bug.

George:  Especially in a garden with mixed fruit. They move from tree to tree.
Terry:  They are very fast. There has been some work done in the Burdekin and Tablelands and we are finding the peak of the fruit spotting bug in September-October and it is causing fruit drop, but it is such a hard pest to control that we are hoping that we can wear this one.  We haven’t found a pheromone for it that seems to work nor can we find lures to trap them and the only chemicals we have are harsh.

Our last pest that happens around flowering time is mango scale. Commercially it is our most severe pest. I think it is a problem because it causes dead material in the trees and just the very efficient nature of this pest.  The females lay 200 eggs at a time and those little crawlers move out over the next 24 hours.  They don’t move very far – they are all males. The females walk a bit further but the males stay close to their mother and in summer the life cycle can be only 40 days so it is a very efficient pest.

Member:  Organic control?
Terry:  With very good pruning and getting rid of the old material out of the tree and with some of these new parasites you will get away from this particular insect. We have imported a parasitic wasp and are doing trials before release.
Member:  Possum control?
Terry:  I have seen tin used successfully around the base to stop them climbing on the trees.
Sheryl:  In a previous newsletter, in Mackay they had an old fishing net which they put at the base of the tree about 90cm high from the ground and about 4 layers.  A possum likes something taut that he can hang onto so the net is loose and the possum does not like being off balance.

Varieties 

Maybe it is more my taste but I actually prefer cultivars like Nam Doc Mai because I can get it to re-flower very nicely into warmer weather like October. As you will see in the latest newsletter it is explained that mangoes are one of those ones that are monoembryonic or polyembryonic.  The monoembryonic ones we definitely graft because they do not come true to type from seed. The polyembryonic ones they do come true to type from seed but we also graft them because we have some data from Nth Qld looking at long-term yields and the actual grafted trees have always out yielded the non grafted trees even for things like Kensington Pride which are polyembryonic.

Dwarf Trees

I have had a little experience with the use of Sabre. I don’t know if Sabre itself is a dwarfing root stock or because on Sabre the tree is productive that it has kept the tree smaller.  The only big root stock trial that has gone on in Australia is the one in Katherine and in that one Sabre came out pretty well.  It has now been replicated by a grower up there who claims to have some very nice dwarfing root stocks but I have not actually seem them. 

Sheryl:  Is it a secret?
Terry:  No, they are being commercialised.

Article compiled by Barbara O’Connor and edited by Sheryl Backhouse.

Authored by: 
Terry Campbell
Sourced from: 
April – May 2009 Newsletter of the Sub-Tropical Fruit Club of Qld Inc.
Date sourced: 
Fri 11 Jan 2013