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Australian Macadamia Society Podcast: Episode 3

In Episode 3 of the AMS podcast, Leoni Kojetin AMS Industry Development Manager, speaks with Australia’s Dr Chris Searle (from MacAvo Consulting) and South Africa’s Mark Penter (from the Agricultural Research Council) about the use of ethrel in macadamias.

Mark and Chris discuss the research in relation to ethrel use in stressed plants and the guidelines in Australia for its use. What are the environmental considerations before using ethrel? What if it is used too late? What about the use of adjuvants? Or the use of ethrel in trees with sticktights? Or in conjunction with tree shakers? Mark and Chris answer these questions and more in this podcast. 

EPISODE 3: The use of ethrel in macadamias

Leoni Kojetin (Australian Macadamia Society). Today I'm with Dr Chris Searle from Australia and Mark Penter from the Agricultural Research Council in South Africa talking about using Ethrel. Mark, what are some of the things that you've been researching in South Africa?

Mark Penter (Agricultural Research Council, South Africa). Recently, our primary work has been on cultivar evaluations and kernel quality, but in the past a big part of our work has been on the use of Ethrel.

Leoni. When you say Ethrel, what is the active ingredient we're talking about?

Mark. In Ethrel or ethephon, as we call it, the active ingredient is 2-Chloroethylphosphonic acid. 

Dr Chris Searle (MacAvo Consulting). What does that break down to in the plant, Mark? 

Mark. The 2-Chloroethylphosphonic acid is not an active ingredient, although it's labelled as such. In the plant, however, it is metabolised to ethylene, and ethylene is the molecule that gives the effect that we're looking for.

Leoni. But ethylene is naturally produced in the plant?

Mark. Yes, it is, particularly under stress conditions, which is why we're talking today about Ethrel use in stressed plants. Naturally, under stress conditions, ethylene is already there, so by adding ethylene, we're not really sure how the plants are going to respond.

Chris. While it's created under stress conditions, it's actually something that we can use to be beneficial and actually help us to drop nuts, isn't it?

Mark. That's correct. In this instance, we were using it to abscise nuts so they fall to the ground in a uniform manner and can be harvested readily, but there are a number of other things that ethylene achieves. In some plants it will induce flowering, while in others it will inhibit flowering. It can be used as a defoliant. So, there's a wide range of applications for ethylene in the agricultural sector.

Leoni. So, it's a natural plant hormone that moves different processes towards ageing. Is that a good way to describe it?

Mark. Or towards senescence, yes.

Leoni. Chris, what are some of the guidelines that we have in Australia for Ethrel use?

Chris. While it is registered in Australia for some macadamia varieties, there are recommendations for different levels to be used for different varieties. And it is a product that must be used with care, and only at certain times of the year. If you put too much in the tank and have a rate that is too high for some varieties, you can end up with severe defoliation. To emphasise, it's a product that must be used carefully and within the guidelines, otherwise you can really have some adverse side effects, such as having excessive leaf drop. There are ways you can manage that, but it's a product that you must be careful with and seek guidance before you use it.

Leoni. Mark, what are some of the environmental considerations with using Ethrel, in South Africa?

Mark. You have got to be a little bit careful about it. For instance, one of our biggest complaints is Ethrel not working as it's intended, in particular, the nuts don't drop. And that is particularly for cooler periods. We advise growers to keep an eye on the mid-range weather forecast, and if there's any potential for a cold front developing, not to apply it, as it will not be effective. Conversely, very hot conditions can give you a much greater effect than expected and you can get excessive leaf drop. So those are two of the more important ones.

Leoni. Chris, we simply have a different rule of thumb in Australia. What is that?

Chris. Well, the old rule used to be, "Use Ethrel up until Anzac Day". And that's because around that time, under our traditional weather conditions, that was when we got the changing seasons. It went from being warm in summer to having cloudy, overcast nights, and warmer nights, to having nights that were clear and with lower night temperatures. And we had that change in season where the humidity would drop out.

Under those conditions, once soil temperatures start to get cooler, Ethrel becomes far less effective. And, as Mark said, you'd see it come through with a cold front, it would work well before the cold front, while the moment the cold front hit, it would almost stop working overnight. And we believe in Australia, this is to do with soil temperature. And that's really quite critical. 

In the last few years, we have seen people use it later into the season because our autumns and warm temperatures have gone on longer, but traditionally it always used to be used up until Anzac Day. 

Mark. We won't really have a cut-off or calendar date as we don't have that sudden change in weather, rather it gets gradually cooler towards winter. So, for us, the cut-off time is closely related to when the trees are getting ready to flower. You don't want to be spraying when the flower buds start to swell. That's too late, but anytime up until then, our growers do tend to use it.

Leoni. And what have you seen in cases when growers have used it to too late, when there's already been bud swell?

Mark. If bud initiation has occurred, which is something you can't see by the way, but if initiation has occurred and the trees are approaching bud swell, but bit hasn't occurred yet, we will generally get a couple of weeks delay in the actual flowering. Where bud swell has occurred, it tends to damage and burn those flowers.

Leoni. Is there a rule of thumb that growers use in terms of maturity?

Mark. The maturity standard for macadamia is 72% oil, which generally equates to dry kernel floating on water. We advise growers who want to apply early in the season to do a maturity test and to have at least 95% of their kernel floating on water. That's the minimum standard, but often the nuts are not sensitised to ethylene at that stage. You can spray and you'll get a very small crop. We generally advise waiting a bit later in the season, which for us is around late April to May. So, your cut-off date is often our starting date.

Chris. We certainly have seen that later application in Australia, with people using it later in the season than they did traditionally. It was always one of those things that growers were quite sensitive about using, because everyone had heard the stories about adverse effects. And we do have some cultivars such as A16, which are very, very insensitive to Ethrel, and then we have other cultivars such as A14, which are incredibly sensitive to it. So, you can apply it to two cultivars on the same day, and get a great response, or nothing at all.

Mark. Having said that, we really only apply to one cultivar and that's Beaumont. Although our growers do use it on other cultivars, they're advised to do so with extreme care because we haven't tested Ethrel on many of our cultivators, and we really don't know what that response will be.

Leoni. But Beaumont is your most prolific cultivar in South Africa?

Mark. Yes. It used to be around 65% of the trees in the ground. Today it's probably around 45%.

Leoni. And do growers get some benefit from the defoliation in Beaumont?

Mark. Yes. Traditionally, Beaumonts not treated with Ethrel are incredibly dense in the centre and tend to hang on to their old leaves for a long period. We found in the early years of our work, that an Ethrel application will drop those older leaves, open up the canopy and create more light inside, and probably assist with flowering and light interception. And for some of our trials, we noticed not as significant, but definitely a trend towards slightly higher yields in Ethrel-treated trees.

Leoni. Mark, are there some considerations or adjuvants that are used in South Africa?

Mark. We generally recommend, at the very minimum, a wetter. If the pH of the tank mix is okay, then just a wetter, otherwise a buffer towards a pH of seven.

Chris. It's the same here (In Australia). We would recommend a wetter, and we have seen probably slightly better impact of using Ethrel when people have used a super wetter such as Duet, which is a silicon wetter that just ensures that you get much more even coverage over your nuts when you apply it. You must be careful with some of those super wetters, but anecdotally, and we really don't have any science behind it, they do seem to give an improved efficacy of using Ethrel.

Leoni. So, Chris, I know that Professor Stephen Trueman has done some research. What has he found with the long-term impacts of Ethrel?

Chris. I think it was part of his PhD program, quite a while ago, where he found essentially the same as what Mark described. It seemed that for A16 and a couple of other of cultivars, using Ethrel, dropped the nuts out of the tree. But, in subsequent years, yield went up. 

It comes back to the likelihood that removing the older leaves allowed better light distribution within the canopy and allowed the light to get through the tree. You've got a more open tree, and possibly less disease. Just ensuring that those old leaves are out of the way means that there is more light, with the result that they saw the yield go up without detrimental effects. However, one must stress that it was using Ethrel within the right range for those cultivators, because there's a benefit from removing some leaves, but there's also a disadvantage if you remove too many.

Leoni. Mark, have you found that when you apply it to stressed trees that already have quite a high natural ethylene?

Mark. Yes. We had one year, a very dry year, 2003, where we applied to both irrigated and non-irrigated orchards. And definitely in the non-irrigated orchards we saw a decline in yield the following year, in contrast with the irrigated orchard in that instance, where there was no effect from the ethylene.

Chris. It's the same here, where the recommendation is, do not apply to stressed trees, particularly trees that you feel are impacted by Phytophthora. That will only drive them further into decline. And that's certainly a recommendation. And they're also largely somewhat insensitive to it when you do apply it.

Mark. I should have said that our recommendation is not to apply to stressed trees, if you can recognise that your trees are under stress. Phytophthora is quite easy to tell, but there are other environmental stresses which are maybe not so easy to determine by eye.

Leoni. What about using Ethrel on trees that naturally tend towards having stick tights?

Mark. The stick tight is a different story. It's not going to respond to the ethylene, so you can apply it, but you're not going to get much benefit, unless there are other nuts in the tree that have not reached a stick tight situation.

Leoni. Chris, do you want to explain what we mean by stick tights?

Chris. Stick tights are those brown nuts, where the husk is still on the tree, slightly open, with the nut hanging inside. The husk, which is brown, is physiologically dead, will not react to the Ethrel. So, we just have to usually get those out with tree shakers to remove the stick tights, to help break the disease cycle.

Leoni. Does it help if you spray the Ethrel before those stick tights form?

Chris. It would be, but we do see some cultivars that are more prone to stick tights. Again, a lot of stick tights are probably the result of stress within the tree. So, trees that have a lot of stick tights are often stressed trees, and because of this you wouldn't actually go and apply the Ethrel anyway. And if you wanted to go down this road, you would need to remove that stress before you apply it.

Leoni. Mark, is stress a difficult thing to gauge in macadamia?

Mark. Yes, and it depends on the sort of stress you're talking about. As I said earlier, if they're under stress of, say, Phytophthora, it's quite easy to see, but environmental stress is a different story, particularly drought stress. They don't wilt readily, and they don't undergo chlorosis in the early parts of drought stress, so it is difficult to tell when your trees are under stress. 

There are some studies going on in South Africa where we see the potential to measure drought stress using stem xylem potential, which is a measurement that almond growers in South Africa currently use, but it's not widely used by macadamia growers. That's something we probably have to adopt and train growers to use. But before that, the studies that are in progress now need to give us some guidelines. So, there's no easy, simple way for a grower to tell that there's a stress situation.

Leoni. And Chris, a lot of growers have used Ethrel in conjunction with tree shakers. Can you describe some of that use?

Chris. In the last two or three years, we've seen a trend towards using the tree shakers, particularly in Bundaberg. What the Ethrel does is it weakens the abscission zone where the nut joins to the stem so that when the tree shaker shakes the tree, the nuts are much more likely to come off. It's like preconditioning the nut, before the tree shaker is used. The tree shakers are a lot more effective when you apply Ethrel before using them as it preconditions the nuts, so they are ready to fall off. That little bit of an extra shake is what gets them off.

Leoni. Are there some varieties that you absolutely wouldn't use Ethrel on?

Chris. I would be quite a wary of using it on A14. You must make sure that the rate is down quite low. I've seen some lovely defoliation when growers have used the wrong rates. And then conversely, if you've got A16, where I've seen people use incredibly high rates, and it just is completely ineffective. Again, it changes with season, but it's also a tool that you need to be experienced in using but build up to it carefully.

Also, if you're a grower in Australia, just be aware that if you do apply it, you're going to get a lot of nuts down in two weeks' time, which needs a lot of preplanning. You've got to look forward in your weather forecast to make sure they're not forecasting rain in two weeks, you need to have enough silo space, and you need to have the people ready to sort the nuts. 

I see quite often, people spray the whole orchard. And then the shed can't keep up with the amount of nuts that are on the ground, and then the harvester can't get round, there's some leaf that's also abscised, it lands on top of the nuts, and now they can't pick them up. And they compounded the whole situation, making it worse.

So, if you are going to use Ethrel, there are a number of things you need beforehand and you have to set yourself up to be aware of it. If you're going to do the orchard, spray one part this week, spray another part next week, and spray another part the week after, so you can actually cope with the nut fall when it comes down.

Leoni. And in South Africa, are there any cultivators that you don't use Ethrel on, Mark?

Mark. We particularly recommend that it's not used on 816. 

We've had some very large leaf drops that at the recommended rate, and the leaves that don't drop generally turn yellow. The trees recover slowly from that, but it's not a desirable situation. And there are a few other instances such as with cultivars like 791 and 788, where Ethrel will actually cause an abscission of young leaves and new leaf flushes. It's critical that growers make a note of what condition their trees are in before they apply it. You don't want to target trees that have the young flush, particularly those cultivars.

Leoni. Thank you so much for your time and your expertise. It's been good chatting to you.