Acotanc: Ora Blanca and Other Citrus in the Tropics

Ora Blanca and Other Citrus in the Tropics

Author: Bruce Toohill
Phone: +61 8-89722590 Fax: +61 8-89722057

Citrus originated in tropical regions, but has mainly been grown in temperate areas of Australia. Now citrus is being grown in several locations in the dry tropics, and new, improved varieties of grapefruit are being developed.

What I am presenting is a brief summary of what is happening in citrus in tropical parts of Australia, most particularly in the northwest, the Kimberleys of WA and the top end of the Northern Territory. Peter Johnson, based at Kununurra is basically the presenter of this data. I left there ten years ago. We both worked for the Agriculture Department in those days. I moved over the border to grow mangoes. I take an interest and he has kindly put my name on his paper.

Citrus, of course, as you well know is fairly well established in temperate Australia. Interestingly enough, citrus comes from the tropical world. A number of us always felt that at least some of the citrus varieties in commercial use are in the wrong part of the country. That doesn't apply to oranges, not necessarily to lemons, but certainly to things like grapefruit, limes, pummelos, and so on. I am sure those of you who come from southern Australia have, at least in the past, eaten very sour, white-fleshed grapefruits. As a result, you know they are not very popular any more. The production of grapefruit in Australia is declining simply because consumption is declining. When I say here it is a new industry to Australia, I am talking specifically about the tropics of the country. The ones in which we are taking an interest, these here in the first line.... Mandarin varieties is already a well established and expanding industry in southeast Queensland. But southeast Queensland is not exactly tropical. It is a rather cool, subtropical environment, so it doesn't really reflect the areas I am going to talk about.

There really is still little known about citrus performance in this part of the world. Years ago, when I first went to Darwin in about '76, the CSIRO had some very small plantings at Darwin and Kununurra, just inside the WA border on the Ord River. Those plants simply showed that citrus will grow there, and grow quite well. It wasn't a particularly good selection, but it was a useful indicator of what was possible. Nowadays, there is a new generation of trials going on.

One of the main points about northern Australia...Darwin is at a latitude of 12.5 degrees South, Katherine, where I live, is about 14 degrees, and Kununurra is about 15.5 degrees South. Broome is about 17.5 degrees, on the west coast. The Burdekin area of N. Queensland is about 20 degrees South. That borders the region I am particularly interested in, and that I know fairly well. So we have high heat units. Our friend, Professor Coronel, was talking about a mean temperature of about 28 degrees in the Philippines all year. Darwin is about 32. Whenever I have a day in the 20s, I feel a little chilly.

With these high heat units, we end up with very high sugar levels and very sweet fruit. Again, to what I usually describe as the European palate, a certain acid level is often preferred. But from my point of view, I much prefer it sweet. When I moved north, I started eating grapefruit again. Even the white-fleshed grapefruit have a far sweeter flavour up there than they do down in the Sunraysia district. High sweetness, low acid, which gives you a very high sugar/acid ratio. At Gingin, which is about is about 80 km north of Perth, where there is a small industry of citrus and where there is a trial of these red-fleshed grapefruit, you have a sugar/acid ratio of not much more than 5. At Carnarvon where my learned friend and mentor from many years ago, John Burt, came from, which is just south of the Tropic of Capricorn, it is about a 50% increase, and in Kununurra it is about another 40% increase again.

In terms of is almost as if we were going to be trying to be presenting a new fruit to the market. You could compare it, perhaps, to the Kiwi fruit. Nobody had ever heard of Chinese Gooseberries, and they dreamed up the name Kiwi fruit. If you can achieve what they did with the Kiwi fruit, with a fruit that looks like a brown piece of excrement, to put it nicely, on the world scene, then the possibility is pretty good for a red-fleshed grapefruit that looks quite nice. (tape ends and is turned over)

...a step up, if you like, from the old ruby grapefruit, which generally has a poor intensity of red in the flesh. These new varieties, at least five, Ray, Rio, RioRed, RayRuby, StarRuby, Henderson and Flame, all have far more intense colour. Where those varieties have been planted down in Sunraysia, Mildura, northwest Victoria, where a lot of citrus is grown, the red comes out as almost a brownish colour. A brownish red, a fairly poor example of what can be achieved. In the north, the colour is really quite intense.

Another aspect of the warm climate is the fact that we end up with a green skin, because cold temperatures de-green the fruit, and we don't have much of that. That is not a problem because you can easily use a de-greening process using ethylene under temperature control. But erratic flowering habits present more of a problem. This is not just a characteristic of citrus, but characteristic of many of the fruits we are growing in that part of the world. We can have very warm, dry winters. In the 80s we had a series of hot, dry years in our part of the world. The 70s were cooler, wetter years and much colder. I remember driving down from Darwin to Kununurra in the middle of the dry season in May or June and absolutely freezing. At night in Darwin it would be 15 degrees and Katherine it might get down to 4 degrees, and Kununurra would be 5 or 6 degrees. In the 80s, we rarely saw temperatures below 10 degrees, and often not below 15, for 7 or 8 years in a row. These sorts of characteristics quite dramatically influence the flowering pattern, therefore, our productivity.

To talk specifically about grapefruit, and red grapefruit. These are the sorts of points that are motivating us to follow up these varieties. The fact that the southern areas don't grow good grapefruit, and that there is an indication of the decline of the industry. Whereas, production in the United States is going to be a lot more, but the interesting thing is that 19% of that industry, involving both domestic consumption and an export trade, particularly into southeast Asia and more recently into Europe. There is certainly scope for us to develop that approach, to produce that range of varieties and target those markets to our advantage. Because there is already a demand for the product. Domestically, we have to create the demand. There is really only a small production of Ruby Red from Carnarvon that has a fairly strong, although small, following in the Perth market. Very few of them find their way into the eastern states. So the Australian market as a whole has a fairly narrow appreciation of red-fleshed grapefruit. Most of it comes from California during their export season. Interestingly enough, the Californian industry is a very big industry, it maintains high quality standards, the fruits all look exactly the same when you open the carton. Some of the WA produce, the old Ruby variety, tended to have thicker skins, more variation in appearance, so that when the ship from the States was late, the Perth market gave pretty much the same price for Carnarvon produce as for US produce. When the ship arrived, there is the American price and there is the Australian price. So there is a demand, but a relatively small one. We are certainly looking to develop that demand.

One of the problems with citrus, in particular, because there has been a lot of research work done on it, is simply the combinations of scion and rootstock choice that you make for the environment in which you want to grow it. I can't stress enough the need to plant what you are interested in in the place where you want to grow it, rather than depend on information from elsewhere, entirely. A good example of that is at Katherine right now there is one fellow planted about 7000 lemons. His name is Leng, and one of his forebears gave the name to the Leng Late Navel Orange selection that came out of Sunraysia many years ago. So he has a family tradition in citrus growing. He chose to plant Lisbon lemon on rough lemon rootstock. The trees are only 4 years old, they are taller than this ceiling. The rootstocks are just chock-a-block with suckers, big thorns on them. He has an incredible difficulty now in dealing with that. He chose that combination. For my money, it is the wrong combination.

We are not sure, at the moment, what the combination should be. The Swingle rootstock seems a better choice of rootstock for the red-fleshed grapefruits. The Henderson and the Rio in these early trial results have also shown some fairly good results, but one year following another year may give quite different results, in these early years. It is premature to jump to any conclusions at the moment.

The Ora Blanca is a white grapefruit-pommelo hybrid. It achieves even higher sugar levels of 22/1 as a ratio, at Kununurra. That is an incredibly sweet fruit. If you tried pommelos, you will appreciate what I am getting at. They are a sweet, delicious fruit, but they are quite difficult, physically, to eat. It is not something you just eat like an orange or a mandarin. Again, that shows the difference between various environments on the outcomes of that particular measuring point. There is good scope to follow up that type of citrus, as well.

Just briefly, other possibilities include limes and lemons. We have a small Tahitian lime industry, especially in N. Queensland and also in Darwin. We have West Indian limes across the north. But the market for these is quite fickle. If you have too much production, the price drops to a very low level and there is no money, so there is no point in doing it. There is definitely a market for summer lemons out of Australia, for the Australian market as well as export. We currently import American lemons at that time, and from some other countries as well. The bulk of Australian lemon production is in the autumn to spring period. January, February, March is a particular time when we can produce our own lemons.

One problem we have in our neck of the woods is the labour and the climate that occurs at that time of the year. It will be the same for grapefruit, although grapefruit can push through to March and April, when these issues aren't so important. It is very hot and wet. Our labour force leaves, goes off on holidays, Christmas, back to wherever they came from. So we have a problem with labour. Secondly, we have quite regular and reliable rainfall in those months of January, February and early March, which can cause water damage to the quality of the fruit, and also marks the fruit on the tree before harvest. That in itself is a big problem that will be part of the success or lack of it as we develop this industry in this part of the world.

Because of the varying climate that we have, the only thing stable about the dry tropics of the north of Australia is its unreliability. The problem is doing a switch to reproduction away from vegetative growth. Very often, there will be a vegetative flush with minimal or no flowering, under the wrong conditions. If you can come back a bit, into the cooler part of the dry season, basically July-August and use drought to drought the plant and then hit it hard with irrigation and a certain amount of nutrient, you can actually kick off flowering whilst the conditions are still coolish. Then you can bring forward the harvest time. That is a method of manipulation that is being looked at. To the extent that it has been done so far, again it has certain unreliable aspects to it as well. But I think, in time, as more happens and more plants are planted out and everyone is trying it, it will come out into a usable methodology.

Most of the mandarin varieties really aren't heat-tolerant enough for our area. Emperor is one that will produce quite well, but it is not really a large, commercial variety. There is an on-going introduction of genetic material, particularly of the mandarin. The quarantine laws in Australia relating to mandarin were relaxed in the last ten years, to allow the introduction of propagules, not seeds. Before that it was only seeds, and they had to be grown out in quarantine for several years to look for viruses and all the rest of it. They are still importantly interested in virus status, and so on, but you can now actually bring in plant material. That allows us, hopefully, to improve our range of material that might suit our part of the world.

Those are the basic summation of areas we need to be looking at. This will go on for some years. I would suggest it will be at least another five years before we can be reasonably sure of variety/rootstock combinations and of what combinations suit what soil types. The growing conditions are an interesting area we will be looking at. We have a long dry season, from about April until late November, December, so we have to irrigate. With citrus, of course, shallow-rooted, they require year-round irrigation. We can use irrigation management to help us make the trees flower when we want them to. That is a very useful ability to be able to time the crop according to when you want to get it off. That is something that is going on with other agronomic practices to give us the outcomes we want.

There are some native citrus in Australia, the Microcitrus genus that occurs across northern Australia. There is the Eremocitrus group that occurs in the arid interior, particularly if you drive from Mildura to Broken Hill, for example, you will go through stands of Eremocitrus, which has a fruit about yay big, mostly seed. It certainly has the smell of citrus when you break it open. I wouldn't particularly recommend eating it. The CSIRO at Mildura in those days collected the genetic material and has it in an arboretum. One of the native citrus is known as Fingerlime. That is the only one I know of that has any sort of commercial use. Certainly, I think it is important to keep that genetic material available so that in time to come it may become useful.