Citrus huanglongbing (HLB), or citrus greening disease, is one of the most destructive diseases of citrus worldwide. Since its discovery in Florida in 2005, citrus acreage in that state has significantly declined and will continue to do so unless a cure is found – and soon.
At the forefront of the fight to save the Florida citrus industry from this seemingly incurable disease is Dr Michael Rogers and his team from the Citrus Research and Education Centre (CREC) at the University of Florida. CREC is the oldest (it celebrates its 100th anniversary in November) and largest off-campus experiment station in the University of Florida’s Institute of Food and Agricultural Sciences. It is unique among research centres in that it focuses on one commodity – citrus.
Here Dr Michael Rogers, CREC’s Director, brings Fruit Juice Focus up to date on the background to HLB, the complexities of combatting the disease and how close they are to finding a cure.
Fruit Juice Focus (FJF): Could you give me some background on greening disease? How it is spread, what the symptoms are and what impact it has on the plants?
Dr Michael Rogers (MR): Citrus greening disease, also known by its Chinese name Huanglongbing or HLB as it is more generally referred to nowadays, is the worse disease worldwide in citrus production and for good reason. It is the most challenging disease of citrus that we have ever faced, or that anybody has ever faced. It is caused by a bacterium Candidatus Liberibacter and the form we have imported into Florida is Asiaticus. The reason for the Candidatus designation is because this bacterium has never been grown in culture before. The bacterium is the pathogen that causes the disease and the bacterium moves from plant to plant spread by the insect the Asian Citrus Psyllid – the scientific name being Diaphorina citri.
The psyllid will move from tree to tree feeding on the leaves and injecting the bacterium into the plants vascular system into what they call the phloem.
In the same way that humans have arteries and veins, so do plants. These are the phloem and xylem where the phloem moves food and nutrients such as sugar and amino acids from leaves to storage organs and the growing parts of the plant, and the xylem transports water and minerals from roots to aerial parts of the plant. The bacterium gets into the phloem in the plant and eventually it moves down to the root system and will kill off a lot of the root system before you even see any symptoms of the disease above ground.
The early symptoms that we have seen mimic nutritional deficiencies in a plant which on the face of it suggests that the grower has not done a good job of fertilizing the plant leading to a magnesium or zinc deficiency. That’s what we focused on early on – looking at symptoms that could have just been an issue of poor fertilisation. But in reality those symptoms are a result of the root system not being able to take up the nutrients it needs because of the effects of the disease. And that’s why it’s such a hard disease to manage because you have the root systems dying off below ground and before you even know it the plant is diseased and by the time you start to see the first symptoms and know what’s going on it can be too late.
FJF: HLB was not discovered in Florida until 2005 – why had the disease not been a problem before?
MR: We found HLB in Florida in 2005 and the chances are that it had already been here for up to ten years before that. But growers didn’t notice or weren’t looking for it and when they saw the symptoms noted above they probably thought ‘Oh, this is a nutritional issue’, because quite often the first trees that were showing symptoms were along the edges of groves and that’s where trees can typically suffer the worst from nutritional problems. For example fertilizer applications that grower’s use are known to interact with the limestone rock embedded in adjacent roads thus reducing their effectiveness.
This is what makes HLB such a challenging disease. It’s not like we are growing a row crop such as wheat or corn that you plough under every year and re-plant. We are trying to grow trees long term that are becoming infected and it’s a difficult to identify and the only real control in the past has been cutting out those diseased trees as soon as they become infected. What makes it very difficult is that they can be infected three years or longer before you know that you have a problem.
FJF: In which country, and when, was the disease first discovered?
MR: HLB was first discovered in China in the late 1800’s but the first public confirmation in the western hemisphere was in 2004 where it was announced that Brazil had found HLB. The story goes that they had probably known that there was an issue down there for some while before they finally identified that it was HLB. It was not causing major losses at that stage but definitely spreading undetected.
This got people’s attention in Florida and we started looking much more closely at what was going on in our own groves and when our state department of plant industries began doing surveys they were now looking specifically for HLB and they eventually found it.
FJF: To what extent do you think the disease is responsible for the decline in orange production in Florida over the past decade or so?
The majority of the decline that we have seen in orange production is definitely directly related to HLB. No question. But the waters have been getting muddied a little because in 2004 we had three hurricanes that hit Florida and so most of our production ended up on the ground having been knocked off the trees by the storms. After that it took a couple of years for our production to try and come back up to where it had been before, so there was this huge drop in 2004 that wasn’t related to HLB. But ironically, shortly after the trees started to recover that’s when we found HLB.
While the production was coming back post hurricane it never went all the way up to where it had been before because growers were initially trying to cut down trees that had HLB in attempting to stave off the disease.
And now where we have got stabilised acreage the trees still aren’t producing very well. As the fruit is getting close to harvest and the trees have had the disease for multiple years the root system can no longer support the fruit load and the fruit begins to drop off the trees before harvest.
We are looking this year at something in the region of 70 million boxes or less of fruit produced in Florida of round oranges which, when you compare that to over 200 million boxes back in 2002/2003, this is a huge decrease. The majority of this fall in output is directly related to HLB
FJF: How has the disease been fought in the past and how has this changed over the past decade or so?
MR: There is a lot of work going on looking for solutions to HLB. A lot of what we have been doing since HLB was first found in Florida in 2005 was really trying to slow down the disease spread, to control the insect that spreads the pathogen. We had a lot of emphasis on psyllid control.
We also had taken a closer look at our citrus nursery industry because a lot of the spread of HLB occurred by moving infected plant material around the state of Florida. Our entire citrus nursery industry grows trees outdoors and you had the psyllid come in and feed and infect those unprotected trees which were then moved to groves throughout the state.
Even more of a problem was the ornamental nursery industry. There is a plant called Orange Jasmine which is a tree that a lot of home owners in Florida plant in the yard. It’s a nice shrub that is easy to care for but it was a host for the psyllid and the pathogen and very common in the nurseries in south Florida where psyllids first became established in the state of Florida.
When the psyllids first came into Florida its population moved throughout the state very rapidly on the Orange Jasmine plant which was shipped from south Florida to retail outlets such as Walmart, one of the US’s biggest retailers, and many other chain stores. A lot of those stores were selling infected plants that had come from down south containing the psyllid.
One of the things we did to control the slow-down and spread of HLB was to take our entire citrus nursery industry and put it indoors growing all our nursery plants under screen. This has definitely helped slow down the spread of the disease.
Growers were also attempting to combat HLB by trying to remove any diseased trees found in their groves. In an ideal world that is exactly what you would do but the problem is because this disease spreads undetected for a while, by the time the growers had located diseased trees in their groves these trees had probably been infected for two or three years. And as time progressed, more trees became infected and needed to be cut down. We had to abandon the tree removal effort because if you removed all the trees that were likely to be infected they would have to cut down all the trees in the grove.
Lately we have focused on psyllid management but all the approaches to managing the disease have been short term while we work on a more long term solution – developing a tree that is resistant to citrus greening.
FJF: The University of Florida has recently announced the development of gene-editing technology to fight HLB – could you tell us more? And can you tell us what other methods are being trialled?
MR: You mention the genetic modification angle but before we go into that that there is another approach that we have been looking at and this is the conventional breeding programme. We have a big citrus breeding programme at the University of Florida and we have a lot of trees in the field that have been evaluated, including what we call ‘crosses’, that have been created by cross pollination over the past 20 or 30 years, and it has become a natural experiment because you have citrus greening showing up on our field sites and moving through the groves with the result that many trees die. But equally we have many that don’t die. So in this process we have identified a number of conventionally bred citrus trees that are unique non-commercial plants, varieties that are not currently grown by the farmer that are now showing some tolerance – not resistance. And by tolerance what I mean is that the trees are lasting longer – surviving a bit longer – and producing fruit for a bit longer. They are probably still going to die from the disease eventually but for a grower it means that they will have several more years of productive life out of a tree before it does die. We have actually released two of those new varieties that are showing tolerance to HLB. They are not the round orange but fresh fruit mandarin varieties – one is called ‘Sugar Belle’ and the other ‘Bingo’.
In addition to the mandarins there are many more new varieties that are being examined but not released yet for commercial breeding which are showing promise. Either for release to the growers or to serve as plants that provide genes that are used later on in genetic modification.
Which brings me back to gene-editing technology, or genetic modification.
We have projects right now that our faculty are working on that are identifying genes in citrus plants that are responsible for disease resistance or tolerance. When they identify a resistance gene they move it from one citrus plant to another to create a resistant variety. Alternatively if it’s a gene that causes HLB then it is more desirable to turn off this problem gene. That’s where the CRISPR technique – an acronym for ‘clustered regularly interspaced short palindromic repeats’, gene editing technique comes in and that’s what I think we are all most excited about.
Generally there’s a lot of negative public perception about genetic modification and people are concerned about things being added to plants and don’t fully understand what it means. With gene editing it involves taking something off or taking something away and people’s perceptions are more excepting of that type of technology and it’s much easier and much quicker to develop.
We have a researcher here who was the first person to use gene editing in citrus and they are very much involved in using CRSPR. This researcher was able to develop a grapefruit variety that is resistant to the disease citrus canker. The reason they started with citrus canker was because this is a disease that they had been working with for a long time and they knew which genes were responsible. If we can turn off the genes for citrus canker then it shows we can replicate this technology for citrus greening once we have identified the genes responsible for that disease. We can basically go in and turn those genes off as well. It’s a kind of ‘proof of concept’ approach.
Now the focus is on identifying genes responsible for citrus greening which is not easy as there are thousands of genes that play some role in the various diseases of citrus and the citrus response to the disease. We have multiple researchers now tasked with narrowing down those potential gene targets to find the ones that are most likely to be involved, editing them out and developing that resistant plant for the future.
Those potential genes that our researchers have already identified are now in the process of being knocked out of the plant. Then it is just a matter of evaluating those plants long term to see if the new plant develops a resistance to the disease.
FJF: How long do you expect it take for these methods to start to bring some normality back to the groves?
MR: We are very much in a bottleneck situation now in terms of time and seeing any return to normality. Let me explain. We are talking about growing a tree from a single cell to something that can be evaluated against disease resistance, and growing that tree once you have edited out those genes to get a tree to a size where you can work with can take a year at least.
It’s a waiting game. You have to do the work, and then you have to wait awhile until you have got a tree that is large enough to work with and that’s where we stand right now. We have a lot of potential plants coming forward but we have to do those tests to see if we had successfully developed a resistant plant.
If one of those plants turns out to be the one, then that’s great. Problem solved! But in the meantime while you are waiting on that batch to come through we have to continue to develop even more batches that can come through if the first batch turns out to be dud. We don’t want to lose time. Say for example we had ten different targets knocked out and we are growing those up and then those ten didn’t work out we would want to have ten more, or a hundred more behind them – keeping the work going as fast as we can and continuing to develop back up batches. We are not sitting back and waiting. We are being proactive and continuing to work so that somewhere down the line we have a solution in the pipeline.
FJF: It must be both an exciting and worrying environment to be working in at the moment?
MR: It is exciting but everybody is very worried about our industry and we are all just hoping that we can find the solution sooner rather than later for the citrus growers. We are running out of time – we have a lot of groves where growers are having to ask themselves every year, or maybe every six months – taking a step back and thinking, can we continue to stay in business? We’ve seen some of the small growers go out of business. They were no longer making enough money to make a living. Some of them are selling their groves off and letting the real estate market move in. Some growers are trying to keep their land in agriculture and are looking at alternative crops like peaches and blueberries. They are sticking with it because they, like us, know that there is going to be an answer sooner or later and farmers want to be ready to go back and plant citrus once we have that solution.
We still have growers who are planting citrus right now using a selection of different root stocks that have been developed from the number of provincial breeding programmes that have been running that seem to be a little more tolerant of greening. They are not resistant but growers are planting them anyway and taking extra special care of those plants and doing a better job of watering and fertilization and just trying to keep the plants producing fruits so they have continuity of fruit for the future until we get that true resistant tree.
FJF: Which other organisations are tasked with tackling the disease and what are the different roles they serve?
MR: There are a number of organisations involved and the federal government is providing funding to help with the research. In Florida, we have our research centre CREC at the UF and we are the lead institution for citrus research in the state and we are the largest research centre in the world dedicated to one crop. Also in Florida, we have the United States Department of Agriculture (USDA) and they have a research plant down here focusing on many different areas of agriculture and there is a team of researchers dedicated to working on citrus.
Outside of Florida in other states in the US there are research teams that are working on different aspects of citrus greening looking at plant genetics and insect genetics. We have grower organisations who are working with us at CREC to help guide the research priorities. Basically everybody both in Florida and the US that is tied to citrus in some way is fulfilling some role in fighting this disease whether it’s providing guidance, research or funding.
FJF: Greening has spread more widely in Brazil now and more recently been identified in Argentina. Do you think these countries have the necessary infrastructure in place to effectively combat the disease?
MR: Yes, we have very good working relationships with both Brazil and Argentina and our researchers are collaborating with researchers from those countries. I do work a lot with some Brazilian organisations and they are right along with us with so many different things relative to this disease. And while they found HLB first in Brazil they are not suffering to quite the extent that we are in Florida.
In Brazil citrus growers are typically that much larger in scale and the farms are bigger than in Florida where we have a lot of smaller farms. We don’t have as much land in Florida as they do in Brazil. Florida is a state whereas Brazil is an entire country and they have been able to move their industry around as needed. The size of the farm plays a huge role in how the disease spreads because typically psyllids will fly into the grove and they will start feeding on the trees on the edge of the grove and literally move inward.
The smaller the grove the quicker that grove will become infected and if you have a much larger scale grove like they have in Brazil where they have maybe thousands upon thousands of hectares versus maybe tens of hundreds of hectares that we have in Florida, they come in and treat their groves with insecticides and they can control the psyllids before they move deeper into the grove.
When you look at the disease spread of HLB it usually starts on the outside edge of a grove and works its way in. That’s why they have been able to slow the spread.
They have done a good job on area wide control and we have replicated one of their programmes in Florida in the larger groves in the southern part of the state with the result that the trees are surviving longer than those in the smaller groves further north in Florida.
FJF: The Citrus Research and Education Centre is celebrating its 100th anniversary this coming November. Do you envisage having any positive results from your research to announce at this special occasion?
MR: We will see! People keep asking me how long it will be before you have an announcement to make. I don’t know – it might be a year or it might be tomorrow. We have a lot of things we are really excited about but we don’t want to get premature talking about things until we know how it’s going to work out.
Growers have to make decisions right now based on the information we currently have. We don’t want to give them a false sense of security. As soon as we know we have something you are going to hear a lot more from us about it but we are working as fast as we can – working all hours.
It’s a race against the clock. We know we will get a solution but will it be in time for our industry?
We are confident that we will!