Florida is the largest producer of citrus in the U.S., with a ten-billion-dollar industry consisting mostly of juice oranges. However, Florida’s citrus trees have suffered from Huanglongbing (HLB) disease since 2005. The infection rate throughout the state has reached close to 100%. Dr. Tripti Vashisth, an assistant professor of horticultural sciences and citrus extension specialist at the UF/IFAS Citrus Research and Education Center, discussed citrus nutrition recommendations during the eighth session of the 2020 BMP Summit.

HLB is spread through the Asian citrus psyllid feeding on the leaves of citrus trees. The psyllid transmits bacteria to the tree, which then spreads to the leaves, fruits, and roots. The tree’s growth arrests and health begins to decline. Fruit quality suffers and the fruits drop from the tree begins. The resulting root loss causes reduction of water and mineral uptake.

Strategies to combat citrus greening include:

• Psyllid control
• Genetic modification of psyllids
• Breeding resistant/tolerant varieties of citrus
• Utilizing biotechnology
• Thermotherapy and antibiotics
• Complete and balanced plant nutrition
• Improving root growth

Changes in leaf coloring can signal an HLB affected tree. The leaves show blotchy mottle/chlorosis, which can be confusing because it is similar to sign of nutrient deficiency. As well as nutrient deficiency is also a secondary effect of HLB. The “pen test” can be done to investigate whether a tree has HLB or only nutrient deficiency. A tree infected with HLB will have blotchy coloring, whereas a nutrient deficient plant will have more uniform coloring.

As early as 2009, growers began nutritional therapy to address nutrient deficiency symptoms. They primarily used foliar fertilization and tested many different recipes. The results of testing these various fertilizers were inconsistent and resulted in confusion. Inconclusive results occurred due to trees taking a long time to show symptoms of HLB. Symptoms are highly variable because the extent of HLB varies greatly.

Another highly variable factor of tree health is nutrition management. There are 14 different nutrients and numerous products, chemical forms, and analyses. Efficacy of the treatment depends on method of application, soil type and pH, irrigation, and time of year.

A citrus grower survey conducted in June 2017 involved eight growers located throughout the state who claimed to be making profit. Three important pieces of information were discovered:

• Every grower agreed that their nutrition management programs are more intensive and better now than pre-HLB.
• Growers who had better nutrition management programs pre-HLB and early on adopted good psyllid control did not see as much decline as some other growers.
• There is no fixed recipe for management. Programs are site-specific, require good observation and analyses, and respond to tree needs.

Growers are trying to save their trees by adopting sufficient nutrient management practices and controlling damage. A constant supply of nutrients is critical. Soil-applied nutrition is best because HLB-affected plants have a small root system and require the extra nutrients. For a soil-applied nutrition program, the tree takes up nutrients that are dissolved in a solution. The plant then distributes the dissolved mineral nutrients throughout the canopy. Mobile and immobile nutrients have equal and uniform distribution to all parts of the plant.

Foliar nutrition programs are not as effective because thick leaf cuticle limits the nutrient uptake. Additionally, a significant amount of foliar spray washes away in soil. The HLB-affected trees have few feeder roots, so they may not catch any nutrients that are carried by water. Immobile nutrients can also get locked in leaves with foliar sprays.

Recent research has shown that an overdose of nutrients (applying a higher dose of nutrients than the recommended rate) has a therapeutic effect on HLB-affected citrus. New research is focused on determining if HLB-affected roots are efficient in nutrient uptake and if there is a preference for certain nutrients. One study examined differences in nutrient utilization between healthy citrus and HLB-affected trees. The results showed that even though the HLB-affected plants were significantly lower in root and shoot biomass, these plants metabolized nutrients at a higher rate and may have higher requirements of certain nutrients. Because HLB-affected plants have higher nutrient uptake efficiency, small root mass is the major limitation in nutrient utilization.

Another interesting finding from this research is that chlorophyll content increases with nutrient availability in HLB-affected plants. The feeder roots also undergo significant structural changes to increase nutrient uptake. The HLB-affected trees pexpress genes that improve nutrient uptake. Both the molecular regulation of transport-related genes and changes in root anatomy contribute to high nutrient uptake. Proper fertilization improves the plant’s defense response.

Soil pH also affects the health of HLB-affected citrus. In Florida, the optimum soil pH range for citrus is 6-6.5. However, a study found that citrus was regulalry grown in soils that were 7 or above. There was no soil acidification in commercial citrus groves pre-HLB, but HLB-symptoms intensify under high soil pH. Yield reduction occurred at a high pH because root density is related to bicarbonate stress.

Researchers studied the effects of irrigation water pH on the performance of both healthy and HLB-affected citrus. They also examined molecular regulation of pH-HLB interactions to determine why soil pH is important for maintaining HLB-affected plants. The experiments showed that HLB-affected plants under pH 8.0 treatment had the highest death rate. On the other hand, HLB-affected plants performed similarly to healthy plants at pH 5.8. The increase in pH resulted in heavy leaf drop and decreased leaf biomass. Overall, healthy trees are not affected by soil pH as much as HLB-affected trees. After observing the molecular response, results showed that greater numbers of genes were differentially regulated at pH 5.8 with HLB, while fewer experienced this at pH 8.0. The most important takeaway is that HLB-affected plants have enhanced tree defense ability at a low pH.

A field trial was also conducted to discover the effect of the controlled release form of mineral nutrients, elevated levels of individual micronutrients, and soil pH amendments (to lower pH). The fertilizer was applied to the soil and provided a constant supply of nutrients and a higher rate of micronutrients. The trial involved testing ten different fertilizer treatments over three years at two different testing sites. The arcadia site had the highest yield with treatment 7 (CRF + Tiger MM + Fe 20%) and the highest yield at the Fort Meade Site was with treatment 9 (CRF + Tiger MM + Mn + B 20%). These results show that the fertilizer program needs to be customized for the site to address specific tree needs.

In summation, applying micronutrients at a 20% higher rate than recommended can improve productivity of HLB-affected trees. Soil applied nutrients are more effective than foliar applications. A constant supply of nutrients and soil acidification is beneficial for tree health. Focus on leaf nutrient analysis than the rate of fertilizer applied because trees perform better when they are in the high end of optimum range.

Written by Natasha Roberts, CLUE Communications Intern

Q&A

Q: What is the acceptance by growers for changes in nutrient application?

A: They are actually very willing. Most of the nutrition work has been grower driven in citrus, and that’s interesting. They are very accepting, but of course it takes time; fertilizer change is just not, “okay do this”, there’s a lot of economics that go into the whole thing.

Q: Have these new recommendations or the changes in nutrient application, have they stabilized yield?

A: So that’s the thing, we have not made any changes in the formal guidelines yet. These are more working guidelines; pH at 5.8 – 6.5 is where we are trying to be. Growers who are able to stay in that range are seeing improvement. But each grove is so different, and if they over acidify and get to say a pH of 5, their calcium and magnesium will have started to leach out of the soil. So we are seeing improvement at a pH of 5.8 – 6.5 once the grower sticks to that, but there are so many more things.

Q: Are you looking at fruit quality in the different treatments of that study?

A: Yes, we are. There were particular treatments in the field study that we saw improvements with more of a consumer, liking. We are not exactly what chemicals are there, but the treatment with more zinc is usually more preferred. We think this is because it has a little bit more sour tasting and has better sweet and sour taste.

Q: Are most growers following current recommendation rates, or do they have their own customized program?

A: They are, and it has to be more customized, just from the results of the field trial – we gave the same treatment at two sites and they gave us different results. So, right now we are trying to educate growers that they need to make a specific program for each grove. There’s a lot more effort in growth management today than there used to be 15 years ago. In short, everybody has a customized program; I do not think any two growers are using exactly same program.

Q: It’s interesting to me, the whole HLB thing is kind of analogous to the situation with COVID-19 that we’re in – at the early days of HLB there was so little known and people were trying all kinds of things. I wonder in horticultural crops, or in agriculture that you’re aware of, is there anything like HLB – a quick industry decimating type pest or disease?

A: Not like HLB, but there are certain diseases such as Xylella which affects the xylem, so that is a bit different. I think nothing is equal to HLB for a couple of reasons. The crop grows on the tree throughout the year and we are in subtropical areas where these insects are also there throughout the year, so it’s difficult to control the evergreen trees. So in potato they have something very similar to HLB but potato is an annual crop, you change the crop and you don’t have to deal with it again. But citrus is just so much more complicated. Now with apple, which is another perennial crop, there are some diseases that are more related to the flowering period, but that’s it. Once you control during the flowering time, you are done. With HLB growers are working 24/7 night and day throughout the year to manage this.

Q1: Actually, I did think of something – the wine grapes, right? The French vines were wiped out in what, the 1800’s?

A1: Yes, but then they learned better management, but yes it did wipe out the industry. For our audience today, I’d just like to say that we have not revised the guidelines specifically, but this is where we are headed. This is what a lot of growers are already doing, but we need so many years of data.

Q2: Along those lines, for the recommendations to be used as a BMP they have to be official recommendations, right? So it has to go through the process of publication, peer-review, and then up through the Plant Nutrient Oversight committed within IFAS, so thank you for mentioning that.

A2: Yes. Whenever we are comparing a healthy and HLB tree, most of the work that we do has to be done in the greenhouse – it’s a completely differently type of system that you’re working with than actual field conditions, so it’s a difficult situation.

Q: What do you recommend now for growers, as far as the pH? I heard you say acidic right?

A: 5.8 – 6.5 is where their soil wetted zone soil pH should be. But that’s the thing, a lot of times the growers go overboard and HLB trees are less forgiving than healthy trees.

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