Growers of one of Florida's signature citrus crops, the grapefruit, may see more production and possibly less of the deadly citrus greening disease. Researchers have worked for four years, growing grapefruit under protective screens on a 1-acre experimental plot of trees at the University of Florida Institute of Food and Agricultural Sciences, and they're seeing encouraging results.
UF/IFAS scientists and a few commercial growers have used the system, known as “CUPS,” or “Citrus Under Protective Screens,” for a few years. They're trying to keep the dangerous Asian citrus psyllid away from citrus trees. Infected psyllids can transmit the deadly greening disease to citrus. So far, so good. They're noticing higher grapefruit yields and no psyllids or greening.
Florida grapefruit production has been drastically reduced by citrus greening, also known as Huanglongbing (HLB). In Florida, grapefruit production has gone down from 40.8 million boxes in 2003-2004 to 4.9 million boxes in 2018-2019, according to the USDA.
Arnold Schumann, a UF/IFAS soil and water sciences professor, leads the “CUPS” experiment at the UF/IFAS Citrus Research and Education Center in Lake Alfred, Florida.
And right now, he sees reason for optimism. Schumann is studying how well grapefruit grows in the 1.3-acre facility at the CREC.
Four years of data show grapefruit that exhibit no signs of greening, Schumann said. Researchers planted ‘Ray Ruby' grapefruit trees in August 2014. By December 2018, the trees had produced 2,100 boxes of grapefruits per acre, Schumann said. That's 525 boxes per acre per year on average, but Schumann notes that trees are less productive in the initial two years after planting. In years 3 and 4, the CUPS grapefruit yields were 797 and 892 boxes per acre, respectively. Currently the average yield for Florida grapefruit is about 166 boxes per acre per year, according to the USDA.
“HLB reduces profits for fresh citrus producers in many ways,” Schumann said “Production costs are higher due to increased needs to use pesticides and fertilizers, and fruit production is harmed by stunted tree growth, reduced fruit set and pre-harvest fruit drop, among other factors.”
The CUPS experiment at the Citrus REC has demonstrated that nearly all those harmful effects of HLB can be addressed, Schumann said.
“During the past five years, we have learned much about optimizing horticultural practices and pest and disease management for red grapefruit grown in CUPS,” he said.
Scientists focus on producing high yields with premium grades for the fresh fruit market.
“Our understanding of fresh fruit quality has been honed by our partnership with the Dundee Citrus Growers Association, which harvested and shipped our CUPS grapefruits and tangerines for the past two seasons,” Schumann said “Most importantly, fruit grown in CUPS should all be ready to sell, and our grapefruit and tangerine harvests have achieved 100 percent pack-out. For grapefruits, the fruit size is very important because it greatly affects the selling price.”
One reason for the good yield is the grapefruit's ability to adapt to the higher daytime temperatures under the protective covers, he said.
Other reasons for the increased productions include:
- High-density planting.
- A hydroponic system with trees growing in pots, instead of soil and inducing early, large blooms.
- Drip fertigation – a combination of fertilizer and irrigation -- applied several times a day.
CUPS hydroponic grapefruit has all the important attributes for fresh fruit production: high yields of HLB-free fruit, large fruit size, consistent yields and early maturity, Schumann said.
“The experiments at the CREC focused on proving that the CUPS concept was viable,” Schumann said. “Trees were grown mostly in containers, using hydroponics and very high-planting densities.
A couple of Florida growers are using the CUPS method for grapefruit, although it's too soon to know their results, Schumann said.
Scientists are not yet recommending the intensive production system used at the CREC experiment for commercial CUPS, although one grower in Hardee County is already experimenting with hydroponics and container-grown grapefruits, tangerines and navels under cover, Schumann said.
“Our aim is to maximize fruit production and quality in commercial CUPS with trees grown in the ground at moderately high-planting densities,” he said. “We want to document the most successful methods in a CUPS production guide and to update it as we learn more.”
Photo: Honey Murcott mandarin trees grow in 7-gallon pots at 1,361 trees per acre in the Citrus Research and Education Center screen house. Photo credit: Schumann, 2017
A local Ojai grower asks why there seems to be more citrus thrips damage to 'Pixie' mandarins this year. Was it because of the extended bloom due to warmer spring last year? The hotter summer up there that was more similar to climate in the Central Valley? Was it due to the Area-Wide Spraying for Asian Citrus Psyllid - ACP? Or is this a remnant of the Thomas Fire that dumped ash all over the county, disrupting biocontrol agents like lady bird beetles?
And what else does fire do to citrus?
This is classic thrips damage. In this area, it is not usually a problem. This year it seems to be more common. It's not always clear what is the main cause of and what all the interactions are that lead up to an outbreak like this. Just that there is damage now that occurred 10 months ago.
It has been a struggle to get through these hot times and now it's getting cooler, it's even rained, and suddenly that beautiful citrus that has just broken color and is an orange globe splits. It's most common in navels, but all citrus that ripen in the fall – tight-skinned satsuma mandarins, early clementines, tangelos and blood oranges. With the hot summer, it seems that a lot of citrus fruit have accelerated their maturity and are ready, ripe and sweet right now, and maybe ready to split.
And that's the problem. Drought stress. Salt stress due to drought. Water stress due to miserly watering. A heat wave in July. And a weird fall with maybe rain and maybe no rain and is ¼ inch considered rain or just a dedusting? Irregular watering is the key to splitting this time of year. The sugar builds, the pressure to suck in water builds and the fruit has been held back by a constrained water pattern and suddenly some water comes and it goes straight to the fruit and Boom, it splits.
Years of drought, and a stressed tree are a perfect set up for a citrus splitting in fall varieties like navel and satsuma. The days have turned cooler and there's less sense on the part of the irrigator to give the tree water and suddenly out of nowhere, there is rain. That wonderful stuff comes down and all seems right with the world, but then you notice that the mandarin fruit are splitting. Rats? Nope, a dehydrated fruit that has taken on more water than its skin can take in and the fruit splits. This is called an abiotic disorder or disease. However, it's not really a disease, but a problem brought on by environmental conditions. Or poor watering practices.
Fruit that is not yet ripe, like ‘Valencias' and later maturing mandarins are fine because they haven't developed the sugar content and have a firmer skin. They then develop during the rainy season when soil moisture is more regular. Or used to be more regular. With dry, warm winters this may become more or a problem in these later varieties, as well.
Several factors contribute to fruit splitting. Studies indicate that changes in weather, including temperature, relative humidity and wind may exaggerate splitting. The amount of water in the tree changes due to the weather condition, which causes the fruit to shrink. Then with rewetting, the fruit swells and bursts. In the navel orange, it usually occurs at the weakest spot, which is the navel. In other fruit, like blood orange, it can occur as a side split, as seen in the photo below.
Proper irrigation and other cultural practices can help reduce fruit spitting. Maintaining adequate but not excessive soil moisture is very important. A large area of soil around a tree should be watered since roots normally grow somewhat beyond the edge of the canopy. Wet the soil to a depth of at least 2 feet, then allow it to become somewhat dry in the top few inches before irrigating again. Applying a layer of coarse organic mulch under the canopy beginning at least a foot from the trunk can help moderate soil moisture and soil temperature variation.
Once split, the fruit is not going to recover. It's best to get it off the tree so that it doesn't rot and encourage rodents.
- Author: Sara Garcia Figuera, Jennifer Reed and Brianna McGuire
Western Plant Protection Network at UC Davis
Early detection technologies (EDTs) are tests that indicate the presence of disease before signs or symptoms of the disease can be seen. In the same way that a doc-tor measures a patient's blood pressure to look for heart problems, a grower might use a trained “sniffer” dog to detect changes in a tree that looks healthy but has huanglongbing (HLB) disease. By using the EDT, the grower is able to uncover HLB earlier, and can decide on an early, cost-saving course of action.
In the case of HLB, there are many EDTs under development. Some of them look for patterns in the microorganisms that live on the citrus leaves (Leveau snapshot); some look for patterns in the chemicals that are produced by the tree in response to HLB (Pourreza, Davis and Slupsky); and others look for the molecules that the bacterium injects in the tree to cause disease (Ma). A description of some of these EDTs can be found on the Science for Citrus Health website.
Why do we need EDTs for HLB?
To understand why EDTs are needed and what their potential value is, it is necessary to understand the difference between the incubation period for a disease and the latent period. The incubation period is the time between exposure to the pathogen and the appearance of symptoms. The latent period is the time between exposure and the newly-infected host becoming infectious. Huanglongbing (HLB) has a long incubation period and a very short latent period, which means that a tree can be dis-eased for a long time without showing any visible symptoms, while being infectious for a large fraction of that time. Even if a tree does not seem diseased, it can serve as a home for the bacterium (Candidatus Liberibacter asiaticus, CLas) that causes HLB. If a psyllid feeds on the infected tissue of a tree (with or without symptoms), CLas that is present in the leaf tissue can be picked up by the insect and transmitted to other trees when the psyllid moves on to feed. Information from an EDT can help a grower detect the disease in a tree a long time before it would be detected by eye. This cuts down the time psyllids are able to feed on it and transmit the disease, slowing the spread of HLB to neighboring trees.
Why is it important to remove infected trees as early as possible?
If a tree that tests positive for CLas is not treated or removed, the bacterium will spread throughout the tree. Over time, an increasing proportion of the tree's tissues will become infected, increasing the chances that a psyllid will become infected upon feeding, and subsequently spread the infection to healthy neighboring trees. If the infected tree is removed, there is no opportunity for psyllids to feed on the infected tissue and spread the disease. Once CLas is detected, tree removal is the only surefire way to prevent the spread of the infection, and it is extremely time-sensitive. The sooner an infected tree is removed, the lower the chances that psyllids will get infected. The savings associated with early infected tree removal will be proportional to the amount of surrounding trees that would have been infected with CLas due to that tree, and the number of months that it would be left on the ground.
Who is working on the project?
Several research teams in different universities and research stations, supported by a variety of funding organizations, have been working on the development of a variety of EDTs. These EDTs, designed under laboratory and greenhouse conditions, are being validated under field conditions in Texas and Florida. In California, where HLB has not been detected in citrus orchards, samples of different citrus varieties have been collected from healthy trees and trees affected by other diseases from all over the state. These samples are being used to calibrate the EDTs, and to test if they can distinguish between healthy and HLB-diseased trees, and between HLB-diseased trees and trees affected by other common citrus diseases. Dr. Neil McRoberts and his team at UC Davis are evaluating the data from these experiments and providing support to the EDT researchers.
What are the challenges and opportunities?
Currently, regulations require HLB infected trees to be removed if a certain amount of CLas DNA is detected in leaf samples through polymerase chain reaction (PCR). However, CLas is unevenly distributed in the sap of citrus trees, and the leaf samples collected might not be PCR-positive even though the bacterium is already present elsewhere in the tree. EDTs offer the possibility to detect infected trees before they are PCR-positive, so they could be removed earlier in the HLB epidemic. Therefore, the value of EDTs relies on the voluntary removal of EDT-positive trees before the law requires them to be removed.
No EDT gives perfect diagnostic results. Sometimes healthy trees will produce EDT scores that look like diseased trees (so-called “false positives”). Removing such trees will result in an immediate financial loss. However, because the economic damage caused by leaving an infected tree in place is much bigger than the value of a healthy tree, using an EDT to guide decisions has the potential to result in a long-term economic benefit to individual growers and communities, by reducing the spread of HLB. Losing a few healthy trees along the way is the unavoidable cost of stopping the disease from spreading. Like-wise, some trees will seem healthy based on EDT scores but might end up showing symptoms (“false negatives”). The proportion of true positives, false positives, true negatives and false negatives represents the accuracy of a diagnostic test. Dr. McRoberts' team is analyzing the accuracy of the EDTs, and preliminary results suggest that the best performing EDTs could be correctly determining the status of the trees 95% of the time.
The results of this analysis could be used to foster the adoption of EDTs among the citrus grower community, promoting the idea that the sooner infected trees are detected and removed, the smaller impact HLB will have on California's citrus production. Unless there is sufficient cooperation in integrated management of HLB by removing infected trees as early as possible, controlling the ACP on an area-wide scale, and using certified plant material, the California citrus industry is likely to suffer un-sustainable economic losses to HLB.