By Christopher Vincent, Anirban Guha, Joon Hyuk Suh and Yu Wang

It may surprise you to learn that citrus trees can get too much sunshine in the Sunshine State. Manipulating the light environment around a plant can have several surprising benefits. Shade can suppress the HLB cycle and enhance citrus health and yield. Overall, a mildly shaded citrus plant is likely to have fewer signs of stress, less HLB-spreading psyllids, less severe HLB symptoms and higher yield.

Our groups have joined with several others at the University of Florida to assess how shade impacts the various dynamics of pests, disease, plant health and horticulture. HLB is spread by Asian citrus psyllids landing on trees. Previous researchers observed that low light reduced psyllids landing on trees in a laboratory and that the shaded sides of trees in the field had fewer psyllids.

In our initial study at a natural forest site, we found it was difficult to find psyllids on feral citrus trees in shaded hammocks, and very few trees were infected with Candidatus Liberibacter asiatius (CLas), the bacterium that causes HLB. We think fewer psyllids land on shaded trees because psyllids use light reflecting off distant trees to locate them. If there is less reflection, the trees are less likely to be spotted. We also found very few symptomatic leaves in the forest site. Leaves showed few signs of stress to their photosynthetic machinery, regardless of whether the trees had HLB.

We followed our forest study by examining sweet orange in the field with 4-year-old Hamlin trees that were all infected and showed strong HLB symptoms. We installed shade netting with 30, 50 or 70% shade over the trees in late 2018 and followed their growth and health over two years.

IMPROVEMENTS
There is evidence that the trees' health has improved, though they are HLB infected. When we performed a whole leaf metabolomic analysis in spring and fall, we found many changes in the metabolic profile, including prominent changes in leaf hormonal balance and the metabolism of sugars and nitrogen. In association with the hormonal changes, the shaded trees have less intense flushes, though their canopies are not less dense. This indicates the leaves likely have longer lifespans in the shade, counteracting an important symptom of HLB: leaf drop.

The shaded leaves had less foliar starch, meaning shade mitigates the typical starch accumulation induced by HLB. Along with the reduction in starch, the shaded leaves also accumulated more of the sugars involved in carbohydrate export from the leaf, indicating that phloem may be functioning more effectively, though this point needs more research.

These results, combined with evidence that leaf water status has improved, suggest that CLas-infected trees in the shade are healthier than in full sun. Thus, it comes as no surprise that shade improved yields. Over the two years of the study, the trees under 30% shade produced twice the yields of those in full sun.

Of course, too much of a good thing is still too much. Increasing shade beyond 30% continued to mitigate stress in the leaves but did not improve yields. Based on work done by our predecessors at the Citrus Research and Education Center, increasing shade intensity too much leads to a reduction in the flowering needed to set a good crop. The shade needs to be sufficiently moderate to help avoid the worst of the sun- and heat-induced stress while still spurring the trees to make fruit.

SHADE OPTIONS
Despite these benefits, there are challenges to using shade in horticulture. Although shade netting is frequently used in some international citrus regions, such as South Africa and Australia, installing large shade structures may not be cost effective for many Florida growers.

A notable exception to this is the construction of many citrus under protective screen (CUPS) structures and individual protective covers that are installed on young trees. Although these are implemented as exclusion netting to prevent the arrival of psyllids on the trees, the nets also provide the environmental benefits of shade, including a warm humid environment that does not overload the leaves with too much light.

Particle films can also provide temporary, sprayable shade. These consist of particles that can be put in suspension and sprayed on leaves, leaving them to dry as a film.

Our ongoing experiments with kaolin particle films have demonstrated that these treatments also reduce disease pressure and leaf water deficit while enhancing tree growth and yield. We studied the application of red and white kaolin particle films over the first four years of a planting, where we saw the kaolin treatments more than quadrupled the yield compared with a treatment that used foliar insecticides to control psyllids.

We have also found that these treatments in young trees lead to larger, denser canopies and help avoid water stress when the soil water depletes. These treatments are cheaper than other approaches to shading trees and may be more effective in keeping Asian citrus psyllid populations low. Thus, particle films may be more accessible to many growers.

There is still much to learn about how different approaches to shade affect the health, ecology and horticulture of citrus. However, recent promising results indicate that manipulation of the light environment in the canopy is an approach worth considering. In the next few years, we hope to research and develop more effective and practical methods that can help growers find a balance of light for improving tree health and yields in the era of HLB.

Christopher Vincent and Yu Wang are assistant professors, Joon Hyuk Suh is a research assistant scientist, and Anirban Guha is a postdoctoral research associate — all at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.

https://citrusindustry.net/2021/04/05/tweaking-the-sunshine-for-better-citrus-health-and-yield/

https://news.ucr.edu/author/jules-bernstein

UC Riverside scientists are betting an ancient solution will solve citrus growers' biggest problem by breeding new fruits with natural resistance to a deadly tree disease.

New hybrid citrus fruit bred for disease resistance and flavor. (Chandrika Ramadugu/UCR)

The hybrid fruits will ideally share the best of their parents' attributes: the tastiness of the best citrus, and the resistance to Huanglongbing, or HLB, displayed by some Australian relatives of citrus. 

There is no truly effective commercial treatment for HLB, also called citrus greening disease, which has destroyed orchards worldwide. The disease has already been detected in California, where 80 percent of the country's fresh citrus is grown. However, it has not yet been detected in a commercial grove. 

To prevent that from happening, the National Institute of Food and Agriculture has awarded a UC Riverside-led research team $4.67 million.  Chandrika Ramadugu, a UCR botanist leading the project, helped identify microcitrus varieties with natural resistance to HLB about eight years ago.

Cross section of a hybrid fruit bred for this project. (Chandrika Ramadugu/UCR)

“HLB is caused by bacteria, so many people are trying to control it with antimicrobial sprays,” Ramadugu said. “We want to incorporate resistance into the citrus trees themselves through breeding, to provide a more sustainable solution.”

Part of the challenge with this approach to solving the HLB problem is that it's possible to breed hybrids that are resistant to the disease but don't taste good, Ramadugu said. “Hence the need to generate a lot of hybrids and screen them for the ones that will be most ideal for the citrus industry.”

Microcitrus, such as the Australian finger lime, tends to have a sharper, more bitter taste than its relative citrus fruits, like oranges. The perfect cross will have just the right mix of genes to give it sweetness and HLB resistance. 

Ramadugu's team includes collaborators from Texas A&M University, the University of Florida, Washington State University and the U.S. Department of Agriculture, as well as scientists from UC Riverside's Department of Botany and Plant Sciences. 

Breeding project team members from UC Riverside's Department of Botany and Plant Sciences. (Chandrika Ramadugu/UCR)

Currently, the team is studying differences in the genetic makeup of the hybrids they've already bred. Analyzing the new plants' DNA will help the team see whether enough disease resistance has been bred into the fruit, but not so much that the flavor is compromised. 

Another challenge with breeding is the time it takes for new citrus varieties to flower naturally, which can be several years. With the help of Sean Cutler, UCR professor of plant cell biology, the team is hoping to accelerate the time it takes for the hybrid plants to bear fruit in a greenhouse. 

This way the hybrids can be analyzed for taste much sooner. Clones of the best hybrid plants will then be grown in Florida and Texas field trials. 

UC Riverside scientists are using a variety of approaches to fight HLB. While some hope that altering soil and root bacteria will improve plants' immunity to the disease, others are trying to improve HLB resistance by tweaking citrus metabolism, or by using an antibacterial peptide to clear HLB from an infected plant. 

The fruit produced through Ramadugu's method will appeal to many consumers because it will not have genes introduced into them by scientists. Breeding has been done for thousands of years to improve crops and is considered a more natural practice. 

Additionally, Ramadugu says she's excited about her approach because it will ultimately produce a product useful for growers and consumers.

Researchers at the California Data Analysis and Tactical Operations Center (DATOC) have analyzed Asian citrus psyllid (ACP) trapping data along major transportation routes before and after tarping regulations for bulk citrus shipments were enacted. The purpose was to determine the effectiveness of the policy.

DATOC is an independent group of scientists sponsored by the Citrus Research Board and the California Citrus Pest and Disease Prevention Program. The group was formed in 2016 to create and amend tactical response plans for huanglongbing (HLB) suppression and management for California citrus.

DATOC found a significant reduction in the rate of ACP finds throughout the San Joaquin Valley (SJV) after tarping regulations went into effect. The SJV contains more than 70% of California's packinghouses. Coastal and Southern California counties ship more than 63 million pounds of bulk citrus into the SJV annually for processing.

In years past, ACP populations have soared as they presumably “hitchhiked” on trucks that weren't properly covered, coming from Southern California into the SJV and threatening the livelihood of commercial groves throughout California along the way. However, after the California Department of Food and Agriculture (CDFA) required tarping in 2017, DATOC data shows that tarping has effectively reduced ACP movement.

While these results are encouraging, scientists say that growers must continue to remain vigilant. In a recent letter, Citrus Pest & Disease Prevention Committee (CPDPC) chairman Jim Gorden stated that ACP populations are expected to “flare up” occasionally, such as the late 2020 ACP detections in Kern, Madera, San Luis Obispo, Santa Barbara, Santa Clara, Tulare, Contra Costa and other counties.

The CPDPC emphasizes that growers, packers, transporters and other stakeholders must continue to stay on top of this elusive ACP pest and the dangerous HLB disease it spreads. The upfront cost to manage ACP is much less than the potential hit to the citrus industry if HLB spreads throughout the state.

In order to move bulk citrus from an ACP regional quarantine zone or a HLB quarantine area under the terms of the permit(s), growers, grove managers, haulers and harvesters must comply with the CDFA's transporting requirement as detailed in their order. Get specific details here.

Source: Citrus Pest & Disease Prevention Program

Author: Jules Bernstein, UCR,Senior Public Information Officer

New research affirms a unique peptide found in an Australian plant can destroy the No. 1 killer of citrus trees worldwide and help prevent infection.

Huanglongbing, HLB, or citrus greening has multiple names, but one ultimate result: bitter and worthless citrus fruits. It has wiped out citrus orchards across the globe, causing billions in annual production losses.

All commercially important citrus varieties are susceptible to it, and there is no effective tool to treat HLB-positive trees, or to prevent new infections.

However, new UC Riverside research shows that a naturally occurring peptide found in HLB-tolerant citrus relatives, such as Australian finger lime, can not only kill the bacteria that causes the disease, it can also activate the plant's own immune system to inhibit new HLB infection. Few treatments can do both.

Research demonstrating the effectiveness of the peptide in greenhouse experiments has just been published in the Proceedings of the National Academy of Sciences.

The disease is caused by a bacterium called CLas that is transmitted to trees by a flying insect. One of the most effective ways to treat it may be through the use of this antimicrobial peptide found in Australian finger lime, a fruit that is a close relative of citrus plants.

"The peptide's corkscrew-like helix structure can quickly puncture the bacterium, causing it to leak fluid and die within half an hour, much faster than antibiotics," explained Hailing Jin, the UCR geneticist who led the research.

When the research team injected the peptide into plants already sick with HLB, the plants survived and grew healthy new shoots. Infected plants that went untreated became sicker and some eventually died.

Arrows point to areas of fluid leakage from the bacterial cell after treatment with the antimicrobial peptide. (Hailing Jin/UCR)

"The treated trees had very low bacteria counts, and one had no detectable bacteria anymore," Jin said. "This shows the peptide can rescue infected plants, which is important as so many trees are already positive."

The team also tested applying the peptide by spraying it. For this experiment, researchers took healthy sweet orange trees and infected them with HLB-positive citrus psyllids -- the insect that transmits CLas.

After spraying at regular intervals, only three of 10 treated trees tested positive for the disease, and none of them died. By comparison, nine of 10 untreated trees became positive, and four of them died.

In addition to its efficacy against the bacterium, the stable anti-microbial peptide, or SAMP, offers a number of benefits over current control methods. For one, as the name implies, it remains stable and active even when used in 130-degree heat, unlike most antibiotic sprays that are heat sensitive -- an important attribute for citrus orchards in hot climates like Florida and parts of California.

In addition, the peptide is much safer for the environment than other synthetic treatments. "Because it's in the finger lime fruit, people have eaten this peptide for hundreds of years," Jin said.

Researchers also identified that one half of the peptide's helix structure is responsible for most of its antimicrobial activity. Since it is only necessary to synthesize half the peptide, this is likely to reduce the cost of large-scale manufacturing.

The SAMP technology has already been licensed by Invaio Sciences, whose proprietary injection technology will further enhance the treatment.

Following the successful greenhouse experiments, the researchers have started field tests of the peptides in Florida. They are also studying whether the peptide can inhibit diseases caused by the same family of bacteria that affect other crops, such as potato and tomato.

"The potential for this discovery to solve such devastating problems with our food supply is extremely exciting," Jin said.

​Untreated citrus plants on the left, as compared to treated ones on the right. (Hailing Jin/UCR)