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.
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.
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.
- Author: Ashley Robinson
The Australian finger lime, a citrus relative, could be a new specialty crop for Florida citrus growers.
Traditionally, finger limes have remained rare in the United States, grown few and far between. However, the fruit's unique tolerance to HLB is becoming increasingly attractive to Florida growers. Manjul Dutt, research assistant scientist at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) believes finger limes could secure Florida's position in the global citrus market.
In the field, finger limes have a low HLB infection rate. Early on, researchers noticed these trees were much more tolerant to HLB than any of the traditional citrus varieties being grown in the state.
“We have a number of theories as to why this finger lime could be tolerant to HLB,” Dutt says. “It could be due to the presence of physical barriers, or it could be due to the presence of certain toxins or certain chemicals in the phloem that the Candidatus Liberibacter asiaticus' (CLas) doesn't like.” CLas is the bacterium that causes HLB.
The young flesh of finger limes contains high levels of anthocyanins, producing a dark-red color on the leaves of the tree. Studies have indicated that insects, including the Asian citrus psyllid, move according to visual cues. It's possible the high levels of anthocyanins can discourage psyllid feeding and thus prevent transmission of HLB.
Additionally, the phloem of the finger lime contains high levels of aldehyde compounds. According to Dutt, citronellol, a compound of growing interest and present in the phloem, has shown to have anti-bacterial activity, which could also be preventing the replication of CLas.
One of the pressing issues limiting commercial production of finger limes in Florida is the lack of knowledge about the crop. Dutt and his team of researchers are currently evaluating different rootstocks in hopes of finding varieties suitable for Florida's growing conditions.
Furthermore, they are developing new cultivars that are crosses between conventional citrus and finger limes to incorporate HLB tolerance into traditional citrus varieties. Dutt says thousands of trees are currently being evaluated and quite a few appear promising
Tapping into their genetics
While finger limes aren't exactly set out to be the new crop replacing Florida's longstanding orange and grapefruit industry, Dutt believes finger lime trees can provide a strong assist. “Hybrids between finger limes and sweet orange down the road may have sweet orange-like traits that can be acceptable to the grower and consumer. It would create a sweet orange-like fruit with finger lime genetics that allow it to be tolerant to HLB,” he says. “Many people in the industry realize it's a long-term process. Some are skeptical but overall, people are hopeful that the finger-lime genetics play an important role in providing HLB-tolerant trees in the future.”
To date, finger limes are more of a niche crop in North America with only a few growers in California, Hawaii and Florida.
In the meantime, Dutt has produced a finger lime hybrid that looks like a larger finger lime. “We'll be releasing it this summer—it's similar to the finger lime but it has more pulp and the same “pearls” that finger limes do,” he says. He adds that it's a commercial release as a niche crop and hopes the limes will be available in stores in the next three to four years
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