UCR wages $11+ million war against citrus greening disease
Three projects win funding to fight tree-killing bacteria
Three projects win funding to fight tree-killing bacteria
With three new grants totaling more than $11 million, UC Riverside is helping lead the fight against citrus greening or Huanglongbing, a disease threatening citrus industries in the U.S. and worldwide.
The disease is from bacteria transmitted to citrus trees by a tiny flying insect, the Asian citrus psyllid. Infected trees produce no fruit, or fruit that is bitter, small and worthless. Despite intensive research for the past 15 years, there is no known cure for it. It has reduced citrus production in Florida by more than 75%, and it has already been detected in Texas and California.
Because California supplies the country with 80% of its fresh citrus, and because 267,000 acres of Golden State lemons, oranges, grapefruits, and mandarins are at stake if operations are permanently lost, the USDA National Institute of Food and Agriculture is making an emergency investment in citrus disease research with three projects at UCR.
These projects focus on instilling tolerance to the disease with three different approaches: below ground, in the rootstocks, above ground in the shoots and branches, and systemically, with a peptide that would move throughout the tree.
The largest of the projects, at $6.8 million, is being led by Danelle Seymour, assistant professor of genetics in the Botany and Plant Sciences Department at UCR. Their focus is on breeding Huanglongbing or HLB-resistant rootstocks, and the project depends on collaboration with Kim Bowman, a citrus breeder at the USDA Agricultural Research Service in Ft. Pierce, Florida.
“In Florida, nearly every single tree is infected. It's terrible for growers, but wonderful for breeding,” Seymour said. “We can't do this research at large scale in California because the disease isn't as widespread here.”
The classic way to improve resistance or encourage new qualities in crops is through genetics, making crosses between one plant that has a favorable trait, and one that doesn't. “We hope the result is better than the parents,” Seymour said. “When you work in wheat or tomato, you can do these crosses and perform evaluations every year. In citrus, it takes 10 - 15 years to evaluate a new generation of trees.”
Because of the long lag time, the research-ready trees in Florida represent an opportunity for Seymour's team to begin examining new crosses now. The breeder, Kim Bowman, has evaluated over 10,000 trees and unique hybrids, from which a handful will be selected for release to growers.
In addition to evaluating these select few new hybrids for their HLB tolerance, the researchers will be watching the Florida-grown trees' responses to the different environmental conditions in California. “Can they perform well in response to different salinity levels in the soil, different humidity, as well as other pests and pathogens that we have here? We'll find out,” Seymour said.
Chandrika Ramadugu, a project scientist also in UCR's Department of Botany and Plant Sciences, is leading a project to develop HLB-resistant scion varieties that can be grafted to rootstocks. A scion is an above-ground portion of a plant, such as a bud or shoot, that can be used for grafting.
For trees, grafting can be equated to an organ transplant. The scion from one tree is attached to the trunk or rootstock of another with the hope of creating a new plant with combined attributes.
With its grant of $3.28 million, this project will analyze second-generation hybrids that are bred for ten years using Australian lime as a source of disease resistance.
Ramadugu will evaluate 24 novel hybrids in California, Florida, and Texas to assess resistance to HLB. Ideally, in addition to having enhanced disease tolerance, the new plants will also be able to produce good-tasting fruit.
There is little genetic diversity in cultivated citrus. When new pathogens arrive, the genetic uniformity can result in disease epidemics and dire consequences for the crop. In addition to the potential benefits of this project for the fight against HLB, the new hybrids may also help protect citrus from other pests and pathogens.
A third project, granted $1.36 million, will utilize a peptide found in Australian finger limes that is known to impart HLB resistance. Led by Hailing Jin, Microbiology & Plant Pathology professor, the project is developing ways to infuse trees with the peptide.
“The antimicrobial peptide in the finger limes are more efficient at killing bacteria as compared to antibiotics currently used in the field, and much more stable at high temperatures,” Jin said.
Because spray applications are expensive, Jin's project aims to spread the peptide throughout the trees' insides. In collaboration with University of Florida professor Svetlana Folimonova, Jin's team utilizes a natural citrus virus with almost no symptoms to deliver the peptide into the trees.
“You infect the tree with the virus, and it will spread in areas where the bacteria reside,” Jin said. “It would move systemically through the tree, and it would be very cost efficient for growers. No need to buy more insecticides.”
These grants were enabled by the 2018 Agricultural Improvement Act, which authorized the Emergency Citrus Disease Research and Development Trust Fund to fight HLB. With these and other projects, the USDA is bringing together the nation's top scientists to find scientifically sound solutions to the problem in a financially and ecologically sustainable way.
hlb tree pulled
Most commonly we associate the malformed fruit we see in lemons to citrus bud mite. But it can also occur on other varieties of citrus in Southern California, including in this case to tangelos. If there's enough fruit to bite into, it's still edible.
Citrus bud mite is very small, elongated and somewhat tapered at the posterior end, and has four legs at the front end near the mouth. Look really closely in this image to see the squiggly little thing that can cause so much damage, those little white, hooked shaped things within the larger buds.
Females lay about 50 eggs mostly in the bud scales of recent growth. Mite numbers peak in summer, and summer and fall blooms are most likely to suffer damage.
Citrus bud mite is primarily a pest of coastal lemons but in recent years has also been found in interior regions of Southern California. The mites feed inside the buds, killing them or causing a rosettelike growth of the subsequent foliage and distortion of flowers and fruit, which may or may not reduce yield, fruit quality, or both.
Recent research has failed to show any consistent harm caused by bud mite feeding under oil spray regimes, especially in Lisbon lemons, and bud mite damage is offset by the negative phytotoxic effects of oil. Research has not been done to determine if abamectin plus oil sprays for bud mite are economically justified. To detect bud mites before damage occurs:
As an alternative to dissecting buds, bud infestation can be estimated from infested fruit buttons.
No bud mite threshold had been established; levels as high as 80% bud infestation have failed to cause consistent or predictable economic losses. If a reduction of bud mite numbers is desired, apply insecticides 2 to 3 months before the bloom that is to be protected.
READ ON For MORE INFORMATION FROM UC IPM
https://ipm.ucanr.edu/agriculture/citrus/citrus-bud-mite/
citrus bud mite pest image
Citrus growers in Santa Barbara County and the surrounding area are invited to attend the upcoming Santa Barbara County Citrus Industry Workshop on December 8, from 9 to 11 a.m. The workshop will be held at the Santa Barbara County Air Pollution Control District Building, located at 260 North San Antonio Road, Suite A, Santa Barbara. There is no cost to attend, but registration is required.
Registration spots are limited. To secure your spot in the workshop, please RSVP to CPDPP Grower Liaison Cressida Silvers at cressidasb@gmail.com, or call 805-681-5600 for more information.
In cooperation with the California Department of Food and Agriculture, the California Citrus Pest and Disease Prevention Program and the Santa Barbara County Agricultural Commissioner's office, this free in-person workshop will be focused on connecting with growers, PAC/QAC/QAL/PCA holders and farm labor contractors in the Santa Barbara County area to discuss a variety of topics that are currently impacting the citrus industry.
Topics of the workshop will include the most recent updates on Huanglongbing (HLB) detections in Ventura County and new changes to HLB quarantine regulations. An update on exotic fruit fly quarantines in southern California will also be presented. 1.5 CEUs will be available.
Questions? Email Cressida Silvers at cressidasb@gmail.com, or call 805-681-5600, for more information and to RSVP.
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citrus industry workshop dec 8
There are at least four species of Phytophthora species (P. citrophthora, P. parasitica, P. syringae, and P. hibernalis) associated with citrus in California and all species can cause various symptoms in citrus including the three main ‘diseases' associated with Phytophthora spp. The three diseases in citrus caused by these fungal-like pathogens are; Phytophthora Root Rot, Phytophthora Brown Rot of citrus fruits both pre-and post-harvest, and Phytophthora gummosis, which causes a canker at the lower area of the tree usually at or around the soil line. These organisms are active within the field essentially all year long so one tree could possibly have all three disease symptoms at one time but this is usually not the situation. These pathogens are also ubiquitous within the soils of California citrus groves so keeping an eye out for these diseases is essential to help manage these citrus issues.
Phytophthora Root Rot (PRR): PRR is caused primarily by P. citrophthora and P. parasitica. The former is most active in the winter with respect to PRR whereas the latter is more active in warm weather so PRR can be found throughout the year. This disease can affect young to mature trees and is often associated with groves that do not have good drainage such as high clay soils. For example, in the Terra Bella area of the San Joaquin Valley, there are areas with high clay soils that lead to problems with PRR due to the lack of drainage which can also lead to additional disease issues such as Dry Root Rot (Fusarium solani). In fact, both pathogens seem to work in tandem because Phytophthora can weaken the trees by destruction of the feeder roots leading to colonization by Fusarium solani and both pathogens can often be isolated in these situations.
Both pathogens are common throughout most citrus grove soils and can survive for years in the soil by producing persistent spores (clamydospores). When moisture is present in the soils, these pathogens can then produce oospores which are the reproductive spore stage. Oospores will differentiate into motile swimming zoospores that are released by the oospores and swim in the free water in the soil towards the primarily feeder roots. These motile zoospores are the infective spores which can decimate the citrus root system leading to potential death of the tree.
Trees that are infected with this disease will often show light green to yellowing of the leaves, thinning of the canopy, and often causes a slow decline of the tree once infected (Fig 1). The trees decline because the feeder roots get destroyed so the plant cannot uptake water and nutrients effectively, thus leading to potential death of the tree. If PRR is the potential suspect of decline, it is possible to dig up roots to evaluate them because this pathogen mostly infects the feeder roots below the soil line within a foot or so from the surface of the soil line. Figure 2 shows what a healthy root system looks like as well as a root system infected by the pathogen. The colonization of the feeder roots is primarily within the root cortex which becomes soft and disintegrates these cells which makes it easy to separate this tissue layer from the stele of the feeder root.
Root rot can also lead to other diseases due to stressing the plants once feeder roots are consumed by Phytophthora. For example, Fusarium solani, which causes dry root rot in citrus is a secondary pathogen that usually only infects citrus once the trees are under stress. Moreover, if the citrus trees are already infected with Citrus tristeza virus (CTV), the combination of CTV and F. solani can play a major role in quick decline of citrus on sour orange rootstock. Other viruses or viroids may also play a role in this interaction but no studies that we are aware of have tested this specifically.
Fig 1. Yellowing and thinning of citrus canopy due to Phytophthora infection.
Figure 2. Healthy root system of a citrus plant (top)
and a root system infected with Phytophthora showing decay of the feeder root system (bottom).
Brown Rot: This disease is caused by the various Phytophthora spp. and is usually associated with mature fruits. However, twigs, leaves, and flowers can also occasionally be infected which can result in death of these tissues. This disease is usually associated with cool and wet conditions. The symptoms can be seen in the field, primarily on low lying fruit because the spores of the pathogen can get dispersed with water and wind and move from the soil to the low-lying fruit in the tree (Fig 3). Therefore, it is recommended to ‘skirt' the trees so that there is no low-lying fruit to get infected. Brown rot can also occur after the fruit is picked (not showing symptoms) so it is also a post-harvest issue as well. In this situation, fruit that does not show obvious symptoms may be picked and stored at the packing house and the disease can spread to healthy fruit during storage.
Fig 3. Brown rot symptoms of Phytophthora on a lemon fruit.
Phytophthora Gummosis (PG): This disease is caused by the various Phytophthora spp. This disease is usually only seen around the soil line to a foot or so above the soil line but could produce a larger canker higher up the trunk (Fig 4). The disease is recognizable because once infected, the tree starts to produce compounds to combat the infection which results in oozing of sap from small infected cracks in the bark which may look as if the tree is bleeding. The bark usually remains firm but dries out and eventually cracks and can slough off the trunk. Sometimes a white crust appearance will also be seen within and around the canker. Once an infection occurs and the tree is not treated, the canker can eventually spread around the circumference of the trunk that can lead to complete girdling of the tree. This can weaken the tree leading to general decline and or kill the tree which can occur within a year under favorable conditions (moist and cool) but usually will take several years of active infection to cause major damage.
Figure 4. Gummosis symptoms on lower trunk of a citrus tree. Note that the scion is more susceptible than the rootstock because most growers use Phytophthora tolerant rootstocks.
General information on control of Phytophthora diseases. If a grower has a field that has had a history of various Phytophthora issues, there is the possibility to do a pre-plant fumigation using metam sodium or chloropicrin. If a grove becomes infected after planting, the most common methods of control are the use of chemicals usually applied through the drip lines. The most common products are Aliette, Ridomil Gold, and Prophyt. In the last several years, another product (Orondis), as well as some other chemistries, have been developed to control Phytophthora diseases. In this study, minimum effective rates to reduce Phytophthora root rot incidence and pathogen soil populations were determined after one and two applications in fall 2016 and summer 2017, respectively, and greenhouse studies confirmed the efficacy of the new fungicides. These findings led to fluopicolide recently receiving a federal and oxathiapiprolin (Orandis) a full registration for use on citrus. The researchers also requested that ethaboxam and mandipropamid also be considered for registration for control of Phytophthora diseases of citrus in CA. These new compounds will provide highly effective treatments and resistance management strategies using rotation and fungicide mixtures for the control of Phytophthora root rot of citrus.
Micronutrient sprays that contain phosphite may also help to control these diseases because this molecule stimulates a systemic inducedresistance response in the citrus trees that helps the plant fight off infections. The new compounds will provide good control when used in a rotation to avoid resistance, as has happened with many older products. For additional information regarding these new options for control of Phytophthora root rot of citrus, see. (https://apsjournals.apsnet.org/doi/10.1094/PDIS-07-18-1152-RE).
More details on Phytophthora and its control can be found at the UC IPM website; https://www2.ipm.ucanr.edu/agriculture/citrus/?src=redirect2refresh.
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hlb defprmed citrus
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