UCR wages $11+ million war against citrus greening disease
Three projects win funding to fight tree-killing bacteria
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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.
The U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) has confirmed positive identifications of Citrus yellow vein clearing virus (CYVCV) in the Hacienda Heights area of Los Angeles County during the California Department of Food and Agriculture's (CDFA) routine multi-pest survey. This is the second area in California where CYVCV has been detected, following the first detections in Tulare County in March 2022.
In response to the additional detections of the virus, CDFA is surveying residential properties within a 1-mile core radius area around the initial find site in Los Angeles County to fully determine the extent of the disease's presence in the area.
Photo credit: https://gd.eppo.int/taxon/CSYV00/photos
CYVCV can be spread by vectors as they move from tree to tree feeding on foliage. The vectors include citrus whitefly, green citrus aphid, melon or cotton aphid, and cowpea aphid, which are all known to be present in California. CYVCV can also be spread through grafting and the movement of infected propagative materials and rootstocks, and contaminated tools and equipment. While there is no treatment for CYVCV, as of now, the best mitigation measures are to control the virus' vectors and sanitize tools and equipment. To the greatest extent possible, growers are encouraged to urge their field crews to clean and sanitize all their equipment thoroughly in between jobs or when moving between groves.
For any questions about CYVCV, please call the CDFA Pest Hotline at 1-800-491-1899 or visit CDFA's website to learn more.
Armillaria mellea is the pathogen that causes root rot of many forest and ornamental and agronomic trees. The pathogen occurs in landscapes and urban soils as well as a natural pathogen in forests and on lands converted to farming. While symptoms can appear suddenly, it is generally considered a slowly developing pathogen that takes years to kill a tree. When infected trees begin to die and are removed from orchards the fungus rapidly colonizes the remaining roots and resides in soil as potential inoculum for trees planted later.
Armillaria is difficult to control. Fungicides are not effective and even the most effective fumigants such as methyl bromide may not completely eradicate Armillaria from soils. Sterol demethylation inhibiting fungicides can be effective in slowing and preventing Armillaria infections in grape and stone fruit crops. Fungicides if used frequently are subject to breakdown in effectiveness through pathogen resistance to the treatment.
Adding uncomposted organic matter to soil provides the carbon and energy necessary to build microbial populations in soil, some of which are antagonistic to Armillaria. Mulches of bark and fresh woody debris of Scots pine increased soil microbial activity and organisms (fungi) eleven years after treatment. In this study both Trichoderma koingii and fresh wood and bark mulches applied to soil increased T. verde. Tree wood products such as yardwste or tree trimmings wastes are effective substrate for culture of many different fungi including Trichoderma spp. Downer et al, found that Armillaria mellea only survived a maximum of 11 days in fresh yardwaste static piles and that recovery of viable Armillaria was negatively associated with increases in Trichoderma in the wood chips.
Trichoderma have long been understood to play a role in control of Armillaria, especially after soil fumigation. Some effectiveness has been shown with Trichoderma in concert with other non-chemical methods.
In this study two potential cultural controls (hole size and yardwaste amending) and one biological treatment (Trichoderma application) on their ability to help peach trees survive Armillaria infested soil.
Peaches, Prunus persica were planted as grafted saplings in an avocado orchard previously infested with Armillaria mellea (Vahl) P.Kumm. Peaches were used because they are so sensitive to the fungus. Trees were planted in large or small holes withor without fresh yardwaste chips added as an amendment and with or without a Trichodermabiocontrol product sprayed into the hole. Trees were monitored for six years -- growth and mortality was tabulated. Six years later 40% of the trees had died from the disease. Trees planted in a large hole were more likely to survive than in a smaller hole (P=0.07) and trees in large holes with fresh organic matter added were the most likely to survive (P=0.04). Trichoderma sprays in the planting hole did not increase survival rates. While growth was initially retarded by adding fresh yardwaste to the hole, in later years none of the treatments affected growth rates
Read on: https://www.plantsciencejournal.com/apdf/jpsp-aid1031.pdf
How the new neonic regulations impact your operations
There are new neonicotinoid regulations effective starting Jan 1, 2024 to protect pollinators. Neonicotinoids are a class of pesticide that are chemically similar to nicotine and that affect insect central nervous systems, resulting in paralysis and death. They are known to negatively impact pollinators, including bees. Neonicotinoid pesticide products include the active ingredients clothianidin, dinotefuran, imidacloprid, and thiamethoxam. The new regulations affect the applications of these pesticides on some crops. Because pesticide labels will not be updated, it is critical that anyone using these products is aware of the regulations.
For crops that are harvested after bloom (e.g. berries, citrus, cucurbits, oilseed crops, pome fruits, stone fruits, tree nuts), neonic use is prohibited during bloom. In addition, there are restrictions that apply if multiple active ingredients are used OR if both soil and foliar applications are used during the growing season. These restrictions are concerned with the application amounts of the active ingredient and vary by crop type. There are also additional restrictions if managed pollinators are being used. Because there are additional restrictions for certain crops, the best way to understand how your crop will be influenced is to examine the regulations for each crop at this site, which has crop-specific factsheets: https://www.cdpr.ca.gov/docs/enforce/neonicotinoid/neonicotinoid_regulations.htm
There is another category of crop, those for which DPR determined that neonic applications might impact pollinators. This crop category includes avocados, figs, coffee, peanuts, and dates. For these crops, neonic use is prohibited during bloom. In addition, only one neonic active ingredient is permitted during the growing season, and one application method (foliar or soil). Neonic use is also prohibited on these crops when managed pollinators are used during the growing season.
Let's talk about which crops are NOT impacted. Those crops that are harvested before bloom, such as herbs, leafy greens, and bulbs (e.g. garlic) are not subject to the new regulations, and growers can still apply neonics according to the current label. Crops that are grown in enclosed spaces (e.g. greenhouses) or under insect structures (e.g. netted blueberries) are exempt from the new regulations.
There is another scenario that is exempt from the regulations -- any applications to crops to control a quarantine pest. This means that any application to control a quarantine-listed fruit fly or Asian Citrus Psyllid (ACP) are exempt from regulations. Let's talk about what this means for a lemon grower in Ventura County. Under the new regulations, they should not apply any neonics while the crop is in bloom, even if only a few flowers are open. This means no neonic applications for much of the year. However, a grower can apply neonics to their lemon bloom AT ANY TIME, including during bloom, if ACP is the listed targeted pest. To achieve this exemption, the grower must obtain a written recommendation from a licensed PCA for the ACP application, and they must keep this recommendation for two years.
If you have questions about the new regulations, feel free to contact me at UCCE Ventura County at hcohen@ucanr.edu.