Posts Tagged: Citrus Greening
Two more trees infected with huanglongbing (HLB) disease were identified and destroyed in the days before UC Cooperative Extension and the Citrus Research Board kicked off their spring Citrus Growers Education Seminar in Exeter June 27. The new infections raise the total number of HLB-infected trees in Los Angeles and Orange counties to 73.
The latest statistic set the stage for spirited discussions about a looming threat that cut Florida citrus production by 60 percent in 15 years. The devastating citrus losses in Florida were recounted by Ed Stover, a plant breeder with USDA Agricultural Research Service in Fort Pierce.
"One of the benefits of coming here is I am reminded how beautiful citrus is," Stover said. "In Florida, there are more than 130,000 acres of abandoned groves." He showed slides of trees with thin canopies, pale leaves and green fruit; in one image the trees were skeletons among tall weeds.
Huanglongbing disease is an incurable condition spread by Asian citrus psyllid (ACP). The psyllid, native of Pakistan, Afghanistan and other Asian regions, was first detected in California in 2008. Everywhere ACP is found, the pests find and spread HLB.
Stover and his colleagues are searching for citrus cultivars that have natural tolerance for the bacteria that causes HLB, but progress is slow. Transgenic citrus, he said, is the best bet for developing citrus with HLB immunity.
"In my opinion, commercial genetically engineered citrus is inevitable, and GE crop concerns will likely decline with time," he said.
In California, the aggressive push to keep psyllid populations low, regulations to limit the spread of psyllids when trucking the fruit, and active scouting for and removal of HLB infected trees in residential areas could buy time for researchers to find a solution before California suffers the fate of Florida citrus growers.
"Be vigilant," Stover said. "As long as you are still making a good return, there is almost no investment too great if it keeps HLB out of California."
Beth Grafton-Cardwell, UCCE citrus entomology specialist and director of the UC Lindcove Research and Extension Center near Exeter, said the prime research in the San Joaquin Valley is aimed at early detection techniques.
Once a tree is infected, it takes nine months to two years for the bacteria to spread throughout the tree, so that when leaves are selected for testing, they detect the bacteria. Capturing and testing psyllids is one way to to find the disease early. Other early detection techniques focus on the microbes, proteins and aromas produced by sick trees.
"These can be measured with leaf test, a VOC (volatile organic compound) sniffer, swab or even dogs," Grafton-Cardwell said. "Scientists are studying every conceivable way to stop the disease."
In the meantime, growers were encouraged to carefully monitor for and treat psyllid populations in their orchards with pesticides. Pesticide treatment recommendations are available on Grafton-Cardwell's Asian Citrus Psyllid Distribution and Management website, http://ucanr.edu/acp.
"We have lots of challenges," Grafton-Cardwell conceded. "We hate disrupting our beautiful integrated pest management program. But monitor your own groves, apply the most effective treatments and remove suspected (infected) trees. Going through the pain up front will save us in the long run."
Funding of the Citrus Research Board is an investment in pertinent research that supports the industry, making the information accessible to all within the industry from pest control advisors to packing houses to farm managers and others within the industry. The goal is to get the research done and then make sure it is used. CRB represents both large and small growers throughout California.
CRB research programs are funded by grower assessments which attract both federal and state funding, funding which represents a third of the total budget. This funding is used to support such projects as, HLB-resistant citrus rootstocks; the development of effective, low-cost HLB early detection technologies to rapidly remove infected trees; improved biocontrol methods for specific insect control like Asian citrus psyllid, as well as others; pre-and post-harvest citrus research to maintain export markets, amongst many other research programs.
The Citrus Research Board also supports the Citrus Clonal Protection Program (CCPP) with the goal of insuring the safe introduction of citrus varieties, disease diagnosis and pathogen elimination of introduced varieties and the maintenance and distribution of introduced varieties. CCPP serves as the primary source of clean, disease-free budwood and new varieties from Florida. This work is a collaboration between the Citrus Nursery Board, the University of California and State and Federal Regulatory agencies. The CCCP has become a major hub of the National Clean Plant Network for Citrus, resulting in the collaboration with 10 citrus centers in nine states and territories with multimillion dollar funding in support of CCCP's operations.
CRB research supports the California Citrus Quality Council (CCQC) with the primary objective of ensuring that California citrus meets domestic and international phytosanitary, food safety, food additive and pesticide residue regulations. CCQC ensures that California citrus growers have access to export markets for their fresh citrus fruit. Exports represent a third of the California citrus grower profits.
CRB-funded research into the California citrus-breeding program has led to the development of the Tango mandarin, along with others. The core breeding program conducts yield trials throughout the state on all varietal types to give growers information on upcoming new varieties and rootstocks. There is ongoing work to incorporate molecular tools to expedite breeding efforts to find plant materials resistant to HLB.
Along with CRB funding for cutting-edge projects for pest and disease control strategies, the CRB-funded CORE IPM Program led by Beth Grafton-Cardwell has responded to citrus grower needs for modifying existing spray schedule to treat Asian citrus psyllid. The program evaluates rotational sprays at appropriate times to avoid pesticide resistance to ACP.
Finally, this CRB-packaged information has been extended to growers through programs, including: The California Citrus Conference, Post-Harvest Conference and Seminar, and Regional Grower Education Seminars. CRB-funded research is compiled in Citrograph Magazine, the only magazine dedicated solely to the California citrus industry.
Three citrus trees that produce inedible fruit at the UC Lindcove Research and Extension Center in Visalia may be a game-changer for the citrus industry, reported Ezra David Romero on Valley Public Radio.
The trees are thought to be resistant to huanglongbing, a severe disease of citrus that has devastated the Florida industry and could become a serious problem in California. The citrus-saving potential of the three 34-year-old trees was outlined in an article by UC Agriculture and Natural Resources writer Hazel White in the most recent issue of California Agriculture journal.
UC Riverside citrus breeder Mikeal Roose collected seed from the trees and will test seedlings as soon as they are large enough.
"So what (breeders) have to do is cross this with some edible varieties and eventually create something that has the gene for resistance, but also the genes for good fruit," said Beth Grafton-Cardwell, Lindcove director and research entomologist.
Huanglongbing disease has cut citrus production in Florida by more than half. It's been found in residential citrus trees in Southern California, but hasn't reached the state's vast commercial orchards yet. Grafton-Cardwell said she expects the disease will arrive in 4 or 5 years.
Citrus huanglongbing (HLB) is a destructive disease with no known cure. To identify sources of (HLB) resistance in the subfamily Aurantioideae to which citrus belongs, we conducted a six-year field trial under natural disease challenge conditions in an HLB endemic region. The study included 65 Citrus accessio9ns and 33 accessions belonging to 20 other closely related genera. For each accession, eight seedling trees were evaluated. Based on quantitative polymerase chain reaction analysis of the pathogen titers and disease symptoms, eight disease-response categories were identified. We report two immune, six resistant and 14 tolerant accessions. Resistance and tolerance observed in different accessions may be attributed to a multitude of factors, including psyllid colonization ability, absence of pathogen multiplication, transient replication of the bacterium, lack of paht\\thogen establishment in the plant, delayed infection, or recovery from infection. Most citrus cultivars were considered susceptible: 15 citrons, lemons and limes retained leaves in spite of the disease status. Resistance and high levels of field tolerance were observed in many non-citrus genera. Disease resistance/tolerance was observed in Australian citrus relative genera Eremocitrus and Microcitrus. which are sexually compatible with citrus and may be useful in future breeding trials to impart HLB resistance to cultivated citrus.
2016 The American Phytopathological Society, vol. 100, N0. 9, p 1858-1868
Photo: Starch Accumulation in Infected Leaves
In a Fort Pierce, FL, field planting, plant growth, and Huanglongbing (HLB)
severity were assessed as indicators of HLB tolerance on progenies of 83 seed-source
accessions of Citrus and Citrus relatives mainly from the Riverside, CA, genebank. The
HLB-associated pathogen [Candidatus Liberibacter asiaticus (CLas)] and vector [asian
citrus psyllid (ACP), Diaphorina citri] were abundant, and trees were naturally
challenged for 6 years before metrics (leaf mottle, percent canopy mottle, overall health,
canopy density, canopy width, canopy height, and trunk diameter) were collected in Oct.
and Nov. 2015. The healthiest trees with low or no HLB symptoms were distant citrus
relatives: Balsamocitrus dawei, Bergera koenigii, Casimiroa edulis, Clausena excavata,
Murraya paniculata, and one accession of Severinia buxifolia. Within Citrus, most of the
healthiest trees with densest canopies, little leaf loss, and greater growth were those with
pedigrees that included Citrus medica (citron). These included progenies of Citrus hybrid
(‘Limon Real'), Citrus limetta, Citrus limettioides, Citrus limonia, C. medica, Citrus
volkameriana, and some Citrus limon accessions. Trees in this category exhibited distinct
leaf-mottle characteristic of HLB and substantial pathogen titers, but maintained dense
canopies and exhibited good growth. Trees from seed-source accessions in the genus
Citrus without citron in their background were generally among the least healthy overall
with less dense canopies. The exceptions were progenies of two Citrus aurantium
accessions, which were markedly healthier than progenies of other Citrus seed-source
accessions not derived from citron. Linear regression analysis, between metrics collected
and pedigree of seed parent, indicated that percentage of citron in the pedigree
significantly correlated with measures of tolerance. Although no commercial Citrus
genotypes yielded progenies with strong HLB resistance, in this field experiment several
progenies maintained dense canopies and good growth, and may be useful for breeding
HLB tolerant cultivars.
HORTSCIENCE 52(1):31–39. 2017. doi: 10.21273/HORTSCI11374-16
Photo: HLB Leaf Symptoms