Posts Tagged: disease
This is a reminder of the complexity of huanglongbing and the bacterial infection it causes. This abstract is from the HLB Conference in Florida last fall.
4.a.5 Symptom variations and molecular markers that illustrate the HLB complexity
Yongping Duan, Marco Pitino, and Cheryl Armstrong
USHRL-ARS-USDA, Fort Pierce, FL 34945, USA
Huanglongbing (HLB) is a devastating bacterial disease of citrus worldwide due to its intracellular and systemic infection. Various HLB symptoms are observed on different species/varieties of citrus plants: from yellow shoots to blotchy mottle on the leaves, from vein yellowing/vein corky to mosaic/green islands similar to zinc deficiency on the leaves, from whitish discoloration to stunted green leaves, etc. These variations of symptoms, which result from a combination of biotic and abiotic stresses, are not only present on individual plants from a variety but also exist on individual branches of an infected plant. Our results indicated that the adaptation of the bacterial populations, such as the dynamics of ‘Candidatus Liberibacter asiaticus' (Las), plays an important role in the induction of various symptoms and that Las mutations as well as the number and recombination events of Las prophages/phages affect this phenomenon. In addition, the selection of the host plants (resistance/tolerance) for the bacterial populations is also critical for symptom expression during disease progression. Based on severity, we divided HLB symptoms into four grades. It is worth noting that the grades of HLB symptom severity show a positive correlation with our newly identified biomarkers from host plants, and that gene expression profiling of different grades of infected leaves rationalized the differentiation based on the dynamics of these biomarkers. Because of these findings, we propose new approaches that allow for rapid selection of variant citrus plants, including bud sports with greater HLB resistance/tolerance.
Non-Technical Summary: Various symptoms of citrus huanglongbing display in different species/varieties of infected citrus plants. These variations of symptoms are not only present on individual plants from a variety, but also exist on individual branches of an infected plant. We have identified some molecular markers from the citrus plants and Las pathogen that illustrate the HLB complexity. Therefore, we propose new approaches that allow for rapid selection of variant citrus plants, including bud sports with greater HLB resistance/tolerance.
Recently, an outbreak of shoot and twig dieback disease of citrus has been occurring in the main citrus growing regions of the Central Valley of California (Fig 1). The causal agents of this disease were identified as species of Colletotrichum, which are well-known pathogens of citrus and other crops causing anthracnose diseases. At this time, it is unclear how wide-spread the disease is in California citrus orchards, but surveys are being conducted to evaluate the spread of this disease in orchards.
The disease was first noticed in 2012 by several growers and nurserymen in various orchards in the Central Valley. Symptoms included leaf chlorosis, crown thinning, gumming on twigs and shoot dieback, and in severe cases, branch dieback of trees (Fig.2). The most characteristic symptoms of this disease are the gum pockets which appear on young shoots either alone or in clusters and the dieback of twigs and shoots (Fig.3). These symptoms were primarily reported from clementine, mandarin, and navel orange varieties. In order to determine the main cause of this disease, field surveys were conducted in several orchards throughout the Central Valley. Isolations from symptomatic plant samples frequently yielded Colletotrichum species.
Field observations indicate that symptoms initially appear during the early summer months and continue to express until the early fall. Trees showing dieback and gumming symptoms characteristic of this disease are usually sporadic within an orchard and generally only a few twigs or shoots are affected within a tree. Morphological and molecular phylogenetic studies allowed the identification of two distinct species of Colletotrichum (Colletotrichum karstii and Colletotrichum gloeosporioides) associated with twig and shoot dieback. Interestingly, these Colletotrichum species were also isolated from cankers in larger branches. Although C. gloeosporioides is known to cause anthracnose on citrus, a post-harvest disease causing fruit decay, it has not been reported to cause shoot dieback of citrus. C. karstii however has not been reported previously from citrus in California and our laboratory is currently conducting field and green house studies to determine the pathogenicity of this species in citrus.
At present, it is unclear how widespread this disease is in California orchards or how many citrus varieties are susceptible to this disease. Pest control advisors are advised to remain alert and monitor citrus trees for the presence of the disease in the Central Valley (particularly clementine, mandarin, and navel varieties) during the early summer months. Continuing research lead by Dr. Akif Eskalen (UC Riverside) in collaboration with Dr. Florent Trouillas (Kearney Agricultural Research and Extension Center), Dr. Greg Douhan (UCCE Farm Advisor Tulare County), and Craig Kallsen (UCCE Farm Advisor in Kern County) is focused on further understanding the biology of the fungal pathogens as well as factors influencing disease expression in order to develop management strategies against this emerging disease.
Shoot dieback symptoms on Clementine
Branch dieback symptoms on Clementine
Gumming symptoms on Clementine
(photos: A. Eskalen)
The recent Huanglongbing Conference in Orlando, FL was chock full of people and ideas. Some of the ideas were still in the fermentation state and some were in practice on farm. One of the ideas that has been put into practice is the use of antibiotics, such as tetracycline and streptomycin to control the bacteria, Candidatus Liberibacter asiaticus (CLas), which causes HLB or citrus greening. This is somewhat disturbing since the rise of antibiotic resistant bacteria which affects humans has been affected by the wide-spread use in animal production facilities. This has led to some food companies to discontinue the sourcing of meat from animals treated with antibiotics for non-health reasons.
Antibiotics are molecules that limit the growth or reproduction of bacteria. They come under the umbrella of bactericides which include antibiotics, but also disinfectants like bleach and copper sulfate and antiseptics like peroxide, iodine and alcohol. Antibiotics when properly used will not harm human tissue and can be derived from bacteria, fungi and synthetically and will often act directly on the bacteria that is causing the disease. Some of these molecules can be simple assemblages of amino acids called peptides (etymology “to digest”) or strings of peptides called proteins. And sometimes they do not work on the bacteria itself, but on steps that lead up to processes that make the bacteria effective at its job.
At the conference, several papers were presented that illustrated this type of antibiotic effect. One of these papers was presented by Robert Shatters for his group. The peptide they are looking at actually inhibits the movement of the CLas bacteria in the gut of the insect, reducing or possibly preventing the transmission of the bacteria to the host plant – citrus.
The following is an abstract from the paper.
Identification of gut epithelium binding peptides that reduce systemic movement of ‘Candidatus' Liberibacter asiaticus within the Asian citrus psyllid vector
Robert G. Shatters, Jr1, Dov Borovsky1, El-Desouky Ammar1, David Hall1, Kasie Sturgeon2, EricaRose Warwick2, Marc Giulianotti3, Radleigh G Santos3 and Clemencia Pinilla4
1USDA, ARS, USHRL, Fort Pierce, FL USA; 2University of Florida, CREC, Lake Alfred, FL USA; 3Torrey Pines Institute for Molecular Studies, Port St Lucie, FL USA; Torrey Pines Institute for Molecular Studies, San Diego, CA USA.
Non-Technical summary: The Asian citrus psyllid is the only known vector of the bacteriumthat causes citrus greening disease. This insect acquires CLas from an infected citrus tree while feeding as a nymph. Transmission to uninfected trees occurs when infected adults emerge and fly off and feed on uninfected trees. Our current understanding of the CLas-psyllid interaction suggests that adults become competent for transmission only after the bacterium moves from the insect gut into the hemolymph and then to the salivary glands. We have identified a set of small peptides that when fed to the psyllid, bind the gut membranes and reduce the ability of the citrus greening bacterium to move from the gut to the salivary glands. These peptides are now being tested to determine if they can be used as an effective way of reducing the spread of citrus greening disease.
This and other paper abstracts will soon be available at: http://irchlb.org/files/33373ab0-7df3-4117-9.pdf
photo: HLB Symptoms
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.
Topics in Subtropics
January - March Topics in Subtropics 2017
In this issue:
- Revisiting an old study on high density citrus orchards
- Shoot and Twig Dieback in Citrus
- Alternative Crops or ......
- Referendum Comments Citrus Research Board