- Author: Ben Faber
The word is getting out. If you have yellowish leaves, cupped/upright and the fruit is small, it may not be Huanglongbing (HLB), but it sure seems like all of my neighbors think so. It could be just lack of water, and in a drought, that could be the most likely cause. But there are other causes of symptoms that might be associated with HLB. Citrus Stubborn Disease causes these symptoms, but also distorted fruit and shriveled, discolored seeds, and bitter fruit like HLB. Under very hot conditions, leaves on some shoots may have misshaped, blunted yellow tips with mottling similar to the nutritional deficiencies seen in HLB. The leaves can have shortened internodes, so there's a bunchy growth habit like in zinc deficiency. Fruit are small, sparse and have early drop. Again, a lot like HLB. Even more so, though Stubborn can cause stunted, thin canopies. Misshapen fruit, though can be caused by bud mites in lemon, chimeras (spontaneous mutations) and Tristeza (a viral disease). Frost damage can add further to the confusion about symptoms that might be associated with HLB.
Citrus Stubborn Disease is a serious disease that leads to reduced fruit quality and yield. It occurs most commonly in oranges, but does show up in other citrus including lemon and mandarin. It's more commonly seen in older trees that were initially propagated with infected tissue, and growing in hot, dry environments. Unlike HLB, it doesn't lead to the death of the tree, just major loss of income. It's caused by a phytoplasma (a bacteria without a cell wall) and is spread by a leafhopper. There are other hosts like mustard and cabbage that can harbor the organism to be spread to new tissue and especially young trees. Warm winters favor the spread of the infected leafhopper, like we have had this winter.
So how do you distinguish between all the possible causes that look like HLB? HLB can be tested for as well as Tristeza. There are no commercial labs that check for Stubborn. It's basically a process of elimination then to decide to test for HLB or to think that the tree has the disease. So, know your trees and their history. Was there a freeze this winter? Know the effects and symptoms of drought and monitor soils and trees for water stress. Check for bud mite. It also takes time from the point of infected Asian Citrus Psyllid invasion to the time when symptoms of HLB start showing up in the tree. So keep your eyes open, but don't assume the worse at this point.
More on Tristeza:
http://www.ipm.ucdavis.edu/PMG/r107101311.html
Stubborn:
http://www.ipm.ucdavis.edu/PMG/r107101211.html
Images:
Misshaped fruit from Stubborn
Discolored seeds from Stubborn
Reduced canopy size from Stubborn
- Author: Nastaran Tofangsazi and Beth Grafton-Cardwell, Department of Entomology, UC Riverside
With the detection of Huanglongbing (HLB) in California in 2012 and 22 additional cases reported during 2015 through June 2016 there is a major concern among citrus growers about the spread of this incurable bacterial disease. The vector of the disease, the Asian citrus psyllid (ACP), is a hardy insect with good dispersal capabilities and can be found in many southern California citrus groves today. With no direct cure for HLB at present, the only option for growers to combat the disease is to control the psyllid. This can prove difficult for conventional citrus growers with broad spectrum insecticides, but for organic citrus growers, which grow an estimated 7% of citrus in California, the task is even more difficult with the currently available options.
Entrust (spinosad) + oil, Pyganic (pyrethrin) + oil, and oil alone are currently the recommended and most widely used insecticide options for organic growers (UC IPM Guidelines for Citrus). While these insecticides are fairly effective in killing ACP if they make direct contact, the residual life of these pesticides is very short (days) compared to conventional insecticides (weeks to months). For example, in our petri dish studies, 10 fl oz/acre Entrust SC + 0.25% Omni supreme spray oil caused 89% mortality, 17 fl oz/acre Pyganic 5.0 EC + 0.25% Omni supreme spray oil caused 73% mortality and 0.25% Omni supreme spray oil caused 42% mortality when 1st-2nd ACP nymphs were exposed to treated leaves one day after application. Nymphal mortality continued to decline for the Entrust + oil treatment (69% mortality) and even more severely declined for Pyganic + oil (27% mortality) 3 days after treatment. In contrast, one-day-old residues of a conventional insecticide, the neonicotinoid 5.5 oz Actara (thiamethoxam), resulted in more than 95% mortality of nymphs and mortality remained high for more than a month.
Studies of grower orchard treatments confirmed laboratory studies that showed a short residual effect of organic treatments (Entrust + oil and oil alone) compared to conventional insecticides (Actara). We monitored changes in population densities of ACP (adults by tap, nymphs and eggs by flush examinations) in the fall of 2015 before and after a grower sprayed separate orchards with one of three insecticides; 1) 1.25% 440 Supreme Spray Oil by ground application (400 gpa), or 2) 9 fl oz Entrust SC + 1% oil by air (50 gpa), or 3) 5.5 oz Actara by air (50 gpa). The oil treatment had little effect on the adult population, but significantly reduced psyllid nymph densities for 17-24 days. Entrust was completely ineffective in controlling psyllid nymphs, but suppressed adult and egg populations for about 14 days. Actara, a conventional insecticide, was the most effective treatment in the study and provided more than 5 weeks of both adult and nymph control. Because of the short residual effect of organic insecticides in citrus, repeat treatments are needed at a frequency of about every 2 weeks for ACP control.
Tamarixia radiata wasps released for biological control of ACP provide 20% to 88% parasitism depending on geographical location and time of year. If there were no disease to be concerned about, this level of parasitism by Tamarixia would be sufficient to protect citrus from the feeding damage of the psyllid. However, the disease spreads rapidly with just a few psyllids and so a greater level of control is needed. Generalist predators, such as lady beetles, lacewings and assassin bugs, also assist with control. Argentine ants can severely disrupt this parasitism by protecting psyllids from natural enemies. Unhappily, Entrust + oil, thought of as a very selective insecticide combination, was found to be highly toxic to adult Tamarixia wasps exposed to 3 day old residues. Thus, the organic insecticide that is the best for controlling the psyllid pest is not compatible with the parasitoid natural enemy, limiting our ability to use integrated strategies to control the psyllid.
At present, it is not mandatory, but is strongly recommended, that all southern California citrus growers treat their orchards in an area wide manner. The area wide program consists of coordinated treatments twice a year (winter and fall), and additional treatments in between. Due to the short residual nature of organic insecticides, organic applications should be applied twice within 10-14 days of each other for every single conventional insecticide application. This is especially important for younger groves as ACP nymphs thrive in new flush. Organic growers have a tough decision to make between treating frequently for ACP and the high cost associated with those treatments or transitioning into conventional management in order to more effectively control ACP. Additional solutions are needed for organic citrus.
UC IPM Guidelines for Citrus: Asian Citrus Psyllid. http://ipm.ucanr.edu/PMG/r107304411.html
- Author: Mark Hoddle
Extension Specialist in Biological Control, Department of Entomology, University of California Riverside, CA 92521mark.hoddle@ucr.edu
Well, what a dramatic title, and it is certainly an unabashed rip off from Winston Churchill too! However, many growers in California view the threat posed to the long term viability and profitability of citrus by Asian citrus psyllid (ACP) and huanglongbing (HLB) as a war! ACP is a small sap sucking insect that is native to the Indian subcontinent and it has emerged as a global threat to citrus because it vectors a bacterium (Candidatus Liberibacter asiaticus [CLas]) that causes a lethal and incurable citrus disease, huanglongbing (sometimes referred to as citrus greening). Trees displaying HLB go into decline and some varieties may die in as little as 5-8 years post-infection. Symptoms may include chlorotic leaves (Fig. 2) and premature fruit drop. Fruit that are retained become misshapen as they develop, ripen irregularly (hence the common name citrus greening), and have a bitter taste. Florida's “war” with ACP and HLB started around 1998 when the psyllid was first found, then in 2005 the first citrus trees with HLB symptoms were discovered. Some economic estimates of the ACP-CLas epidemic in Florida suggest that more than 8,000 jobs have been lost, production has declined by 23%, and revenues have dropped by 16% .
ACP was first detected in southern California in 2008, and in 2012, the first citrus trees with HLB symptoms were found in Hacienda Heights in Los Angeles County. Subsequently, HLB has been confirmed in San Gabriel, also in LA County. Detections of ACP in the San Joaquin Valley (SJV) are increasing, especially around Bakersfield, and these finds make the largest citrus production areas in California highly vulnerable to CLas. One response to the ACP invasion has been to go after the vector in urban residential areas where ACP enjoys a relatively good life on backyard citrus trees that are treated infrequently with pesticides. The intention of this urban-residential-ACP control program was to knock back psyllid numbers to low levels with the goal of reducing the rate of spread of ACP and CLas through urban areas and into commercial citrus production zones. Vector reduction have been attempted in two different ways; spraying insecticides and biological control. The California Department of Food and Agriculture aggressively pursued residential spraying of citrus during the initial stages of the ACP campaign in southern California, but this quickly became unsustainable due to cost and the speed at which ACP was spreading through the urban landscape. Spraying is still used in areas were ACP populations are small and localized, such as those being found in parts of the SJV.
The second approach has been to run a classical biological control program targeting ACP. Classical biological control introduces natural enemies from the home range of the pest into the invaded area with the goal of establishing these beneficial agents so that they permanently suppress pest populations to less damaging levels. This approach necessitated searches for ACP natural enemies in the native range of the pest, a process referred to as foreign exploration. Because ACP has a huge putative native range, foreign exploration was conducted in Punjab Pakistan (Fig. 3), an area with ~70% climate match with the major citrus production areas in the SJV. A good climate match, in theory, should result in natural enemies that are pre-adapted to California's hot dry summers and cool damp winters. From September 2010 to April 2013, six expeditions searching for ACP natural enemies were conducted in Pakistan. The University of Agriculture Faisalabad (UAF) was the home base for this project. Collaboration with UAF Faculty was excellent, and the Vice Chancellor of UAF, Dr. Iqrar Khan, is a UC Riverside graduate in plant pathology, and Mike Roose was Iqrar's major professor!
Two species of parasitoid, Tamarixia radiata and Diaphorencyrtus aligarhensis, were found attacking ACP in Pakistan. Both species were returned to the Insectary and Quarantine Facility at UC Riverside. Mandatory host range and host specificity tests were conducted over a 2-3 year period and results indicated that both species likely posed little environmental risk to California and it was concluded that both species offered significant benefits because of their ability to parasitize and feed on ACP nymphs. Both of these processes kill ACP nymphs. Consequently, USDA-APHIS issued release permits and CDFA took over the mass rearing of these parasitoids. As of June 2016, more than 3 million Tamarixia and 170,000 Diaphorencyrtus have been released in southern California.
Initial results of the ACP biocontrol program are promising. Tamarixia appears to have established widely in southern California, and in combination with other species of natural enemies, especially generalist predators like lacewing and syrphid fly larvae, significant reductions in ACP populations have been documented. Diaphorencyrtus lags behind Tamarixia because it was the second parasitoid out of the quarantine pipeline, but multiple recoveries have been made at about 60% of sites where this species has been released. However, the impact and rate of spread of Diaphorencyrtus and whether it can compete successfully with Tamarixia is unknown and the subject of Citrus Research Board (CRB) and USDA-MAC sponsored research.
Although the major biocontrol efforts have centered on ACP in residential areas in southern California, the focus of the biocontrol program, especially with respect to Tamarixia, is beginning to face northwards, and battle lines are being drawn around and through Bakersfield and Tulare. The redirection of effort has been the subject of intense discussion at recent CRB BioControl Taskforce meetings. The emerging consensus is that movement of biocontrol agents into urban areas north of the Tehachapi mountains is needed. It is highly likely that in late 2016 or early 2017 Pakistani mercenaries will be released for the first time into the SJV to hunt down ACP.Asian citrus psyllid, Diaphorina citri, is an invasive pest in California. Immature psyllids, the nymphs (A) acquire the HLB-causing bacteria when feeding on infected plants. (B) Adult psyllids can carry bacteria between trees and inoculate healthy plants when feeding (Photos by Mike Lewis, Center for Invasive Species Research, UC Riverside)
ACP and Nymph
Tamarixia
- Author: Ben Faber
Beth Grafton-Cardwell our IPM Specialist who is Lindcove Research and Extension Director and a UC Riverside Entomologist recently gave a talk on the different approaches being taken to confront Asian Citrus Psyllid and Huanglongbing. A summary of a lot of her points is available at:
http://ucanr.edu/repositoryfiles/ACP_Fact_Sheet-101755.pdf
She brought up some points that I think need to be better known. Some early detection techniques are being developed so that infected trees can be quickly identified and removed so that they do serve as a reservoir of inoculum that can increase the spread of the bacterial disease. These techniques are based on measureable levels of different chemicals.
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Sick trees produce different volatiles (Volatile Organic Compounds can be “sniffed” by machines or trained dogs)
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Sick trees produce proteins that can be measured
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Trees produce small RNAs in their defense response that can be measured
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The bacteria produce proteins that can be measured
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The micro-organisms associated with sick trees are different than those associated with healthy trees and these can be measured.
All of these techniques are being tested out right now and being refined. It will mean faster diseased tree identification and removal. This will still mean chemical control of the psyllid to control disease spread.
Early detection is just one of the techniques being employed to fight this insect/disease complex. Every conceivable possibility is being explored, including:
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Psyllid traps – attract and kill
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Psyllid deterrents – chemicals that would drive psyllids away from citrus
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Antibiotic treatments to control the tree infection
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Heat treating trees to destroy the bacteria in the tree
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Resistant rootstocks and scions (traditional breeding and genetic engineering)
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Utilize an altered citrus tristeza virus to introduce anti-HLB genes into plants (Genetically Engineered)
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Altering the psyllid so it can't vector the disease and releasing the ‘nupsyllid' to replace the wild ones (GE)
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Treat trees with chemicals (interference RNAs) that prevent the psyllid from picking up the disease
Wow. Many of these are a long way off, but some might be coming out soon. We still need to deal with the psyllid so that when infected insects become more widespread, the disease will not spread as fast as it has in Florida and other citrus growing areas.
- Author: Ben Faber
What this means is that there is different feeding behavior on different scion varieties that is unaffected by the rootstocks used in this study. This does not mean "a" rootstock can not have an effect, just that the ones used in this trial did not.
EFFECT OF DIFFERENT CITRUS SCION AND ROOTSTOCKS
COMBINATION ON FEEDING OF Diaphorina citri
Alves GA1, Beloti VH1, Carvalho SA2 & Yamamoto PT1
1Escola Superior de Agricultura ‘Luiz de Queiroz'/
Universidade de São Paulo, Piracicaba, SP, Brazil; 2Instituto
Agronômico (IAC), Centro de Citricultura, Cordeirópolis, SP,
Brazil; e-mail: gralves@usp.br
The Asian citrus psyllid (ACP) is the vector of bacteria associated to the huanglongbing and has a host range of more than 50 species of the Rutaceae family. The knowledge about the feeding behavior in different hosts can show useful aspects for future studies of plant resistance and ACP management. Therefore, was evaluated the effect of different combinations of scion and rootstock of citrus in the feeding of ACP adults. For this, we tested Valencia, Pera and Hamlin sweet orange, Ponkan mandarin and Sicilian lemon grafted on Rangpur lime and Sunki mandarin rootstocks.
The flushes were individualized with cages made of transparent plastic cup and “voile” tissue. To collect the honeydew, discs of filter paper were placed at the base of each flush. The adults fed for a period of 72 h. After this, the discs were removed and immersed on the ninhydrin solution. After 24 h, the drops area of honeydew was determined using the Quant software. The feeding was more intense on sweet orange varieties, with a highest value observed to Valencia (0.902 cm2) and the smaller area to Ponkan mandarin(0.269 cm2). Unlike observed for scion varieties, when different rootstocks for the same scion variety were tested, no difference was observed in the consumption of ACP.
Rootstocks can have a tremendous effect, but not in this case with the rootstocks used