HLB in Texas: Steps and challenges to curb this threat

HLB in Texas: Steps and challenges to curb this threat Mamoudou Sétamou, John da Graça and Ray Prewett Background Citrus trees are part of the landscape in Texas, but commercial citrus is mostly confined to the three southernmost counties of the state. The Texas citrus industry is a multimillion-dollar business supplying essentially fresh fruit to the market. The quality and reputation of our red grapefruit, the official state fruit of Texas, go beyond our borders. In recent years, the sustainability of our citrus industry is being threatened by citrus greening disease, or huanglongbing (HLB), and its vector, the Asian citrus psyllid (ACP). ACP has become a major pest in Texas in the last decade. It is economically important because it is a known vector of the putative bacterial causal agent of HLB. There is no known cure for the disease, and infected trees slowly decline and ultimately die, hence the disease s nickname of industry killer. Sprawling citrus orchards mixed with abundant backyard citrus trees that are largely unmanaged in the commercial citrus producing areas of south Texas make a perfect environment for the explosive spread of the psyllid in Texas. nymphs, and leaf tissue with suspect HLB symptoms collected during these surveys were sent to the USDA-APHIS diagnostic laboratory in Raleigh, NC, for qpcr testing for the causal Liberibacter bacteria; none was positive. With the detection of HLB in 2008 in Louisiana, a bordering state of Texas to the east, three major developments took place to strengthen mitigation efforts of HLB in our state. The first was the certification of TAMUK-CC by USDA as an HLB diagnostic laboratory, meaning that insect and tissue testing for Liberibacter could be done in Weslaco. Only those samples which give a possible positive reading are sent to the USDA for confirmation in case of a first detection. The second was the launch of an intensified survey in all ecological settings where citrus is found in Texas. A commercial grove survey was led by TAMUK-CC, and a sentinel residential tree survey was conducted by USDA-APHIS. In this latter survey, cities in the Lower Rio Grande Valley (LRGV) were divided into 1-square-mile grids with 72 sentinel trees per grid. Special attention was given to cities close to the border with Mexico. USDA-APHIS also added commercial orchards to its survey. The Texas Department of Agriculture The short-term strategy in Texas has been to slow the spread of the disease by reducing the ACP population to the lowest possible level. Growers and stakeholders can no longer ignore its threat. Soon after its first detection in 2001, the potential impact of ACP was underestimated, and efforts were not directed toward controlling its population and spread. At that time, HLB was not confirmed in the U.S. so while the insect s presence was noted, limited attention was given to it. However, ACP pest status quickly changed with the detection of HLB in Florida in the fall of 2005. In response to this wake up call, the Texas A&M University-Kingsville Citrus Center (TAMUK-CC), in collaboration with USDA-APHIS and the Texas citrus industry, initiated statewide surveys in 2006 for the early detection of HLB and implementation of mitigation efforts. Concurrently, ACP pesticide testing began at TAMUK- CC to identify the most effective pesticides that can be incorporated into our citrus production systems for both conventional and organic growers. We learned that the psyllid had not only become endemic in the commercial citrus belt of Texas but also from Big Bend in the west to the upper Gulf of Mexico coast in the east. During the next two years, samples of psyllid adults and (TDA) concentrated its efforts on collecting samples from nurseries. Psyllid samples and leaf tissue with suspect foliar symptoms were collected and tested at the Citrus Center. The third effort was the development of a psyllid control program on an area-wide basis for commercial growers. For the last few years, warned by the devastation caused by HLB in Florida, Texas has been preparing and taking steps for a proactive mitigation of HLB. We have been wondering when our time would come; that time has come. On January 13, 2012, citrus greening was confirmed in the Lone Star State. Development of an ACP area-wide management program in Texas In accordance with the recommendation of the National Science Foundation, measures to reduce the incidence and spread of HLB were set forth to protect the Texas citrus industry. Since HLB was not known to occur in Texas as of 2008, and only its psyllid vector was present, restriction on the movement of all plants in the family Rutaceae and an implementation of an aggressive vector control program 32 Citrograph September/October 2012

were decided to protect the state citrus industry. TDA adopted regulations to restrict movement of citrus nursery stock into the citrus zone -- the eight southern-most counties known to produce commercial citrus. Concurrently with all the regulations to restrict plant movement, TAMUK- CC and USDA APHIS-CPHST partnered to develop a psyllid area-wide management program adapted for the Texas citrus production system. A twostep approach was used in the development of this program. In a multi-locational trial covering 800 acres in 2008, we demonstrated that effective control of psyllid is achieved when chemical sprays are applied when no young flush shoots are present and consequently, there are no immature psyllid developing on trees. Thus, we established the importance of spray application during the dormant season No. of ACP adults/trap from early November to early February in south Texas and again prior to major flush cycles during the active growing season to target psyllid adults before they lay eggs on new flush growth. However, due to the frequent movement of adult psyllids between groves, effective psyllid control can only be achieved if spray applications are done in a coordinated fashion for groves within a specific geographic area. The larger 10 8 6 4 2 0 1/1/2007 7/1/2007 1/1/2008 7/1/2008 1/ 1/2009 7/1/2009 1/1/2010 7/1/2010 1/1/2011 7/1/2011 1/1/2012 Fig. 2. Asian citrus psyllid (ACP) population trends in commercial citrus groves in Texas. Effect of coordinated dormant sprays in a voluntary area-wide management implemented by growers. (Arrows indicate coordinated dormant sprays). the area covered within a short period of time (preferably within two weeks), the better the level of control achieved. Validation tests for the area-wide management program were performed in grower participatory trials over a two-year period in 2009 and 2010. Approximately 1,400 acres and 4,674 acres belonging to 17 growers and 53 growers, respectively in 2009 and 2010, were included in this validation pilot project. September/October 2012 Citrograph 33

34 Citrograph September/October 2012 Two dormant spray applications (one in November and another one in early February) to target overwintering psyllid populations were applied in a coordinated fashion within a two-week window for each spray. Due to the diversity of ongoing grove care practices such as hedging, fertilization, irrigation, and to some extent harvesting, it was more difficult for growers to coordinate spray application during the active growing season. Thus, growers made their active growing season spray decisions based on adult psyllid numbers and tree phenology of a specific grove. Outreach and adoption of the psyllid area-wide management by growers Concurrently with the development of the area-wide management program, an outreach program was initiated to educate growers on the threat of HLB and the need to control its psyllid vector. From October 2009 to January 2010, several grower meetings were organized where details of the area-wide management program were presented, and growers were encouraged to voluntarily adopt and implement this proactive psyllid control program in their groves. Sign-up and reporting forms were filled out by growers participating in the voluntary area-wide management program. A coordination team was created under the leadership of the Texas Citrus Mutual. Growers report their acreage sprayed during the dormant season and their pesticide spray records to that team. This coordination team also organized grower refresher meetings twice a year, in October and January, to reinforce the need of not letting our guard down in the battle against HLB and psyllid. From the beginning of the program in January 2010 to the last coordinated dormant spray, a satisfactory participation rate has been observed, but efforts are still needed to cover the entire commercial citrus acreage. Figure 1 presents the total acreage covered during the different coordinated sprays in commercial citrus. Despite a moderate start of ca. 57%, total acreage under voluntary area-wide management of psyllid exceeded 80% in the last two years. Monitoring of psyllid population has been ongoing in 20 sentinel groves from 2006 to 2008, and 33 additional groves from the pilot project were added to the sample pool in 2009. Fig. 1. Grower participation in the voluntary psyllid area-wide management program in Texas from 2010 to 2012. Values represent the acreage sprayed at least once during the dormant season relative to total commercial citrus acreage in Texas.

From 2010, a total of 62 groves were monitored biweekly for psyllid. All but 12 of these groves are under area-wide management. Adult psyllid populations were monitored using yellow sticky cards. Figure 2 summarizes the trend in psyllid populations in Texas citrus groves from 2006, when no specific control was directed at psyllid, to 2012 when area-wide management is being implemented at the grower level. It is very clear that psyllid populations have generally been on the decline in commercial citrus from the onset of the area-wide management program in Texas. Despite this substantial decrease in psyllid populations, more efforts are needed to get the buy-in of all growers and to ensure that psyllid control is done season-long. Nursery practices Nursery production in Texas has traditionally been in open fields. Since the late 1990s, the Citrus Center has managed a mandatory budwood certification scheme. With the threat from HLB, a collection of foundation trees was placed in a screenhouse in 2008. A grant from the US Department of Commerce was obtained to construct a greenhouse complex for budwood increase production which is now being populated with plants propagated from the protected foundation trees. Following the Florida example, a committee of nurserymen and growers has been formed to plan for future nursery practices. They have agreed that nurseries producing trees for orchards should be under screen by September 2013. At Fig. 3. First HLB-positive tree detected in a commercial Valencia orange grove in Texas, January 2012. the moment, this is a voluntary agreement, but discussions with TDA and nurseries producing for the homeowner market are ongoing. Detection of citrus greening in Texas In late December 2011, USDA-APHIS inspectors collected leaves from a 6-year Valencia orange tree with suspicious leaf symptoms located near the southeast corner of an orchard in San Juan, TX, just six miles north of the Rio Grande. The Citrus Center lab performed qpcr on the sample, and obtained a result indicating the presence of the HLB September/October 2012 Citrograph 35

Fig. 4. Distribution of HLB-infected trees in commercial Valencia orange (left) and Rio Red grapefruit (right) blocks suspicious samples collected but tested negative for Candidatus in Texas, June 2012 ( indicates HLB-positive trees, Liberibacter asiaticus, healthy tree with no apparent symptoms and tested negative for Liberibacter, and indicates newly planted healthy citrus trees; the arrow indicates the first HLB-positive tree found in the grove). Picture courtesy of USDA-APHIS on data collected by Texas &M University-Kingsville Citrus Center. bacterium. USDA-APHIS was immediately informed, the tree was resampled, and confirmation qpcr was performed in Beltsville. On Friday, January 13, the official confirmation of the disease in Texas was announced. TDA established a 5-mile radius emergency quarantine zone around the site which prohibited the movement of nursery stock out of the area, and required that fruit harvested had to be free of leaf material. Growers voluntarily suspended harvesting until a protocol was developed and issued by TDA. The initial confirmed tree appears to have been the first one infected in the orchard (Figure 3). It was smaller than other trees and had classic symptoms of blotchy mottle, corky veins, lopsided fruit with aborted seed and vascular browning, and twig die back on all sides. It had most likely been infected for approximately 4 years. Both affected orchards belong to the same grower who has initiated a program to remove the infected trees. He also intensified psyllid control under the guidance of the Citrus Center, ensuring that other pests were also controlled. An intensive survey of all the orchards as well as residential citrus in the quarantine zone was initiated in a combined operation by USDA-APHIS and TDA. Some additional infected trees were identified in the Valencia orchard, as well as in a 5-year-old grapefruit orchard immediately to the east. No other infected trees in orchards or dooryards have been detected. The Citrus Center conducted monthly tree-by-tree surveys in the two orchards and found 55 infected Valencia and 18 grapefruit trees. In each orchard, most infected trees are in clusters (Figure 4). There are some unusual aspects to the Texas HLB situation. The average time for HLB confirmation after the first detection of ACP in several nearby states and countries has been about 7 years; in Texas, it was 11 years. In Belize, Mexico, Louisiana and California, the first positive detection was in psyllid samples collected from dooryard trees. In Texas, the first detection was in an orchard tree; since January 2012, nearly 5,000 psyllid samples have been tested, but only three have been confirmed as positive. Both affected orchards belong to the same grower who has initiated a program to remove the infected trees. He also intensified psyllid control under the guidance of the Citrus Center, ensuring that other pests were also controlled. Conclusion and future plans Texas has taken proactive steps that have proven successful in reducing ACP populations in commercial citrus groves through an area-wide management program. This program was initiated shortly after citrus greening was discovered in Louisiana and is being ramped up to a higher level now that the disease is present in the state. Texas growers have been serious about ACP, and that effort has provided Texas with a head start in slowing the spread of the disease. Experience with this disease around the world is that the best outcome that can be achieved is to slow the spread of the disease; eradication has not been successful, although Louisiana could turn out to be the exception in efforts to eradicate the disease. The short-term strategy in Texas has been to slow the spread of the disease by reducing the ACP population to the lowest possible level. For the long term, Texas is counting on the development of trees resistant to the disease as the most sustainable long-term solution, and while the focus of this article is on the short-term strategy, Texas is also at the forefront of developing disease-resistant trees. Growers in the immediate area around the site where the disease was detected are very motivated to aggressively control ACP. The initial reaction from growers throughout 36 Citrograph September/October 2012

the Rio Grande Valley was that the disease could be widespread, but because of the intensive surveying before and after the detection of the disease, growers are realizing that this might not be the case. There has been a tendency for our growers to let their guard down a bit in recent months, but the industry leaders are working hard to ramp up the ACP control effort in all commercial citrus. The goal going forward is to ramp up ACP control throughout the commercial production area but particularly in the quarantined area where the disease has been identified. The trees where the disease was initially detected have been removed. The citrus industry is working with the grower to continue to remove all subsequently identified infected trees. While many in the industry would like for the trees to have been removed as soon as they tested positive, the grower has been extremely diligent and consistent in monitoring and controlling the vector. The first round of an intensified survey in the quarantined area has been completed, and so far no additional tissue has tested positive except the trees in the first two groves confirmed early this year. These survey efforts are continuing. What challenges lie ahead for Texas? If you compare the situation in Texas to that of Florida and Brazil, some of the obvious differences are that grapefruit is the dominant citrus type in Texas grown on over 70% of the total acreage and grapefruit tends to harbor less psyllids than sweet oranges, limes or lemons. Most of our citrus acreage is irrigated by flood irrigation. With this type of irrigation, large amounts of water are applied less frequently than is the case with other types of irrigation, including drip and micro-sprinkler systems. Our scientists have observed that less frequent irrigation and less annual rainfall means fewer synchronized flush cycles which confine psyllid reproduction to specific times of the year, and this is generally associated with lower ACP populations. One of the biggest challenges in Texas is the small size of the average block of citrus. Smaller blocks of citrus means Texas will have to deal with more edges than citrus production areas where the blocks are much larger. Research in Texas and in other areas has clearly demonstrated that ACP and HLB are concentrated on the edges of groves more than in the interior of groves. Texas and California have a common challenge with ACP being widespread in dooryard citrus that is near commercial citrus. A large amount of the commercial citrus in Texas is interspersed with a wide variety of citrus in backyards. On the other hand, in the case of Florida, their focus is more on addressing abandoned groves more so than backyard citrus because much of their commercial citrus generally has more separation from backyard citrus. Growers in Texas have a strong incentive to control ACP while homeowners are generally not as motivated, at least by economics, to control this pest. Most homeowners are not on a regular pest control regime; however, they are very receptive to having treatment conducted on their property. The challenge in implementing a treatment program in backyards is the lack of state or federal resources to pay for such treatments. Biocontrol strategies of ACP using parasitoids and entomopathogenic fungi are in the field testing stage in Texas and will be an important component in the near future for psyllid control in non-commercial citrus and in organic production. September/October 2012 Citrograph 37

HLB is widespread in Mexico. Although it has not been confirmed in Tamaulipas, the state immediately across the Rio Grande River, drug violence along the border has made early detection of HLB very challenging in Mexico. Backyard citrus is very common right across the Rio Grande River in cities like Reynosa and Matamoros, but the larger plantings of citrus in Mexico are over a 100 miles south of the border. The Rio Grande Valley is a major and rapidly growing corridor for the importation of commercial citrus, particularly limes, from areas in Mexico known to be infected with HLB. There is not any hard evidence that the HLB infection in Texas came from Mexico, but the two infected groves are right along a road that is a major transportation corridor for trucks carrying limes from Mexico. Most of these trucks are enclosed; however, in years past, some have not been enclosed refrigerated trucks. Therefore, Mexico is one possible source for the infection in Texas. Our commitment to citrus growers goes beyond products At Syngenta, we re proud of the role we play in keeping citrus groves healthy and productive whether it s through a broad portfolio of products, research to meet new challenges or direct technical support right in your grove. For us, it s not just about selling you a product. It s about working together for the future of the citrus industry. Come hear about the latest in citrus research at the California Citrus Conference in Porterville, CA (Porterville Fairgrounds) October 10-11, 2012. Visit the Syngenta booth, K11. Linda LaVanne Southern Coastal Counties Southern California Counties Ph: 805-402-4770 Roger Williams Kern County Ph: 661-747-9243 Kyle Shore Tulare County Kings County Ph: 559-786-5438 2012 Syngenta. Important: Always read and follow all bag tag and label instructions before buying or using Syngenta products. The instructions contain important conditions of sale, including limitations of warranty and remedy. All products may not be registered for sale or use in all states or counties. Please check with your state or local extension service before buying or using Syngenta products. The Alliance Frame, the Purpose icon and the Syngenta logo are trademarks of a Syngenta Group Company. MW 1LMC2027 7/12 The average time for HLB confirmation after the first detection of ACP in several nearby states and countries has been about 7 years; in Texas, it was 11 years. In Belize, Mexico, Louisiana and California, the first positive detection was in psyllid samples collected from dooryard trees. In Texas, the first detection was in an orchard tree; since January 2012, nearly 5,000 psyllid samples have been tested, but only three have been confirmed as positive. APHIS has attempted a traceback of backyard trees suspected of being infected. So far there is no hard evidence of where the infection in Texas originated. The Texas citrus industry has been advocating for more inspections of citrus from Mexico, and so far only limited improvements to the basic inspection procedures have been implemented. Customs and Border Protection in the Department of Homeland Security have done some blitzes, but there has not been a major change in the inspection protocol even though the industry would like to see additional safeguarding steps taken. Texas has learned from the experience in Florida and elsewhere. The industry has invested a significant amount of funds to provide clean budwood source material to commercial nurseries. There is a general agreement in the industry that all commercial citrus nurseries should be producing nursery stock from enclosed structures by September 2013. ACP control will continue to be an important key to slowing the spread of the disease. Have the survey efforts in Texas been successful in detecting the disease early? Unfortunately, with the latency of the disease, it is difficult to provide an unambiguous answer to that question. Here is what we do know. Texas growers have the advantage of learning important lessons from other areas that have been dealing with this disease. We know we are not doing everything that we could be doing, but we are in the fight with HLB for the long haul and so far we have several reasons to be optimistic about the future of Texas citrus. Dr. Mamoudou Sétamou is Associate Professor of Entomology at the Texas A & M University-Kingsville Citrus Center in Weslaco, Texas, and Dr. John da Graça is the Director of the TAMUK Citrus Center. Ray Prewett is President of Texas Citrus Mutual in Mission, Texas. 38 Citrograph September/October 2012