By Marcelo Pedreira Miranda, Haroldo Xavier Linhares Volpe, Renato Beozzo Bassanezi, Ivaldo Sala and Juliano Ayres
Huanglongbing (HLB), or citrus greening, is a notable threat to the citrus industry worldwide and was reported in the central region of São Paulo state (SPS) in Brazil in 2004. HLB then spread quickly throughout SPS, requiring the citrus industry to adapt rapidly to prevent the devastating impacts on citrus production.
According to a survey conducted by Fundecitrus in 2020, 20.87 percent of the orange trees in SPS and the west–southwest regions of Minas Gerais state showed symptoms of HLB. This compares favorably with other countries where disease incidence is much higher. The difference may be partially explained by the previous experience of Brazilian citrus growers with citrus variegated chlorosis (CVC), a disease caused by the bacterium Xylella fastidiosa and spread by leafhoppers.
The measures used to manage CVC were improved for HLB management and included: 1) nursery citrus tree production under an insect-proof screenhouse, 2) removal of symptomatic trees in the field and 3) insect vector control. In HLB management, due to the high dispersal ability of the Asian citrus psyllid (ACP) and the difficulty in preventing primary infections, removal of inoculum sources in areas surrounding citrus farms is essential.
MONITORING AND REGIONAL MANAGEMENT
In SPS, ACP monitoring is considered an important component of HLB management; use of yellow sticky traps is the primary method. Traps are mostly set on orchard borders, particularly along property perimeters, and are checked weekly and replaced biweekly. The detection of a single psyllid is sufficient to trigger ACP control measures.
In SPS, the establishment of coordinated regional psyllid management has proved essential for successful HLB containment. In 2011, Fundecitrus developed the Psyllid Alert System (PAS) to coordinate simultaneous, area-wide insecticide application based on the regional ACP population and citrus vegetative stage. Currently, PAS is available for 12 regions of SPS and the west-southwest of Minas Gerais state, with approximately 31,700 yellow sticky traps (92 percent from growers and 8 percent from Fundecitrus) distributed in 1,498 orchards, for a total monitored area of 280,774 hectares.
INSECTICIDE USE
Chemical control is the most effective and widely used method for ACP management. Citrus growers in SPS must only use insecticides from the “ProteCitrus” (Citrus Protection Products) list to comply with international regulations. Chemical control of ACP is dependent on tree age, as follows:
- Nursery trees — Application of systemic insecticides by drench one to five days before planting
- Young orchard (≤3 years old) — Application of systemic insecticides as a soil drench or directly onto the trunk of the citrus trees plus contact insecticides sprayed on the foliage
- Mature orchard (>3 years old) — Only contact insecticides sprayed on the foliage
The spray frequency is determined for each block of trees based on tree age, flushing period, ACP monitoring, HLB incidence and block location. In general, blocks with young and flushing trees and located on the perimeter of the farm are sprayed more frequently.
In SPS, the spray volume used for ACP control is based on the tree-row volume rather than tree height and/or age. It has been demonstrated that a reduction in spray volume used by citrus growers, from 70 milliliters spray mixture per cubic meter of tree canopy to 25 to 40 milliliters per cubic meter, maintaining the same insecticide concentration (grams of active ingredient per liter of water), does not affect control efficacy against ACP.
Therefore, it was possible to reduce the water required for spraying and insecticide rates per hectare by up to 64 percent and management costs by 40 percent. Operational efficiency was also improved by spraying a greater area in less time. This is an important consideration in HLB management, as some sites of the farm (e.g., orchard borders) require higher spraying frequency.
EXTERNAL MANAGEMENT MEASURES
The incidence of HLB in SPS ranges from 0.08 to 60.46 percent. In regions with high HLB incidence, even in orchards with rigorous symptomatic tree removal and frequent ACP control, the disease incidence could increase due to the constant dispersal of ACP from unmanaged areas (backyards and/or abandoned orchards) to commercial orchards. According to PAS data, 90 to 95 percent of the ACP population in SPS is from unmanaged areas.
To reduce the primary spread, Fundecitrus, in partnership with citrus growers, is extending the HLB management measures to areas within 3.1 miles of commercial orchards. These measures include: 1) replacement of ACP host plants by ornamental or other fruit trees, mainly in backyards, 2) insecticide application, mainly in abandoned orchards and 3) release of the parasitoid Tamarixia radiata in backyards and abandoned orchards.
The Fundecitrus Extension service team makes the first contact between growers and the owners of these unmanaged areas. Citrus growers are then responsible for the eradication/replacement of ACP host plants and/or ACP control.
Currently, some citrus growers and companies have specific teams to conduct these external measures. Since August 2018, 1,056,371 citrus and orange jasmine trees from backyards in rural and residential areas and 527,563 citrus trees from abandoned groves in different regions of SPS have been eradicated. The integration of HLB control measures performed inside and outside of farms has led to a significant decrease in the annual HLB infection rate on many of the participating farms. Additionally, the cost of the external measures only represents 10 to 15 percent of the total farm HLB management cost.
SUSTAINABLE APPROACHES
The SPS citrus industry is investigating more sustainable approaches for HLB management. There are currently six facilities rearing T. radiata, one from Fundecitrusand five from citrus growers, producing approximately 16,800,000 parasitoids per year to be released in unmanaged areas.
Other promising alternative ACP control methods being studied include the use of processed kaolin in citrus borders and biological insecticides based on the entomopathogenic fungus Cordyceps fumosorosea. In the future, these more sustainable management methods will replace some chemical insecticide applications.
Marcelo Pedreira Miranda, Haroldo Xavier Linhares Volpe and Renato Beozzo Bassaneziare scientific researchers, Ivaldo Salais an agronomist, and Juliano Ayresis general manager — all at Fundecitrus in Brazil.