Cultivating Citrus Resilience to HLB in California
Ashraf El-Kereamy
Cooperative Extension Specialist at UCR
Director of UC Lindcove Research and Extension Center
Huanglongbing (HLB), also known as citrus greening disease, is caused by the bacterium Candidatus Liberibacter asiaticus, which is transmitted by the Asian citrus psyllid. The bacterium harms citrus trees in several ways, leading to severe physiological and structural damage.
After the infection, the HLB bacterium invades the phloem, the vascular tissue responsible for transporting nutrients throughout the tree. HLB causes a disruption in nutrient uptake and transport within the tree. This leads to nutrient deficiencies, particularly in essential elements such as nitrogen, phosphorus, and potassium. As a result, the tree experiences stunted growth, yellowing of leaves (chlorosis), and overall nutrient imbalance. The bacterium affects the root system, leading to root loss and dieback. This compromises the tree's ability to take up water and nutrients from the soil, exacerbating the impact of nutrient deficiencies and water stress. Additionally, it affects the photosynthetic process in citrus trees. The bacterium's presence disrupts chloroplast function, reducing the tree's ability to convert sunlight into energy. This results in decreased photosynthesis, further contributing to nutrient deficiencies and overall tree decline. One of the characteristic symptoms of HLB is the yellowing of leaves, often referred to as citrus greening. The bacterium affects the chlorophyll content in leaves, leading to a mottled appearance. This yellowing occurs unevenly on the tree and is a clear indicator of HLB infection. HLB-infected trees experience stunted growth due to the disruption of normal physiological processes. The overall health of the tree declines, leading to a reduction in fruit production and quality. Infected trees may produce smaller, misshapen, and discolored fruit. HLB-infected trees often exhibit premature fruit drop, where fruit drops from the tree before reaching maturity. This is a significant economic impact for citrus growers, as it reduces the yield and marketable fruit. Over time, HLB-infected trees undergo a general decline in health. The combination of nutrient deficiencies, reduced photosynthesis, and compromised vascular function can lead to the death of the tree. The lifespan of infected trees is significantly shortened compared to healthy trees.
It's important to note that there is currently no cure for HLB, and managing the disease involves a combination of strategies. While researchers from all over the world are working hard to get a cure for that disease, citrus growers are increasingly turning to cultural practices (a diverse set of agricultural strategies) as a sustainable and holistic approach to mitigate the profound impact of this relentless disease. Cultural practices encompass a nuanced and multifaceted array of agricultural techniques and approaches used in citrus groves. In the realm of citrus and HLB management, a comprehensive strategy is imperative to create an environment that not only minimizes the likelihood of disease transmission but also nurtures the robustness of citrus trees and makes them more resilient to HLB.
Various agencies in California are making significant efforts on Psyllid monitoring and management, a first defense line to stop the spread of the disease. Regular Monitoring: Instituting a meticulous and frequent monitoring system for Asian citrus psyllid populations is paramount. This proactive approach enables growers to detect infestations early, facilitating targeted interventions to halt the spread of HLB. In addition to Psyllid monitoring and management, harnessing the power of nature through the introduction of beneficial insects establishes a delicate ecological balance. These natural predators act as biological control agents, keeping psyllid populations in check without resorting to chemical pesticides. Swift identification and removal of HLB-infected trees are critical components of disease management. The prompt destruction of infected trees eliminates potential bacterium reservoirs, mitigating the risk of further psyllid transmission to healthy trees. Utilizing beneficial microorganisms as soil inoculants enhances root health and improves nutrient uptake, potentially bolstering the tree's ability to withstand HLB stress.
Another method is to isolate the trees from the Psyllid through growing the Citrus trees Under Protective Screen (CUPS). The system is a closed screenhouse built on a number of acres from 1-5 acres. The system is currently used by some growers in Florida; however, the response of the Californian varieties to grow under such conditions is not known. The first CUPS structure was established at UC Lindcove research and extension center, and we are currently evaluating the performance of some varieties under this structure.
Recognizing that weeds can serve as alternative hosts for psyllids, implementing rigorous weed management practices eradicates potential habitats and minimizes the presence of psyllids in citrus groves. Studies in Florida and California showed a beneficial effect of using wood chip mulching on root growth and weed control. We are collecting more data on these trials, and it will be available for the growers during the next season.
Managing nutrients with precision is vital for supporting citrus tree health. Tailoring fertilization to the unique needs of the orchard ensures trees receive optimal amounts of essential nutrients like nitrogen, phosphorus, and potassium. This fortification enhances natural defenses against HLB. Additionally, implementing consistent and well-timed irrigation practices is crucial. Effective water management alleviates stress on citrus trees, reducing susceptibility to HLB and other stress-related diseases.
Further, incorporating HLB-resistant rootstocks is a crucial aspect of disease management. Selecting rootstocks that exhibit resistance or tolerance to the bacterium contributes significantly to orchard health. Ongoing research endeavors focus on identifying citrus varieties that exhibit inherent resistance or tolerance to HLB. The strategic planting of these varieties adds an extra layer of protection against the disease and its associated symptoms, contributing to the overall health and productivity of citrus orchards. Currently, we are evaluating the effect of a number of HLB resistant rootstocks on the tango mandarin growing under California conditions. These rootstocks include US942, US897, and X639. The evaluation of the resistant rootstock and varieties will continue in collaboration with worldwide researchers.
In the face of the formidable challenge presented by HLB disease, cultural practices emerge as a promising, sustainable, and sophisticated approach to mitigation. By embracing a comprehensive strategy that intertwines vigilant psyllid management, optimization of tree health, meticulous sanitation measures, selective varietal planting, and collaborative initiatives, citrus growers can fortify the resilience of their orchards. The integration of cultural practices, including rootstock selection and proper fertilization, not only contributes to HLB control but also nurtures the long-term sustainability of citrus farming, ensuring a future where citrus crops not only survive but thrive in the face of adversity. As our understanding of cultural practices evolves, the citrus industry can anticipate a resilient, vibrant, and flourishing future.
