On Guard Against Citrus Greening

Q. How is citrus greening, or Huanglongbing (HLB), impacting growers in your region?

A. John Taylor, CCA, agronomic service representative in Florida, Syngenta: Citrus greening, or HLB, has been the single most devastating challenge to befall the Florida citrus industry. It has reduced yields by 50% or more, doubled production costs, and reduced the overall citrus-bearing acreage in Florida, which at one time was more than 800,000 acres, to around 400,000 acres today. Additional ripple effects include the loss of critical infrastructure, growing reluctance to replace existing trees or plant new ones, and declining consumer demand due to increased commodity prices.

A. Christine May, PCA, agronomic service representative in Southern California and Arizona, Syngenta: California and Arizona do not have citrus greening systemically across all citrus-growing regions. Therefore, a large part of the impact has been from quarantines that restrict movement of fruit between different growing regions, as well as “spray and move” requirements prior to harvest. At this point, citrus greening has not caused commercial losses of fruit in my region. Most of the economic impact has been from additional sprays to keep populations of the Asian citrus psyllid (ACP) — a major vector of the disease — low.

Q. What role does scouting play in managing citrus greening in your region?

A. Taylor: We no longer scout for citrus greening in Florida. Early in the disease outbreak, the industry actively scouted and removed infected trees. This entailed robust field scouting efforts coupled with real-time PCR [polymerase chain reaction] testing for positive confirmation. Unfortunately, in Florida, the disease was fairly well distributed when it was initially detected, so scouting and tree removal proved ineffective at preventing spread and establishment. The latency period between infection and symptom expression — combined with the fact that the pathogen is insect-vectored — means field scouting is only a viable strategy early in the disease introduction and establishment.

The economic threshold for ACP and all insects that vector disease is technically zero. ... Slowing or stopping this spread within California and Arizona is important for the overall economics of citrus production.

A. May: Because citrus greening in my region is not well-established, scouting can be a useful tool. I suggest that Western citrus growers go to the University of Florida IFAS Extension sites for recommendations. Additionally, growers can go to their county’s ACP Task Force meetings, where they can find out about scouting workshops.

Q. Does controlling ACP benefit trees either before or after they’re infected with citrus greening?

A. Taylor: That’s a question that research has yet to conclusively vet. Early indications seem to show that trees repeatedly infected by hot psyllids see more rapid disease progression and tree decline. Since citrus greening is now endemic in Florida, growers have largely abandoned intensive monthly sprays in mature groves. Where young trees are being established, growers maintain intensive psyllid management practices for the first three years of a tree’s life to help delay infection. Since it only takes one hot psyllid to infect a tree, the action threshold is zero. In our subtropical growing environment, attempting to eradicate psyllids is not biologically or economically feasible. As a result, most growers have evolved into maintaining psyllids at low levels. Minecto^® Pro insecticide is one tool that offers growers extended residual control of ACP and other citrus pests.

A. May: Controlling ACP has an important economic impact, regardless of the status of citrus greening infection. The economic threshold for ACP and all insects that vector disease is technically zero. If a block is infected, a single ACP can transmit HLB to another block or another grower’s trees. Slowing or stopping this spread within California and Arizona is important for the overall economics of citrus production. While trees can remain in production with HLB, yield and quality are reduced, which over time reduces the production of the entire industry. Systemic infection within the industry would impact growers and fruit packers if the yield decreases enough.

Q. Why is root health important, and what can growers do to improve it?

A. Taylor: Citrus greening infects the root system of trees early in the disease cycle. It causes as much as 30% to 40% loss of the fibrous root mass, which is then reflected in the various above-ground symptoms commonly associated with the disease. This rapid loss of root mass also negatively interacts with other common root issues, such as Phytophthora. The interaction of citrus greening with other biological stresses magnifies the impact of each disease and hastens the decline of the trees. As root mass declines, trees become more sensitive to water quality, fertilizer quality and stress events, such as drought and cold. Without a healthy root system, trees lack the physical ability to efficiently collect water and nutrients as well as store carbohydrates to facilitate normal growth patterns and full crop loads.

Citrus greening, or HLB, has been the single most devastating challenge to befall the Florida citrus industry.

Root health is challenging for growers to manage because the root system is out of sight and challenging to monitor. Growers have learned to use soil sampling to monitor pathogens like Phytophthora and track root mass trends. Additionally, more attention is being paid to water quality, fertilizer quality and other root health impacts, including nematodes, root weevils and organic soil amendments. Root health is not a single-factor equation. Growers should focus on identifying stressors and opportunities for improvement and then focus their resources on the factors they can effectively manage.

A. May: Root health is an important part of managing HLB and keeping trees in production. HLB infection blocks the phloem of the tree, which transports nutrients. Roots are the basis for nutrient uptake. Keeping them healthy and protected from other diseases can maximize their uptake ability, enabling the tree to get the most nutrients possible. The combination of HLB and root diseases like Phytophthora can be very detrimental to nutrient uptake. Syngenta offers two products that help manage Phytophthora infections and, therefore, improve root health: Ridomil^® Gold SL and Orondis^® fungicides.

For more information on managing citrus pests, speak with your Syngenta representative or go to the Syngenta citrus crop page.