Optimizing citrus tree root health and controlling insect vectors with neonicotinoid insecticides may play key roles in managing the dreaded citrus greening disease, said Florida-based Syngenta agronomic service representative John Taylor. The incurable bacterial disease, also known as Huanglongbing (HLB), has ravaged Asian citrus for years and is now threatening U.S. groves from Florida to California. The bacterial infection from the disease inhibits proper nutrient uptake, which results in plant death in as little as five years.
“For now, there is no cure for HLB, and no resistant citrus varieties are available,” Taylor said. “Management is difficult, but certain strategies can slow the spread of the disease, including managing psyllids with neonicotinoids, promoting root health, planting disease-free nursery stock and removing infected trees.”
When it comes to fighting HLB, growers face unique challenges. For starters, HLB-infected citrus trees do not show external symptoms during the first year of infection, so there is a long period of time when a grower cannot visually detect an infected tree—but the tree is still a source of bacteria that can spread to other trees via the Asian citrus psyllid (Diaphorina citri). These insects are necessary vectors for spreading HLB, and once infected, psyllids are disease carriers for the rest of their lives.
Managing psyllids with neonicotinoid insecticides has become one of the primary strategies to prevent the spread of HLB. These insecticides are anti-feedants, so psyllids do not want to feed on treated trees. This is an important factor in preventing HLB spread because the psyllid needs to feed to pick up and disperse the bacteria.
“We recommend that Florida citrus growers soil-apply neonicotinoids every six weeks, and in between make foliar applications of insecticides with different modes of action,” Taylor says. “Thiamethoxam, the active ingredient in Platinum 75 SG insecticide, is a neonicotinoid with good solubility that allows for faster uptake by trees, which makes it a good choice during those times of year that are a little dry or when trees are less active.”
Taylor added that protecting roots from other fungal diseases like Phytophthora, may help slow the development of HLB. Studies show that roots infected with Phytophthora may interact with the HLB root bacterial infection to further damage roots and accelerate tree decline.
“To delay crop decline from a tandem of Phytophthora and HLB, growers can implement a root health treatment program with Ridomil Gold SL fungicide,” Taylor said. “Ridomil Gold has direct fungicidal activity against the Phytophthora root rot disease. Roots rapidly absorb the fungicide, which is then translocated throughout the root system, promoting root health and crop development.”
Another critical point for growers is to start with disease-free plants. In response, Florida has established a production model where all citrus seedlings must be produced in enclosed greenhouses. The overall U.S. citrus nursery industry is moving in that direction as well.
Identifying and removing infected trees are big challenges. In Florida, the disease was so widespread by the time it was detected, eradication was impractical. In California and Texas, researchers are pursuing more efficient ways to detect the disease in trees not yet exhibiting symptoms and are hoping early awareness of HLB will prevent the rapid advancement of the disease that Florida has faced.
“Many a boogeyman has spooked citrus growers over the years,” Taylor said. “But citrus greening isn’t folklore. It has the potential to put people out of business, so taking the right steps to decelerate its spread is the best defense, for now.”
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