The Asian citrus psyllid (ACP), or Diaphorina citri, is a pest that carries the bacteria Candidatus Liberibacter, which causes huanglongbing disease (also known as citrus greening disease). The ACP originated in Asia, but has since spread to the southern and western United States. When the ACP feeds on citrus tree shoots, Candidatus Liberibacter infects the tree. Plant tips wilt and recede, and as the disease progresses, infected trees may experience discoloration of fruit, premature defoliation, stunted growth, loss of root mass, and in severe cases, death of the entire tree.
‘Huanglongbing’ means “yellow dragon disease,” which describes one of the most destructive symptoms of the disease.
Early on in the progression of huanglongbing (HLB) disease, infected leaves’ veins often appear blotched yellow, leading many growers to believe that nutrient deficiencies are to blame. However, nutrient-deficient trees exhibit symmetrical yellow staining, while HLB is distinguished by asymmetrical yellow blotching on the leaf vein. Trees stricken with the disease are prone to flowering during the off-season. These blossoms typically fall off.
The most notable and financially destructive symptom of HLB is the production of small, irregularly shaped green fruit. While the top of infected fruits will turn a more normal color when ripe, the side opposite the stem remains green.
Asian citrus psyllids transmit huanglongbing disease when they feed on citrus tree leaves.
Psyllids are plant-eating insects which are monophagous, meaning they only feed on one kind of plant. In this case, citrus trees. While these citrus-plant-eating bugs are an annoyance to growers, what makes them a harmful pest is their ability to transmit Candidatus Liberibacter, a bacterial plant pathogen deadly to citrus trees. Psyllids take the bacteria into their body when they feed on an infected tree, and they secrete the bacteria via saliva into healthy plants the next time they feed. Psyllids can easily fly from orchard to orchard and reproduce in a matter of weeks, which means huanglongbing disease can spread quickly.
There are three forms of the Candidatus bacteria pathogen: Candidatus Liberibacter asiaticus (carried by the psyllid in Asia), Candidatus Liberibacter africanus (carried by the psyllid in Africa) and Candidatus Liberibacter americanus (carried by the psyllid in America).
While the Asian form of the pathogen is heat tolerant—meaning the disease requires temperatures around 95°F—the American form of the pathogen is heat sensitive, meaning that the Asian citrus psyllid can only pass on the bacteria and cause huanglongbing disease at temperatures between 68 and 77°F.
In the United States, the ACP population is at a peak in the months of October, November, December, and then again in May and August. Because citrus trees generally bloom in early spring, the prevalence of ACPs in the fall and winter months is, literally, a deadly combination: the pests disrupt the trees’ off-season, and pose a threat to the growth of fruits come springtime.
Huanglongbing disease is not curable—treatment regimes generally focus on controlling the psyllid population.
There is currently no known cure for citrus trees infected with HLB—the ability to eliminate the disease comes down to managing psyllids populations.
In California, efforts have been made to monitor the geographic spread of psyllid populations to determine where best to use chemical agents to reduce the insects’ ability to spread the pathogen. The California Department of Food and Agriculture (CDFA) has requested that growers who detect the presence of psyllids to contact the CDFA’s Exotic Pest Hotline at 1-800-491-1899.
Experiments with trimming limbs that show symptoms of HLB have found that pruning is ineffective in controlling the disease, especially in trees with more advanced symptoms. Any citrus tree which shows symptomology of HLB must be considered to be incurable, and should be removed from the orchard as soon as possible. Because symptoms of HLB can take two years or longer to become visible from the time of infection, it’s advisable that trees adjacent to an infected tree also be removed and destroyed, as they can act as reservoirs and vectors for the disease long before symptoms become apparent.
The CDFA has seen success in controlling ACP populations with the implementation of a two-pronged chemical control solution. When the presence of psyllids is confirmed, all citrus trees are treated with a pyrethroid-based insecticide, which kills juvenile and adult psyllids on contact. In conjunction with this, a systemic insecticide is applied to the soil, which kills ACPs in the nymph stage, which hide away in young leaf tissue where they can’t be treated with foliar pesticides.
University of California researchers have also been successful in controlling ACP levels by breeding and releasing Tamarixia radiata, a species of wasp which feeds upon ACP nymphs. But researchers have found that successfully controlling ACP populations through biological measures like T. radiata requires careful control of ant populations as well, as local ant populations will harbor and protect psyllids, killing T. radiata and other antagonistic species in order to feed on the honeydew produced by ACPs.
Lastly, ensuring balanced nutrition, particularly adequate supplementation of boron and calcium, will encourage the growth of hardy plant tissues that are more resistant to pests and disease.