The Challenges of Managing Citrus Huanglongbing: A Question of Delivery to Phloem and Roots

Huanglongbing (HLB) is a bacterial disease of citrus that has caused a loss of ~60% of the citrus crop in Florida, ever since it was found in the state in 2005. Candidatus Liberibacter asiaticus, the causal agent of HLB, is transmitted by the Asian Citrus Psyllid that injects it directly into the phloem, or sugar transport system, of the plant. The phloem is in the veins of leaves and underneath the bark. The pathogen moves through the phloem into the roots. Once in the roots it causes significant dieback of roots that reduces water and nutrient uptake, reducing canopy vigor and causing leaf and fruit drop. The systemic nature of HLB and its protected environment make the disease particularly hard to manage. Even foliar bacterial diseases of plants, such as citrus canker, are difficult to control. The only traditional treatment is spraying a protective layer of copper on the foliage and fruit. Nonphytotoxic antibiotics can pause disease, but they require costly and labor intensive trunk injections to get them into the plant phloem. Plants lack an adaptive immune system, so bacteriostatic antibiotics only stabilize the tree, but do not cure the infection. The need to regularly inject antibiotics will increase the cost of treatment. Development of a systemic bactericide that could be delivered to phloem tissue by spray or drench application would greatly assist growers trying to save their trees from HLB.