The 2012 ASHS Annual Conference

A severely debilitating and currently incurable disease called Huanglongbing (HLB) (a.k.a. “Citrus Greening”) is a serious threat to the Florida citrus industry. All commercial citrus cultivars are to some extent susceptible to the causal agent, a type of phloem-limited bacterium designated Candidatus Liberibacter asiaticus (C-Las). The classic symptom of the disease is blotchy mottling or variegated chlorosis of leaves and tree decline. Recent microarray analysis studies have revealed genes involved in sieve pore plugging are up-regulated in HLB-symptomatic trees. HLB-related phloem plugging interrupts the communication between sources and sink organs, and nutrient transport. Since there is not cure, current management strategies have focused on preventing infection by eliminating the inoculum (infected trees), vector control, production of pathogen-free nursery plants, and nutritional programs to treat symptoms. The goal of this project was to understand the effects of salicylic acid (SA), a common nutritional program ingredient, on the growth of citrus plants. In other plant species, SA is known to induce systemic acquired resistance (SAR); however, based on field observations, SA appears to be acting as a plant growth regulator (PGR) in HLB-affected trees. In light of this knowledge, the central hypothesis of this project was that the application of SA induces budbreak and the development of new shoots in citrus. New growth apparently helps to alleviate HLB symptoms by generating new, functional phloem that helps restore plant function until it is populated by C-Las. Young healthy citrus trees of ‘Hamlin’ and ‘Midsweet’ sweet orange were maintained in a growth chamber for six weeks. Different trials consisted of different sources of SA applied as foliar sprays. Growth measurements were taken weekly, photosynthesis and respiration rates, chlorophyll content, electrolyte leakage percentage and leaf sap pH were assessed at different points in time, including at the end of the experiment. Plants were destructively harvested and leaf, shoot and root dry mass determined. Total non-structural carbohydrate content and nutrient content were analyzed. These experiments, using healthy citrus trees, did not show a significant effect of SA on budbreak or shoot growth. This indicates that SA may only be effective at inducing budbreak and shoot growth in stressed trees (e.g., HLB-affected). More detailed experiments are planned using HLB-affected trees.