Foliar phosphate applications influence organic acid composition in the citrus phloem and could reduce HLB disease by limiting CLas titers

Citrus greening disease is now completely endemic to Florida citrus trees, having spread rapidly across all counties in the past 20 years and causing devastating economic losses. The disease’s etiological agent is the phloem-restricted bacterium, Candidatus Liberibacter asiaticus (CLas). Liberibacter crescens, the closest culturable relative, was discovered to prefer citrate as its most effective carbon and energy source. Citrus plants load free-citrate in response to phosphorus deficiency. In Florida’s calcareous soils, supplemental phosphate fertilization is very low due to the assumption that it is readily available for plants through the soil. It is likely that citrus trees are loading citrate to mine phosphorus from the soil, which could inadvertently exacerbate CLas infection. In greenhouse experiments, foliar sprayed phosphate is taken up by the plants, resulting in decreased free-citrate levels and changes in fumaric and succinic acids in the phloem, in addition to delaying titer detection of CLas. Our field experiments - where mature trees have a well-established HLB infection that’s maintained by infected psyllids – exhibited lower CLas titer for some of the phosphate treatments across time.