Fe²⁺ Ions Alleviate the Symptom of Citrus Greening Disease

Citrus greening (CG) is among the most devastating citrus diseases worldwide. CG-infected trees exhibit interveinal chlorotic leaves due to iron (Fe) deficiency derived from CG; thus, Fe content is lower in infected leaves than in healthy leaves. In this study, we demonstrated that the foliar application of Fe²⁺ relieves the symptom of CG infection in citrus trees. We applied Fe²⁺ and citrate to the leaves of infected rough lemon plants. Following this treatment, a reduction in the number of yellow symptomatic leaves was observed, and their growth was restored. Using chlorophyll content as an index, we screened for effective Fe complexes and found that a high ratio of citrate to Fe²⁺ in the applied solution led to effects against CG in Shikuwasa trees. A high proportion of Fe²⁺ to total Fe was another key factor explaining the effectiveness of the solution in CG infection, indicating the importance of Fe²⁺ absorption into plant cells. We confirmed the proportion of Fe²⁺ to total Fe through the high correlation of reflectometry data via a triazine reaction and X-ray absorption fine structure analysis. These results demonstrate that the foliar application of a high-Fe²⁺ citrate solution can restore the growth of CG diseased trees.