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.

An epidemiological investigation was conducted in the Amami Islands of Kagoshima Prefecture, Japan, to determine the pathogenesis of citrus greening disease, also known as citrus Huanglongbing (HLB) disease, especially in relation to trace metal nutrition. We sampled 11 sites with acidic soil and 46 sites with alkaline soil in private gardens on the islands of Tokunoshima and Amami Oshima. At the acidic sites, no HLB-infected citrus trees were detected using PCR amplification. In the private gardens, 38.6% of the trees tested were infected with HLB disease. By comparing soil pH values in those private gardens, we found that HLB infection was related to soil pH. Among trees growing in alkaline soil, 47.8% were infected, whereas none of the trees growing in acidic soil in the private gardens were infected. When comparing a distribution map of HLB-diseased trees with a soil component map of the five Amami Islands, HLB disease was not detected in acidic soil, but the distribution of HLB-diseased trees was similar to the location of alkaline soil. Microelement analysis of leaves from trees grown in alkaline soil revealed significantly lower levels of manganese (71.9%), copper (70.1%), zinc (52.3%), and iron (40.9%) compared with the leaves from trees grown in acidic soil. These results suggest that HLB disease prefers citrus growing in alkaline soil where the concentrations of Fe and Zn nutrients are reduced.