Could tree density play a role in managing Citrus Huanglongbing epidemics

Citrus Huanglongbing (HLB) management relies on adoption of preventive measures including the use of healthy nursery trees, area-wide control of the vector Diaphorina citri, and eradication of symptomatic trees. Cultural practices including tree density could contribute to HLB management. We investigated the role of tree density on the incidence of HLB using a historical dataset from commercial orchards in Sao Paulo State, Brazil. The HLB incidence from 2013 to 2015 was compared through 495 blocks of different tree ages (3 to 13 years old), tree localization within the farm (edge or non-edge), and tree densities (from 220 to 714 trees·ha−1) according to the tree spacing used. Diaphorina citri was controlled rigorously in all commercial blocks using drench applications of systemic insecticides and foliar sprays of contact insecticides. The results indicated that cumulative HLB incidence was predominantly lower at higher tree densities. In addition to vector control and removal of HLB-symptomatic trees, planting of trees in high densities may reduce the proportions of HLB-infected trees, thereby contributing to improved profitability for farmers in HLB-endemic sites. In particular, this management practice should be applied at the edges of farms.