Temporal Progress of Huanglongbing Epidemics and the Effect of Non-commercial Inoculum Sources on Citrus Orchards in São Paulo State, Brazil.

Huanglongbing (HLB) incidence is increasing and threatening citrus production in Sao Paulo State, Brazil, despite multiple efforts to control the disease and its vector, the Asian citrus psyllid (ACP) (Diaphorina citri). The objective of this research was to study the temporal dynamics of HLB epidemics, under intensive disease management, in 177 individual commercial citrus blocks on a single property in Sao Paulo State. The effect of internal and external sources of HLB-associated bacteria and its vector were explored based on the disease epidemics and vector dynamics in the studied area. To manage HLB, the property owner used healthy nursery plants, eradicated symptomatic trees, and insecticides to control ACP. Logistic and Gompertz models were fitted to the data to describe dynamics of HLB incidence for all blocks. The average ACP per yellow sticky trap was determined for the same blocks for a period of four consecutive years. Both logistic and Gompertz models described the HLB epidemics well, although the Gompertz model provided a slightly better fit. Disease progress rates, HLB incidences and average ACP per trap in the 177 blocks were low compared with reports in the literature. HLB incidence and number of ACP per trap were higher (p-value ≤0.05) in part of citrus blocks located around the edge of the property. A large number of non-commercial trees was found near the property and are a potential primary inoculum source of HLB-associated bacteria, accounting for the higher incidence of HLB and ACP per trap in blocks located closest to the property´s edge. These results support the recommended preventive measures to HLB management and the necessity of external actions, to include trees in commercial orchards, and non-commercial trees located near commercial citrus properties, in an attempt to maximize the effectiveness of these preventive measures.