Frequency of symptomatic trees removal in small citrus blocks on citrus huanglongbing epidemics

Abstract

Among the recommended measures for citrus Huanglongbing (HLB) management, the systematic elimination of symptomatic trees is the most argued and difficult to be accepted and accomplished by citrus growers. Elimination of recently affected HLB trees represents a short term yield loss and cost increase due to the need of frequent inspections and removal operations. This work aimed to evaluate the effect of different frequencies of inoculum reduction applied at individual citrus blocks scale (or local inoculum reduction) on HLB temporal progress. Eight experiments were carried out in new planted and older citrus blocks with 504–1290 trees/plot. In all experiments, inspections to detect symptomatic trees were done in a fortnightly or monthly frequency. The treatments of frequencies of local inoculum reduction varied from fortnightly to 6 months. Annual disease progress rate was estimated by logistic model for each plot. No difference on HLB progress rates among treatments was observed, except in experiments 1 and 3 where less frequent tree removal resulted in higher disease progress rate. This ineffectiveness of local inoculum reduction on the disease progress rate was explained by the higher weight of primary spread on HLB epidemics than the secondary spread within plots associated with small size and narrow shape of treated plots (except for experiments 1 and 3), high dispersal capacity of HLB-insect vector among plots and groves, and strong control of psyllid within the plots (except for experiment 1, with poor insecticide spray program). Also, the high amount of inspections to detect symptomatic trees before the eradication treatment, which reduced the escapes (asymptomatic and non visual detectable diseased trees) contributed for these results. It is important to note that these results were obtained with only small citrus plots (0.8–2.9 ha) and they cannot be extended to larger groves and farms amenable to HLB management by the symptomatic tree removal and vector control.