Abstract

In managing plant diseases, especially those of perennial and plantation crops, there is often a tension between regulators seeking to destroy infected plants to prevent the further spread of the disease and growers seeking to retain infected plants to maximise their yields. This has been the case for the Huanglongbing (‘citrus greening’) in Brazil. In an effort to prevent Huanglongbing’s spread, the Brazilian government has regulated that if 28% of block of trees is found to be symptomatic then the whole block will be destroyed. This rule is based on the assumption that a 28% detectable prevalence is likely to mean that the actual prevalence is 100%. We use a mathematical model to evaluate this assumption, and show that the relationship between detectable and actual levels of infection is much wider than allowed for in the Government scheme. There is a high probability that orchards with levels of symptomatic plants substantially below 28% have a very high (> 90%) prevalence of infected plants. Rather than setting an arbitrary prevalence threshold that would be considered acceptable by both growers and the regulator, future regulations should instead focus on co-ordinated spraying amongst owners to control the insect vector of HLB on a regional level.