Mexico is the latest to succumb to the inevitable spread and establishment of huanglongbing (HLB) – the devastating disease of citrus crops. Mexican authorities in the states of Jalisco, Michoacán and Colima have warned growers that HLB – otherwise known as citrus greening – is here to stay.
HLB was first detected in Mexico in July 2009 and it’s clear now that it won’t be going away. Some estimates suggest HLB could affect 60% of Mexico’s citrus industry, an industry which provides employment for 70,000 farmers. SENASICA, the National Service for Animal and Plant Health, Food Safety and Quality, has invested over US$20 million to control the disease. An economic impact assessment report prepared this year by consultants specializing in the economics and finance of plant health programmes indicated total losses to the Mexican economy could amount to almost $7000 million.
Mexico’s not the first to report the alarming consequences of HLB infection. The disease has been known in Brazil for some years and is spreading throughout the region. Florida first reported the disease over 5 years ago and data from 2010 showed 18% of trees in Florida are infected – for more information on the disease in Florida click here. HLB was first reported in Nicaragua and Honduras in 2010 and in Costa Rica in February this year. A recent outbreak in the Dominican Republic is said to have destroyed 34,000 trees and left 5000 unemployed.
HLB causes severe yield losses and eventually kills citrus trees. Fruits from HLB-affected trees have a bitter, salty taste and the name citrus greening refers to the bottom half of the fruits which remain green. The disease is caused by bacteria (Candidatus Liberibacter asiaticus, africanus or americanus) which are transmitted by insect vectors, primarily the Asian citrus psyllid, Diaphorina citri, but also the African citrus psyllid, Trioza erytreae.
New developments in the fight against citrus greening include a new technique for detecting symptoms before the disease is widespread, new methods for controlling the insect vector including a potential biocontrol agent, Tamarixia radiata, which parasitizes the Asian citrus psyllid, and improving plant nutrition to counteract the obstruction to nutrient flow caused by the phloem-inhabiting bacteria. Removal of infected or abandoned citrus trees might help – but who pays the price for removing affected citrus groves, particularly if they are still partly productive? Ultimately, control of HLB is likely to lie in the development of resistant cultivars(1) – some natural resistance has been observed, and bacterial, psyllid and some citrus genomes are near completion or completed.
Is HLB on its way to Europe? An Italian participant at a conference session on emerging pests and diseases claimed the relevant authorities were not giving enough priority to vigilance in this area – ‘when greening comes in, citrus goes out’. The Asian citrus psyllid vector has already been detected in the Canary Islands. But will Europe find the financial resources to support extra vigilance?
(1) Gottwald, T. R. Current epidemiological understanding of citrus huanglongbing. Annual Review of Phytopathology (2010) 48, pp 119-139.
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1 November 2019