03/07/2019 3 min

Huanglongbing disease: a major threat to citrus crops

Huanglongbing disease is one of the most significant threats to citrus crops in the world today. While it affects many producing countries, particularly in South-East Asia, America and Africa, the Mediterranean region is one of the only regions still free of the disease. ANSES has carried out a risk analysis for the European Union area and concludes that there is a high risk if the bacteria responsible for the disease were to be introduced. To date, there are no effective measures for eradicating the disease once it has appeared. The Agency therefore reiterates the importance of ensuring strict compliance with plant import regulations to prevent the introduction and spread of the disease. It also recommends that surveillance be stepped up to enable rapid detection of any possible outbreak, and that measures be developed – in particular biological control – to counter insect vectors or the use of resistant plant varieties.

Huanglongbing disease (HLB), which is caused by different species of the bacterium Candidatus Liberibacter spp. (CL spp.), mainly affects plants of the Rutaceae family and in particular the genus Citrus. These bacteria are transmitted to citrus crops by two insect vectors, psyllids of the species Diaphorina citri and Trioza erytreae.

As HLB is considered to be one of the major health hazards for citrus crops, all imports of Citrus plants intended for planting are prohibited by European regulations because they can potentially carry the disease. The purpose of the ANSES expert appraisal was to assess the risks of entry, establishment and spread of HLB and its impacts in the European Union.

A disease that threatens citrus crops

The main hosts of CL spp. bacteria are citrus plants, which are cultivated intensively in countries in the south of the European Union. All citrus-producing regions (southern Portugal, southern and eastern Spain, Corsica, southern Italy, Sicily, Greece, Croatia, Cyprus and Malta) are therefore threatened.

Moreover, observations made regarding HLB in its current areas of distribution show that for the moment, there are no measures capable of effectively eradicating or reducing the impact of HLB on these crops once the disease has become established. The disease causes significant yield losses, a decrease in fruit quality and leads to tree death.

A high probability of disease establishment and spread

Following its expert appraisal, ANSES concludes that the probability of disease establishment is high due to several factors: the favourable climate, the presence of citrus fruits, the potential adaptability of the bacteria and the ability of vector insects to establish themselves outside their area of origin. In addition, the rate of spread of CL spp. bacteria is considered to be high, particularly through the transport of contaminated plant material.

The insect vector Trioza erytreae is already found in Portugal and Spain and could spread the disease. This situation is worrying. Indeed, the disease can appear several years after the establishment of the insect vector in a given region, making it difficult to detect the bacteria at an early stage. This was the case in Florida, Cuba and Mexico, where the bacterium was detected 7 years later, in Brazil, 62 years later, and Martinique, 2 years later.

Ensure strict compliance with regulations to prevent the entry of the disease

In order to prevent the entry of HLB and the insect vector Diaphorina citri, which has not yet been found in the European Union, ANSES recommends strict compliance with the regulations in force prohibiting the import of Citrus plants intended for planting (plants, budwood, etc.) and compliance with the special requirements for other imports (other host plants intended for planting and products intended for consumption).

The Agency also recommends actions to raise awareness among passengers about the risks associated with the introduction of plants of the Rutaceae family for ornamental, production or consumption purposes. These plants may not fulfil the plant health requirements defined by the regulations and may therefore cross borders without being inspected.

In the event of an introduction, the Agency recommends the deployment of emergency plans for the monitoring, eradication and containment of HLB and psyllids (also known as contingency plans), particularly in the threatened area. It also recommends strengthening monitoring plans with particular attention being paid to asymptomatic host plants and isolated outbreaks of vector insects.

In addition, ANSES emphasises the need to restrict the movement of plants within the European Union between areas contaminated by psyllid insects and uncontaminated areas, to limit the spread of the disease. It also stresses the need to raise awareness among nurseries of the risks related to Rutaceae sold on the Internet or between private individuals.

Finally, noting that there are no effective means of limiting the impact of HLB once the disease has been established, ANSES stresses the need to pursue research programmes on the selection of plant species or varieties that are at least partially resistant to HLB, and on biological control of insect vectors.