SA citrus producers must guard against devastating diseases

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Citrus huanglongbing, better known as HLB, is currently regarded as the most destructive disease of citrus, having caused the near-collapse of the Florida citrus industry over the past decade. This disease, which was first described in China, gives the appearance of a yellow dragon being draped over citrus trees due to the mottling symptom associated with it.

citrus diseases
Citrus greening disease (Laf) yellowing symptoms observed on citrus in South Africa. Source: R Roberts.

The causal bacterial agent of HLB, Candidatus Liberibacter asiaticus (Las), and its vector, Diaphorina citri, has to date not yet been reported with commercial citrus in South Africa. However, HLB has been identified in Asia, the Americas, Mauritius, Réunion Island, and two African countries, Ethiopia and Kenya.

Despite its absence, a relative of Las, ‘Ca. L. africanus’ (Laf), is no stranger to the South African citrus industry, having been associated with citrus greening disease since the late 1920s. It is also known that the vector of Laf, Trioze erytreae, is capable of transmitting Las, meaning that Las can spread in South African orchards in the absence of D. citri.

Blotchy mottle is a symptom of HLB. Source: https://www.latimes.com/food/la-fo-citrus-greening-20190329-story.html.

As the symptoms caused by both are similar, these two must be rapidly distinguished by molecular technologies, to ensure that relevant steps can be taken to limit the spread of HLB, should it enter the country.

The Agricultural Research Council’s Tropical and Subtropical Crops (ARC-TSC) division, in collaboration with the Department of Agriculture, Land Reform and Rural Development (DALRRD), as well as Citrus Research International (CRI), is geared towards testing citrus samples for HLB and distinguishing Las from Laf. This partnership will facilitate the broader testing of citrus for HLB, which helps guard the citrus industry against HLB. – Press release, ARC