African citrus disease fears

C&I Issue 4, 2021

Read time: 2 mins

Shem Oirere

South Africa has published a new notice on measures for controlling the spread of Asian citrus greening disease, also known as Huanglongbing (HLB) that has already been reported in some African countries.

The Food and Agriculture Organization (FAO) estimates losses of up to $403/ha based on a previous study on Brazilian citrus farms. According to FAO, the disease destroys nearly 100m trees globally, while the bacterium responsible, Candidatutus Liberibacter asiaticus, affects all species of citrus although ‘there is no known cure’.

Currently, South Africa has devoted 86,808ha of land to citrus growing, nearly 6% up from the 81,603ha in 2018.

The new control measures are not driven by any occurrence of the greening disease in South Africa, according to Angela Didiza of South Africa’s Agriculture Department of Agriculture, Land Reform and Rural Development, which signed the control measures against both HLB and the Asian citrus psyllid (ACP), Diaphorina citri – the insect pest responsible.

Didiza said HLB has been detected in Ethiopia and Kenya while the insect that transmits it has been detected in Kenya and Tanzania.

The new measures require citrus producers in South Africa to immediately notify the ministry whenever there is an occurrence or suspected outbreak of the disease for quick emergency response.

Citrus greening can also be caused by two other bacteria - Ca. L. africanus, which is spread by the psyllid Trioza erytreae, and Ca. L. americanus that is transmitted via ACP.

Citrus plants affected by the greening disease usually exhibit yellow shoots, asymmetric, mottled leaves, small upright chlorotic leaves and out of phase flushing.

South Africa and other citrus producing countries of Zimbabwe, Malawi, Burundi, Kenya, Somalia, Ethiopia, Cameroon and Madagascar are perennially affected by Ca. L. africanus. FAO says both this bacterium and its vector are sensitive to heat and thrive best at temperatures below 32°C.

Ca. L. africanus blocks the phloem, the vascular tissue that transports and distributes organic nutrients, hence causing loss of leaves, deformation of fruits that become bitter and hard or eventual death of the tree.

Controlling the spread of HLB would lead to the affected areas being declared quarantine areas, destruction of plants or parts of plants infected, prohibiting movement of infected plants and treatment of all infected plants in the quarantine area.

South Africa and Egypt are the leading suppliers of citrus to the EU, especially Netherlands and Portugal that have stringent requirements for existing maximum residue levels (MRLs) for various agrochemical products on fresh produce.

The alert on the possible outbreak of Asian citrus greening disease comes at a time when the Southern African Development Community (SADC) is grappling with other citrus pests and diseases such as Citrus black spot, False codling moth and fruit flies.

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