Pest poses new threat to citrus fruit farming

Scientists have put orange, lemon and tangerine farmers on high alert following the discovery in the country of a foreign pest transmitting the dreaded Asian citrus greening disease.

Dr Sunday Ekesi, the head of research at International Centre of Insect Physiology and Ecology (Icipe) says their researches in Kilifi, Kwale, and Taita Taveta counties have confirmed that the Asian citrus psyllid pest is on the prowl and has attacked orange,tangerinee and lemon orchards in the country’s biggest citrus belt along the Coast.

Icipe and Kenya Plant Health Inspectorate Service (Kephis) conducted the studies informed by the 2015 discovery of the dreaded pest in Morogoro, Tanzania.

"The presence of the Asian citrus psyllid in Kenya means that citrus farmers are exposed to the dreaded greening disease," Dr Ekesi, who is leading a study to map the pest’s footprint in the country and the region and developing and testing workable interventions, tells Seeds of Gold during a meeting in Mombasa.

The disease, which is also known as Huanglongbing or yellow dragon, is one of the biggest threats to the citrus industry worldwide.

It is caused by the bacterium Candidatus liberibacter asiaticus, which is transmitted by the Asian citrus psyllid, Diaphorina citri, a tiny, wedge-shaped brown and white mottled insect.

Dr Ekesi says that Candidatus liberibacter asiaticus is also transmitted by African citrus psyllid, a pest which is largely responsible for the mild African citrus greening disease.

Though there is no case of the Asian citrus greening disease reported in the country yet, Dr Ekesi is concerned that “the multiple vectors means that citrus farmers in Africa are more exposed to the disease as compared to their counterparts in other parts of the world”.

Symptoms of the citrus greening disease include small leaves with yellow mottling, enlarged leaf veins, and small fruits which are green in colour even when they are ripe.

No market value

Fruits born of affected trees are misshapen, and produce little juice which is bitter, meaning that the produce has no market value as it cannot be used as a fresh fruit or for juice.

Sickly trees produce yellow shoots which dieback, wither and eventually die in years. Citrus farmers in the country have been grappling with African citrus greening disease, which has made the highlands unsuitable for growing the fruits.

The disease has been blamed for the increase in the cost of producing citrus fruits and reduction in yields in the country’s citrus belts.

Therefore, the possibility of the Asian citrus greening disease scares stakeholders as the disease is highly devastating and incurable.

The government imposed a quarantine against citrus fruits in 1995 in bid to defeat the African citrus greening disease.

“Citrus farmers were advised to uproot their trees to insulate themselves from the greening disease that had devastated the crop in previous years and which had defeated known conventional interventions,” says Nichodemus Ngeka, the head of Agriculture and Food Authority in Makueni County.

Some farmers in the Ukambani citrus belt complied with the government directive, and stopped farming the fruit.

The Asian greening disease is more notorious and has devastating effects on the industry going by the experiences in China and the US, the world's leading producers of citrus fruits.

Citrus greening disease

When it struck the Florida citrus belt in the US, it reduced the citrus yield by more than 70 per cent, according to Dr Susan Halbert, an entomologist who discovered the pest in Florida, 20 years ago, and who has studied it ever since.

There is no known cure for citrus greening disease currently, and this is why scientists recommend that farmers should act swiftly once the pest attacks their trees or as soon as they detect the symptoms of the disease. They recommend cutting down and disposing off the infected trees.

This leaves farmers with prevention and proper management as the only ways togo aroundd the threat the disease.

In the hard hit Kwale, the agriculture executive Joanne Nyamasyo says that the county government has been advising farmers against moving uninspected planting materials from one place to another to avoid spreading the disease, a strategy approved by scientists.

The disease spreads through infected citrus plants and planting materials such as seedlings, according to Dr Michael Njunie, the head of Kenya Agriculture and Livestock Research Organisation station at Maguga.

Challenges in control

Something else which makes the disease difficult to control is the fact that it can take up to six years for it to show symptoms, according to Dr Halbert.

Though spraying citrus trees with various pesticides works, Icipe and its partners are currently studying the factors that influence the changes in the population of Asian psyllid, its geographical distribution in Kenya and beyond to develop control strategies that are less reliant on harmful pesticides.

The scientists are developing and testing the use of several non-synthetic-chemical alternatives such as intercropping citrus with guava, and the use of natural attractants and repellents of the pest, according to Dr Ekesi.

Control methods

Other methods identified for controlling the Asian pysillid pest is installation of wind breakers on the windward side of orchards, and the introduction of natural enemies of the pest such as the Lady bird which studies have shown to be a repellent to the Asian citrus pysillid.

One of the innovative strategies the Chinese use to fight the pest is to delay the seedlings in nurseries until they are 2-3 years before transplanting to the fields.
"Delaying the transplanting discourages the invasion of pests since they are more attracted to young plants than they are to older trees," says Dr Yijing Cen, an entomologist at South China Agricultural University in Tianhe District.
Scientists, hope that with these interventions farmers will be able to manage the Asian citrus psyllid thus keep at bay the dreaded Asian citrus greening disease.