Nepal | June 03, 2020

Citrus greening: Nepal’s groves under threat

Kiran Timilsina
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The disease entered the country because of the weak quarantine regulations. It is projected that virtually all the mandarin hubs in Nepal will be infected by citrus greening in 10 to 15 years, posing an incurable threat to citrus enterprises

Illustration: Ratna Sagar Shrestha/THT

Nepali citrus is a Rs 2,470 million per year industry. Cultivation of citrus fruits has been the bread and butter for people living in the mid-hills for ages. In Nepal, among all the citrus fruits, mandarin types of citrus occupy about 64 per cent and 68 per cent of the total citrus growing area and production, respectively, according to recent statistics on agriculture. But the Nepali citrus industry is grappling with the most serious threat in history: a bacterial disease with no cure has infected most of the citrus growing hubs and is yet to receive any national attention.

The world’s most destructive citrus disease, ‘citrus greening’, is threatening the groves of Nepal. The disease originated more than a century ago in southern China, where it is named ‘Huanglongbing’ (HLB), or ‘yellow shoot disease’, after its typical symptom. Since it was first recognised in Nepal – near Pokhara – in 1968, the disease has spread all over.

The disease entered the country because of the weak quarantine regulations. It is projected that virtually all the mandarin hubs in Nepal will be infected in 10 to 15 years, posing an incurable threat to citrus enterprises. The form in Nepal is caused by a bacterium, Candidatus liberibacter asiaticus, transmitted by an insect, Asian citrus psyllid, which thrives on young citrus leaves.

The disease obstructs the flow of nutrients in citrus trees. Greening slowly kills a citrus tree. The infection reveals itself first with the mottling of leaves, then stunting of shoots, gradual death of the branches and eventually small deformed fruits with bitter juice. The disease lies dormant for two to five years and spreads by an insect. It lives on citrus trees, depositing bacteria that gradually starve the trees. The insect flies from tree to tree, leaving a trail of infection. The infected tree may survive for several years, but its death is inevitable. The greening will surely destroy the citrus industry slowly if steps are not taken soon.

HLB has progressed from an “acute to chronic disease” throughout the nation and has caused the Nepali citrus industry increasing loss to the grower in terms of revenue. It has mostly squeezed the Nepali orange crop. Sweet oranges, mandarin and mandarin hybrids are most susceptible to the disease. The disease has wreaked havoc to the citrus business in Nepal, leading to a massive economic hit and has eliminated many related jobs.

In order to offset the citrus losses, quite a number of growers are venturing into different crops. Anxiety can be clearly observed in the orange growers of the mid-hills, following the noticeable sharp decline seen in the productivity from 12 metric tons a hectare to 8 metric tons in six years. Nepal has not been able to meet the citrus fruit demand and relies on imported oranges to satisfy internal demand.

There is still no permanent solution to fighting citrus greening, but researchers and growers are trying various methods, and large sums of money are being spent for this. Globally, the methods used to control the disease – spraying to kill the psyllids and removing the infected trees – have proved inadequate. Replanting young vigorous trees have also been not effective because of the attack by the psyllids.

In the case of Nepal, to save our citrus industry, Nepali citrus fruit growers and the government should channel money into research programmes to identify both short-term and long-term solutions. At present, the only way to protect the trees is to control the psyllid population and destroy the infected trees. All those farmers who have planted citrus trees should keep an eye on the psyllids and control them as soon as they are sighted by using a number of recommended control methods available to avoid infection of a healthy citrus tree. A national level programme should be launched to replace the affected citrus trees with vigorous cultivars, grown through selective breeding, that show some tolerance to the HLB, although no real resistance exists till date.

The growers must be supplemented with programmes that include scouting for the infected trees, tree removal, and nutrition and pesticide programmes to manage the disease. The government and related research organisations should invest in research against the disease. The research must focus on rapid detection of disease and replacement with possible less susceptible cultivars. The research groups must work with the growers, investigating possible ways of combatting citrus greening.

A pilot study at the national level should be launched directed at surveying all citrus growing areas, and collection and certification of suspected samples should be done. Citrus growers should be well informed about vector control. The government and private sector should be investing in the production of certified HLB-free shoot tip through grafting.

Rejuvenation of citrus decline is possible, however, it is only possible if the related stakeholders are dedicated to a positive outcome. The agriculture sector has been given due priority by all the economic plans, still, the expected outcomes have not been seen. Despite this, there is a sense of hope that the citrus industry will survive the challenge. To bring about a golden revolution, the nation needs to wake up instantly to carry out rapid diagnosis and awareness programmes. The only worry is that this might be a little too late already.

Timilsina is a researcher in Nepal’s citrus fruits


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