Management of Weligama coconut leaf wilt disease:

The way forward

Dr Priyanthie Fernando and Dr Chithranganie Jayasekera

Today, the coconut industry of Sri Lanka is facing a serious threat by an incurable disease, the Weligama coconut leaf wilt disease (WCLWD). While confined to the Southern Province and spreading therein, causing crop losses, alarms the possible invasion to rest of the country’s coconut plantations.

WCLWD is a disease caused by a bacteria-like germ called ‘phytoplasma’ spreading from tree-to-tree by plant feeding insects (vectors). The insect vectors when feeding on a diseased palm, ingest the ‘phytoplasma’, multiply in the insect body and introduce to healthy palms in subsequent feedings.

Generally, more than a single insect vector is involved in disease transmission. So far, no curative treatment has been found in the world for phytoplasma-borne diseases in any plant. Also, control of the insect vectors to the extent of successfully managing such diseases has been futile.

Hence, diseased plants are capable of spreading the disease throughout its lifespan. The ‘phytoplasma’, which is distributed in the entire plant manifests the physiological processes causing symptoms such as yellows, little leaf, inflorescence decay and abnormal growth etc.

Different ‘phytoplasmas’ vary in its effect in plants. The one causing Lethal Yellowing disease of coconut in the Caribbean and Africa kills the palm within six months of infection.

In contrast, ‘phytoplasmas’ inflicting Root (Kerala) wilt disease in India, which is very much similar to WCLWD and WCLWD do not kill the palm, but reduces its’ vigour resulting in gradual reduction in yield. Currently, around 40 percent nut loss is observed in severely affected palms.

As soon as WCLWD was diagnosed as a phytoplasma-borne disease, the Coconut Research Institute consulted many local and overseas scientists and a Steering Committee was set up to formulate management strategies. Two options were available, either eradicate or live with the disease.

Generally, in short-term crops the best practised method is to destroy the diseased plants. However, considering the impact of removing a large number of palms (nearly 100,000) in the social and economic status of people in the Southern Province recommendations related to the second option was made.

Measures were taken to prevent the spread of the disease to other areas of the country by maintaining a 3 km wide disease-free boundary around the diseased area and preventing transport of palms and their live parts (except, de-husked coconut and de-husked arecanut) out of the diseased area.

Many experiences on the nature of the disease and its spread was gained in the last two years of implementing the program. The disease incidence within the diseased area has increased up to nearly 350,000 palms distributed in 65 percent area of the Southern Province, which is 5.8 percent of the total coconut extent of Sri Lanka.

Although, the disease-free boundary mainly prevented the gradual spread of the disease beyond it, isolated incidences in few places quite far from the boundary were reported. Similar observations have been made in Root (Kerala) wilt affected States in India, where over 50 km jumps of the disease has occurred.

The behaviour of insect vectors is responsible for this nature of spread in phytoplasma-borne diseases.

Although, the insect vectors are slow-fliers spreading the disease slowly, they could be carried away by wind currents to far places to build up isolated disease incidences. Also, it was found that severity of WCLWD within the diseased area progresses causing up to a 40 percent yield loss presently.

The above evidences clearly reflect the danger of the disease and the risk of its spread to other areas.

Hence, more stringent measures to combat WCLWD are urgent. Currently, we are at an important juncture in deciding whether we let the whole country invaded by WCLWD in years to come or employ tough actions to safeguard coconut industry.

It is argued that India is living with the Root (Kerala) wilt disease for so many years still having coconut in their home gardens and plantations and why not we? It should be noted that the situations in India and Sri Lanka is different.

By the time the Root wilt disease was confirmed as a phytoplasma-borne disease in about 15 years ago, the 150-year old disease has spread throughout the Kerala State and in some parts of the adjoining States.

Hence, India had no alternative other than adopting measures to prolong the life span of infected palms. The disease was so widely spread there the removal of diseased palms in order to minimize disease density has been too late.

However, India is also promoting removal of diseased palms in the border areas and planting with disease resistant coconut varieties.

In Sri Lanka, the disease is still confined to a small area of the country and being the third largest plantation crop and a major component of Sri Lankan food basket and energy source of our daily diet it is not too late to take actions to minimize the disease incidence and spread in order to safeguard our coconut industry.

Considering all the above factors, the Coconut Research Institute with the concurrence of the Steering Committee on WCLWD recently recommended removal of diseased palms. It is planned to remove 20,000 palms in 2010 commencing from the areas adjoining the boundary inwards.

It should be recognized that this is not a simple task and many issues have to be considered in the process of marking infected palms. It is important that diseased palms are identified correctly.

The early symptoms of flattening and inward bending of leaflets occurring at various degrees is difficult to detect unless by a trained eye and the grower hesitates to accept that the palms are diseased at this stage. Currently, all diseased palms, except the high bearing ones at the early stage of disease are being marked for removal.

Since, the disease generally takes up to about three years to show symptoms (incubation period) the symptomless infected palms and the infected palms at the very early stage of disease that are not been marked at the first round will be marked in the subsequent rounds. It is expected that systematic removal of diseased palms over a period of about five years could curtail the disease incidence.

It has been decided to pay Rs 500 for the diseased palms less than five years old, Rs 2,000 for diseased palms of 5-45 years age and Rs 1000 for diseased palms over 45 years as a relief payment. Because this payment provided to the grower is not sufficient to compensate his economic loss, a crop diversification program has been planned.

Depending on the suitability of each area for different cash crops such as tea, rubber, cinnamon, cashew, fruit crops and animal husbandry and growers’ preferences assistance will be provided to compensate their lost income. However, many growers prefer to establish coconut again.

In India, it has been found that a dwarf green variety of coconut, which is very much similar to Sri Lankan dwarf green variety show high tolerance to Root (Kerala) wilt disease.

Investigations have begun to determine the level of tolerance in the Sri Lankan dwarf green variety and possibility of issuing seedlings to the home gardens and coast line of affected areas in near future.

Also, research has already commenced identifying other tolerant varieties/cultivars/hybrids for introduction in these areas within the next three years. In this direction available varieties and hybrids of coconut in Sri Lanka have been planted in the diseased area to screen for disease tolerance/resistance.

Also, palms without disease symptoms have been observed in midst of heavily infected plantations. These palms will be selected for breeding of resistant hybrids and hopefully in next five-seven years it would be possible to introduce a coconut cultivar resistant to the WCLWD in the Southern province.

Over the last three years, the Coconut Research Institute has involved in extensive research studies to develop reliable diagnostic methods, understand disease spread and impact of the disease on physiology, nutrient status and yield of the palm, insect vectors transmitting the disease and possibility in using agronomic methods to reduce the impact of the disease.

Several advances have been made to-date. Also, several Universities of Sri Lanka and Department of Agriculture have joined with CRI to develop disease diagnosis tools, insect vector studies and determining factors affecting the disease.

Recently, Plantation Industries Ministry established a special Project Unit to undertake all aspects and accelerate in the removal of diseased palms, conduct crop diversification program, implementation of plant quarantine regulations and undertake awareness programs to carry out the proposed control methods of WCLWD more effectively and efficiently. The Project Office is located at the Regional Office of the Coconut Cultivation Board at Nupe, Matara.

The decision to remove diseased palms in the Southern province should not be taken as a destructive effort. It is an effort for a worthy cause to protect our entire coconut industry and hand over this precious industry to our future generations.