Pestalotiopsis leaf disease will accelerate declining rate of natural rubber production
By Emeritus Professor Asoka Nugawela
Former Director, Rubber Research Institute of Sri Lanka and Chair Professor of Plantation Management,
Wayamba University of Sri Lanka
The natural rubber production in the country declined from 89 thousand metric tons in 2015 to 70 thousand metric tons in 2022 according to available statistics. This level of natural rubber production is insufficient to meet the demand for this raw material in the rubber product manufacturing sector of the country. Therefore, annually around 50 thousand metric tons of natural rubber are imported spending around 85 million US$.
Further, non-availability of the raw material in sufficient quantities is a hindrance to further growth of the rubber product manufacturing sector in the country. This is the scenario of the natural rubber industry in the country, which earned more than 1 b US$ per annum a couple of years ago, is facing today.
Why does production decline?
The national productivity of rubber cultivations is currently in the range of 650-700 kilos per hectare per year. Countries like India and Thailand records productivity levels of around 1,400 kgs per hectare per year. The relevant research institutes advocates that the potential productivity is around 2,500 to 3,000 kilos hectare year. So, in Sri Lanka a huge yield gap is evident.
Lower rate of technology adoption is a major reason for the yield gap. Death of plants due to pest and diseases, unproductive trees due to incidence of tapping panel dryness, extreme weather conditions negatively impacting the harvesting process and high cost of agro-chemicals prohibiting their use also contribute to yield gap.
In addition to the above factors causing the yield gap, it is now increasingly becoming evident that the Pestalotiopsis leaf disease, first recorded in Sri Lanka in 2019 is further widening the yield gap accelerating the declining rate of national rubber production in the country.
Impact of Pestalotiopsis on yield
Yield data collected from a Regional Plantation Company having around 3,000 hectares of mature rubber cultivated in both intermediate and wet zones of the country was used to analyse impact of Pestalotiopsis leaf disease on rubber yield/productivity.
In the estates located in wet zone of the country, the Pestalotiopsis disease had been prevalent since 2019. Further, since the first reporting of the disease in 2019, its severity has increased. Each year apart from the normal natural leaf fall which take place in January/February, two other abnormal leaf falls occurred subsequent to both SW and NE monsoonal rains.
Therefore, three rounds of complete leaf fall have occurred in a year in rubber cultivations located in the wetter areas of the country since 2019. This scenario is despite the adoption all recommended good agricultural practices and also adopting recommended Pestalotiopsis management programs. However, the estates in the intermediate zone have not been affected by this disease. But even in the intermediate zone, in the year 2023 a mild incidence of Pestalotiopsis was evident in an area, which receives a relatively high level of rainfall.
Yield data gathered reveals that in estates located in the Pestalotiopsis disease prone wet zone a declining trend in land productivity/yield is apparent from financial year 2021/22 (Table 1). It should be mentioned that the yield decline in 2021/22 financial year is a combination of both Pestalotiopsis disease and a management decision not to undertake yield stimulation in fields tapped under low intensity tapping systems. This has caused a yield decline in the non Pestalotiopsis disease prone intermediate zone as well in 2021/22 (Table 1).
The mean land productivity/yield in the Pestalotiopsis prone wet zone reveals that the declining trend in yield increases year by year and in financial year 2023/24 the decline is more than 45% relative to the pre Pestalotiopsis period, i.e., 1,294 to 700 kgs/ha/year. The rubber growers expect the yield decline to be even higher in the coming years if no effective system is available to control this disease. It is very clearly evident that in the intermediate zone where Pestalotiopsis disease severity is marginal there is no such decline in land productivity/yield during this period (Table 1).
Sustainability
With the drop in yield likely to increase from the present 45% whilst the costs are continuously escalating, the rubber cultivations in the wetter region of the country will no longer be economically viable. It should also be emphasised that the young rubber clearings are also severely affected by this disease. The growth rates of young rubber plants have reduced which will lead to longer immature/uneconomical periods resulting in high capital costs.
Under such economic situations rubber growers will find it hard to invest on replanting, soil and moisture conservation, nutrient management and also to offer work to the workers threatening the sustainability of the industry under climatic conditions conducive to severe incidence of Pestalotiopsis. The income levels of the rubber harvesters too have dropped due to lower productivity affecting their livelihood and quality of life.
Way forward
Attempts are being made globally to find an effective chemical control method to combat this disease but without succes. Other options being looked at are breeding and selection for resistance and nutrient management. Finding solutions other than chemical control is going to be a mid/long term strategy. Further, there is no guarantee that a solution would be found.
In the light of this situation, the authorities should consider other possible interventions to resolve this issue, which is seriously threatening the viability of rubber cultivations in the wetter areas of traditional rubber growing regions. One such intervention to be considered seriously by relevant authorities/policy makers is to promote and support the cultivation of rubber in the intermediate zone of the country in order to escape from the disease.
