The Hevea rubber tree native to tropical rain forests of South America was the second alternative crop available to fill the vacuum created by the collapse of the coffee plantation industry in Sri Lanka due to the spread of coffee rust epidemic towards the latter part of the 19th century.
With this background, rubber industry began to develop and by 1890s planters realised the potentialities of rubber thus, commercial planting of rubber commenced. By the year 1890, this agricultural commodity was cultivated over 121ha and extent increased to some 16,000ha by 1905.
The first abnormality of the rubber tree was brought to the notice of the mycologist of the Government of Ceylon with the beginning of the 20th century as a canker on stem and branches. J. B. Carruthers identified the causative agent of this disorder as the fungus Nectria and his findings were disclosed at a special general meeting of Kalutara Planters’ Association at the Tebuwana Rest House on October 31, 1903.
In 1905, Petch, then botanist and mycologist to the Government of Ceylon, discovered six fungal species pathogenic to leaf and two root disease causing fungi which were believed to have spread to cultivated rubber from the uncleared stumps of the jungle. Subsequently, a suspected canker condition and a fungal pathogen which was responsible for fruit rot were also brought to the notice of the planting community. This eminent mycologist compiled the first ever scientific review on diseases of the rubber tree in 1911. A descriptive account on the diseases and their management was published during the year 1921 in the book entitled The Diseases and Pests of the Rubber Tree.
Weir and Murray, two other Hevea pathologists also contributed significantly to the diagnosis and control of Hevea diseases before the 2nd World War. Since the middle of the 20th century, several valuable reviews have been offered on the disease situation of the rubber tree in Sri Lanka. The publications of Chee (1976), Sharples (1936) and Siripathi Rao (1975) from Malaysia and Steinmann (1925) from Indonesia also remain as useful sources of information as all important diseases of rubber tree are generally common to the rubber growing countries in South East Asia including Sri Lanka.
The maladies identified as the economically threatening diseases at the beginning of the 20th century were the Oidium leaf fall (OLF), Colletotrichum Leaf Disease (CLD), Phytophthora Leaf Fall (PLF), Bark Rot (BR) and White Root Disease (WRD). These diseases were present island-wide and a considerable effort was made to control them.
With regard to the management strategies, chemical control was widely practised since the beginning of the 20th century. Dusting of sulphur and copper during the refoliation period and South West monsoon, respectively, was a common scenario in the rubber plantations of Sri Lanka during that period.
Until the middle of the 20th century, recommendations have been made without paying much attention to economic benefits and environmental pollution hazards. However, this attitude was changed during 1960’s and pathologists initiated extensive research projects with a view to minimising the application of chemicals to the environment. As a result, high priority was given to study the biology, epidemiology, host parasite relations and disease resistance. On the basis of these findings, it has been possible to reduce the number of applications of fungicides to control diseases or even totally eliminate the use of chemicals in some instances (Table 1). Further, it is worthy to note that in diseases where chemical control is still recommended, less hazardous alternatives have been discovered during the recent past (Table 1).
The first ever check list of pathogens associated with the rubber tree was launched in 2001, after nearly a century of the first disease discovery. The available on pathogenic records surveyed the available literature since the beginning of the 20th century was surveyed and every effort has been made to present a complete list of pathogens documented on the rubber tree in Sri Lanka. According to his compilation, 63 pathogens have been recorded comprising 60 fungi, a nematode, a virus and an alga since the first report of the pest attack on rubber in 1903. However, it has been shown that only around 20 diseases are commonly seen in plantations and nurseries today. The virulence and the incidence of these diseases vary in each year depending on the climatic conditions prevail. It is worthy to note that the type of clone planted, elevation and cultural conditions under which the crop is grown and microclimate of the site also play a significant role in disease development and spread. This ‘Check List of Rubber Pathogens’ serves not only as a useful guide for the search of pathogens recorded on para rubber tree but also provides ready assistance for the diagnosis of the diseases presently found in plantations and nurseries.
Breeding for resistance
One of the interesting features of the disease scenario of the rubber tree is the considerable change taken place in the relative importance of the diseases during the last several decades. Presently, traditional rubber diseases like OLF, CLD, PLF and BR have become less significant and cause minimal damage to the rubber plantations in Sri Lanka. The secret behind this is the success of breeding programmes in1960’s with the aim of producing tolerant clones to the destructive diseases present during that era.
Thanks to the effort of breeders and pathologists, presently RRISL is in a position to recommend Hevea clones resistant to most of the economically important diseases. The present replanting trend shows that only the resistant clones are being accepted by the growers. As a result, it seems that chances of disease epidemics and subsequent yield losses due to several significant pathogens would be very remote during the new millennium. Further, the dream of the expansion of rubber cultivation to high elevations (above 300m) has now become a reality merely due to the development of Oidium tolerant clones by the Sri Lankan breeders.
However, root diseases especially white root disease will continue to pose a challenge during both mature and immature stages of rubber. The traditional method of application of collar protectant after exposing the diseased root system has been replaced by the less laborious soil drenching techniques when treating the significant root diseases.
Unfortunately, a few of these new breeds too succumbed to new pathogens threatening the natural rubber plantation industry in the island.
Expansion of rubber cultivation in to new localities and non adoption of correct cultural practices during the establishment and management of nurseries also predisposed the rubber plants to new pathogenic attacks. The pathogens discovered during the recent past are highlighted in the Table 2 with references to first published reports. Presently the spread of these diseases is being monitored very closely as they may gain prominence at any time due to various factors. Another interesting feature was the spread of some maladies at epidemic levels during the recent past which were considered as insignificant problems during the 20th century. One of the examples is the cockchafer grub attack spread in Avissawella and Ratnapura regions devastating young clearings in the years 2004 and 2005.
Corynespora leaf fall
One of the important events in the disease scenario of the rubber tree which affected the Sri Lankan rubber growers was the spread of Corynespora leaf fall (CLF) during the latter part of the 1980s. This disease was first detected in several poly bag nurseries in Kalutara and Galle districts. Within two years CLF had spread to all rubber growing districts severely affecting RRIC 103, one of the prestigious clones produced by the Sri Lankan scientists. By 1988, around 4,500ha representing 2.2% of the total area under rubber in Sri Lanka was severely affected causing repeated defoliations and die back of trees.
The stake holders who had planted their holdings with the clone RRIC 103 expecting higher yield and income, obliged to uproot and destroy their plantations due to the RRISL decision. As a result, the rubber growers especially the smallholders had to suffer considerable hardships. Apart from the economic loss, growers who had planted RRIC 103 became rather reluctant to cultivate RRIC clones during the CLFD epidemic period.
The government of Sri Lanka considered this issue seriously and appealed to the World Bank to pay compensation to the growers as RRIC 103 was released to the farmers based on a recommendation made by them. Food and Agriculture Organisation (FAO) favourably considered the facts presented by the Sri Lankan government and agreed to reimburse the cost incurred in the relief payments. A sum of around US$ 600,000 was paid to the affected smallholders as a relief payment with the primary aim of restoring confidence in new RRIC clones.
Today, nearly 25 years after the first epidemic, CLF affected area is restricted only to around 20ha in Sri Lanka (total area under rubber is 120,000ha). Most of these lands represent clearings kept for experimental purposes without uprooting during the second epidemic. The secret behind this success story is the selection of clones through intensive screening during the breeding programmes since 1985. Since the establishment of hand pollinated (HP) seedlings, it takes more than 15 years to promote a clone to Group 1 recommendation level. This process involves around seven steps and genetic materials are being closely monitored for CLF susceptibility in each step and only disease tolerant clones and clones with mild infections are promoted to the next category. As a result, Sri Lanka, the worst affected island in the late 20th century now enjoys a CLF free new millennium.
However, rubber growers should be extremely cautious as break down of the disease resistance in tolerant clones can take place at any time and as a result disease may spread to other outstanding clones and destroy them at any moment.
Diseases of quarantine importance
South East Asian rubber growing countries including Sri Lanka are free from certain important insect pests and diseases which affect the South American region, the motherland of rubber tree. The most devastating disease of quarantine significance is the South American Leaf Blight caused by the fungus Microcyclus ulei (Table 3)
Today, the potential threat of this disease has increased tremendously due to high speed air travel, increased trade and movement of tourists between American tropics and Sri Lanka. Though strict quarantine measures have been imposed to protect the territory of Sri Lanka, quarantine failures can occur at any time. However, steps have been taken to educate the Extension Officers, Quarantine Officers, Custom Officers and rubber growers on the symptomatology of the disease for early detection and rubber growing area is monitored regularly for any disease outbreak.
Plant Protection Ordinance Act No. 35 of 1999 provides the legal basis for the protection of Sri Lankan territory against dangerous pests and diseases.
Several diseases which were of minor importance in the past have now gained considerable importance in many countries. For example, Corenespora leaf fall disease which was considered as a minor leaf disease in seedling nurseries in 1958, has assumed epidemic proportions in several rubber growing countries such as Malaysia, Thailand, Indonesia, besides Sri Lanka during the latter part of 1980.
Tar spot, a relatively insignificant disease which normally develops slowly on old leaves, now occurs widely and causes leaf fall in Brazil and Colombia. Two other diseases, Phytophthora leaf wither and Thanatephorus cucumeris, which were of little concern a decade ago in Brazil, now require regular spraying to prevent extensive leaf fall and die-back.
The changing climate will also have an influence on the development and spread of diseases unknown to the industry. Therefore, strict vigilance on the part of the rubber industry will help the research to investigate and recommend suitable remedial measures, before a major calamity sets-in and the industry is expected to up-root and burn such infected trees as was in the case of the spread of Corenespora leaf fall disease in Sri Lanka and also the current recommendation of the Coconut Research Institute of Sri Lanka to prevent the spread of ‘Weligama Coconut Leaf Wilt Disease’.