Bevelopments in rubber diseases management

Most of t he major diseases of Hevea brasiliensis are of worldwide distribution (with the notable exception of South American leaf blight, against which strict quarantine regulations are enforced by rubber-growing countries outside the Americas to prevent the unauthorised import of Hevea) but their local severity and importance vary from one region to another.

Developments

However, since the latter part of the 20th century, there has been a changed scene in the maladies of the rubber tree. This is mainly due to the production of clones that can resist the common diseases and the well-acceptance of the new genetic materials by the growers. However, some of these breeds together with traditional clones succumbed to new diseases, thereby threatening the world natural rubber industry.

Other factors which contributed to the spread of new diseases are the expansion of rubber cultivation to the new localities and the non-adoption of proper cultural practices by the growers. Presently, two diseases, namely, the South American Leaf Blight and Corynespora Leaf Fall are the most threatening maladies affecting the rubber tree.

Background

The rubber tree Hevea brasiliensis, a jungle plant native to the tropical rain forests of South America, was introduced to Asia during the latter part of the 19th century. From this single introduction by Sir Henry Wickham, massive South East Asian and South Asian rubber plantations were developed which span more than nine million hectares today. As with any other agricultural crop, the incidence of the number of diseases increased with domestication and so far more than hundred pathogens have been identified as capable of attacking the rubber tree.

Within a few decades after the first disease record, more than fifty fungal pathogens were identified in rubber plantations and. The maladies identified as the economically important at the beginning of the 20th century were the Oidium leaf fall (OLF), Collectotrichum leaf diseases (CLD), Phytophthora leaf fall (PLF), Bark rot (BR) and White root disease (WRD). A considerable effort was made to control these diseases which were present worldwide.

As part of the management strategies, chemical control was widely practised since the beginning of the 20th century. Dusting of sulphur and copper in the refoliation period and the South West monsoon period respectively, was common in the rubber plantations of most of the leading rubber growing countries in Asia.

Until the middle of the 20th century, recommendations on disease management have been made without paying much attention to economic benefits and environmental pollution hazards. However, this attitude was changed during the middle of the 20th century 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 the study of 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, or even totally eliminate the use of chemicals in some instances, to control the diseases. Though more than 80% of the common diseases share the same geographical distribution, the economic threat of each disease varies from one country to another, based on the micro-climate within the same country and the type of clones cultivated.

Changing scenario

One of the interesting features is the considerable change taken place in the relative importance of the diseases during the last several decades. Presently, some of the traditional diseases like OLF, CLD, PLF and BR have become less significant and cause minimal damage to the rubber plantations. The secret behind this is the success of the breeding programmes that have been in progress since the middle of the 20th century with the aim of producing clones tolerant to the destructive diseases present during that era.

Due to the effort of breeders and pathologists, the leading rubber growing countries are presently in a position to recommend Hevea clones which can resist most of the economically important diseases. The present replanting trend shows that only the clones that can resist diseases are being accepted by the growers. As a result, it seems that the chances of occurring disease epidemics and the subsequent yield losses due to several significant pathogens will be very remote in the future. However, root diseases, especially white root disease, will continue to pose a challenge during both mature and immature stages of rubber.

Integrated approach

Corynespora Leaf Fall (CLF) is a challenging disease, spread with the introduction of new breeds. Unfortunately, towards the end of the 20th century, a few of the new high-yielding breeds together with traditional clones succumbed to a new type of pathogen called Corynespora cassiicola, threatening world rubber production. The disease was reported first in India in 1958 and later in Malaysia, Sri Lanka, Indonesia, Thailand and the African countries. It has now become a serious threat to the world natural rubber industry. Since its first epidemic in the latter part of the 1980s, many outstanding clones, namely, RRI 600, RRII 105, RRIC 103, PB 260, RRIC 110, GT1 and IAR 873, succumbed to the disease.

One of the unique features of this pathogen is the production of different types of symptoms depending on the type of the clone and maturity state of the plant. The typical symptom which is described as railway track appearance or fish bone pattern is unique for the Hevea clones RRIC 103, RRIC 52, RRIM 600, IAN 873, RRIM 725 and seedlings in nurseries. The symptoms produced on the leaves of the clone RRIC 110 are either irregular or polyhedral in shape and surrounded by an extended yellow halo when the leaflet is viewed against the light. Appearance of blackish linear lesions on the midrib of leaflets is the common symptom of the clone RRIC 133. Lesions produced by Corynespora cassiicola on the clone RRIC 132 is more or less similar to the lesions of bird’s eye spot disease caused by Dreshslera hevea. However, this diversity of symptom production has become a limiting factor to an early diagnosis which is essential for the efficient management of the disease.

Disease management

Over the last two decades, CLF has emerged as the most devastating leaf disease of rubber plantations in the Asian and African continents. Chemical control, the most popular technique in disease management, is not practical due to the dense canopy of the trees reaching a height of about 30 metres, undulating terrain of rubber lands, lack of appropriate machinery, high cost of labour and chemicals besides environmental concerns. Presently, no chemical control is practised in any part of the world except in India where affected rubber plantations are sprayed with fungicides with a view to reducing the inoculum potential and arresting disease spread.

Today, attention is paid all over the world to develop clones having resistance to CLF as the main tool to manage the disease. For instance, the secret behind the success of controlling CLF in Sri Lanka (the country worst affected during the first epidemic) was the development of new clones and the intensive screening of such new breeds before releasing them to the growers. However, there is always a danger of breaking down of resistance due to development of new races of the pathogen as observed with RRIC 110 and RRIM 600 clearings during the second world wide epidemic.

Nurseries

Another challenge is the high susceptibility of the plant to Corynespora leaf fall disease, especially in the juvenile stage. The clones which are highly resistant in the field succumb to the disease in polybag stage in the nurseries. Fortunately a protocol based on fungicide application is already available to combat this problem successfully

South American Leaf Blight

The South American Leaf Blight (SALB) is a disease that has changed the history of the world rubber plantation industry and continues to threaten its future. Potential threat of the diseases of quarantine importance to the African and Asian continents has increased tremendously with the beginning of the new millennium. The tremendous speeding up of air travelling and the establishment of direct air links between the SALB-endemic countries and leading rubber producing countries in Asia have greatly reduced travelling time, resulting in the survival of the spores and the spread of the disease into new areas. If the pathogen crosses the Pacific Ocean, the chances of the disease reaching epidemic levels in Asia are very high as almost all the clones planted in this region are susceptible to SALB. Moreover, the climatic conditions in these countries are extremely favourable for the establishment and spread of the pathogen.

Milestone in SALB research

One of the breakthroughs in SALB research during the recent past has been the development of high-yielding, diseaseresistant genetic materials through the CIRAD-Michelin-IRRDB co-operation. This achievement is the fulfillment of a long-felt need and it gives us confidence in the re-establishment of economical rubber cultivation in its motherland. These resistant clones are now maintained at CIRAD glasshouses in France with a view to distributing planting materials among the member countries. It is hoped that all IRRDB-member countries in the Asian and African continents will take the advantage of this golden opportunity and collaborate with CIRAD-MIchelin-IRRDB to evaluate these genetic materials for their agronomic performance and resistance to major diseases present in respective countries.

Conclusion

Intensive research activities carried out over a century by the scientists have provided wealth of information on biology and epidemiology of the Hevea pathogens. Based on these information, a combination of crop sanitation, disease resistance, cultural, biological and chemical procedures are being recommended for the management of the maladies of the rubber tree. It is well-accepted that this integrated approach minimises the disease incidence at a very low cost with minimum damage to the environment. Today, the rubber pathologists are wellequipped with the basic information on maladies affecting the rubber tree. They are now in a position to fight the everchanging challenges of the Hevea tree diseases that can help for a sustainable growth of the rubber plantation industry. (The writer can be contacted via

treecrops@gmail.com)