New smart clones can fight most diseases
The majority of diseases affecting Hevea rubber tree, considered as economically important, were recorded with the beginning of the 20th century. Since then, more than 40 significant diseases have been discovered and most of these find a world-wide distribution today.
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, three diseases, namely, the South American Leaf Blight, Corynespora Leaf Fall and White Root Disease are the most threatening maladies affecting the rubber tree.
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.

The story begins

The first abnormality of the rubber tree that came to the notice of the pathologists was a shining black encrustation that appeared on the underside of the rubber leaf in the Amazon Valley. In 1900, the mycologist P. Hennings identified the causative agent as the fungus Phyllachora huberi and published the information in Hedwigia beiblatt Bd. This is believed to be the first record of a parasitic fungus attacking Hevea rubber.

The diseases of historical importance and current status

Within a few decades after the first disease record, more than fifty fungal pathogens were identified in rubber plantations and, in 1911, the first ever scientific review entitled The Physiology and Disease of Hevea brasiliensis was published by the eminent plant pathologist T. Petch.
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 disease scenario

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, 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.
Thanks 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.
(Will continue in the next issue)