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.
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.
The only hope for the long-term management of the disease is the use of disease-resistant clones. 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.
Crown budding or base budding of highly susceptible clones with resistant genetic material is also a successful strategy in the management of new epidemics. Base budding could be performed up to the second year after planting whilst crown budding is recommended for 2–3 year old clearings.
Adoption of cultural practices recommended to minimise plant stress also reduces incidence of the disease. Balanced fertiliser application, weed-free environment and clearings with mixed clones help plants to withstand the disease up to a limited extent.
Management in 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. Increased trade and movement of tourists and businessmen between the American tropics and the Asian countries can possibly introduce devastating diseases like the South American Leaf Blight into the Asian territory. 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, disease-resistant genetic materials through the CIRAD-Michelin-IRRDB co-operation. This achievement is the fulfilment 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.
White root disease
White root disease, a malady reported with the domestication of the rubber tree, has now become the most devastating root disease of Hevea rubber in both the African and Asian continents (except in India). The most seriously affected countries are Indonesia, Malaysia, Sri Lanka, Thailand, Phillippines, Gabon, Cameroon, Ivory Coast and Nigeria.
This silent killer has already destroyed 5–10% of the rubber lands in severely affected countries resulting huge bare patches and significant reduction in productivity. For instance, area affected in Indonesia alone is more than 80,000 ha and the estimated economic loss in year 2007 was over $ 200 million. Today, with the attractive rubber prices, this figure would be several-fold higher and this disease continues to remain as an increasing threat to the future development of the rubber plantation industry.
Identity of the pathogen
Presently, the causative agent of the white root disease is referred to as Rigidopurus microporus. The original name Fomes semitostus was changed from time to time.
The initial above ground symptom is the development of a slight off-green colour in the foliage, which later turns yellowish. The leaves become leathery and curves downward giving typical buckling appearance. In some instances, the trees flower premature or during the off season. At the advanced stages, leaves turn brown in colour and eventually fall off resulting in die-back of the affected tree. However, a marked variation in the above ground symptoms has been noticed during the recent past on certain clones (eg. RRIC 100). Instead of typical yellowing, the foliage of these affected trees suddenly becomes reddish brown and totally defoliates within few days.
The most obvious symptoms are produced on the roots and at the collar region and these can be used to distinguish the white root disease from other root diseases. The fungus produces whitish, profusely branched rhizomorphs (stands of mycelia) on the affected roots. These are firmly attached to the surface of the roots and turn pale yellow when old. At the final stages, rhizomorphs advance up to the collar region.
Characteristic fructifications of the fungus appear only at the latter stages of the infection. Though the disease could be easily confirmed from these fructifications, the diseased plants are beyond treatment at this stage. Fructification of Rigidoporus microporus is an unstalked bracket produced at the collar region in tiers or on the large exposed stumps during wet weather. Typical fructifications are semi-circular, about 20 cm in diameter, concentrically zoned and grown at horizontally from the wood.
When fresh, the upper surface is yellowish orange with a lighter colour margin whilst the lower surface, is orange brown with large number of minute pores.
It has a wide host range which includes shrubs, creepers, woody horticultural and jungle plants. This is one of the major reasons why white root disease has become extremely troublesome. All these hosts act as centres of infection from which the disease can spread on to rubber. Economically important hosts include tea, coffee, coconut, cacao, oil palm, Ficus species, jack, mango, bread fruit, rambutan, durian, bamboo, manioc, avocado, mongosteen, camphor, albizzia and crotolaria.
Management of the disease
The researches carried out for more than a century have provided a wealth of information on the control of the white root disease. Presently, technology is available to prevent and manage this deadly disease. However, the existing technology is not fully implemented in rubber plantations, especially in smallholdings, due to several limitations.
It is well-established fact that there should be a source of inoculum (infected rubber plant/alternate host plant or their root debris) for the spread of the disease to a healthy tree. The technologies recommended in prevention are based on the basic principle of removing all possible root debris from the replanting or new planting site.
Carrying out a regular inspection of the clearings to detect the infections and drenching of recommended fungicides to the affected plants, including the two neighbouring trees, are recommended in controlling the disease today. The success of the treatment totally depends on the early detection of the infections.
Towards a disease- free world
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 well-equipped with the basic information on maladies affecting the rubber tree. They are now in a position to fight the ever-changing challenges of the Hevea tree diseases that can help for a sustainable growth of the rubber plantation industry.