Rubber Research Institute of Sri Lanka

Recommendations have been made by the Rubber Research Institute of Sri Lanka (RRISL) for proper nursery management in order to raise high quality planting materials1. However, some of the recommendations have not been practiced by the growers. This may be due to economic reasons, lack of awareness, or to minimize costs and increase profit margins. Sometimes the non-adoption of recommended practices results in unexpected problems leading to complete destruction of their nurseries and field plants. A classic example of this is the spread of a collar rot disease in seed germination beds and a disease condition similar to target leaf spots devastating over 100,000 seedlings in several commercial nurseries in Sri Lanka in the late 1980s and early 1990s. Recently target leaf spot disease was found to have spread to field plants of the immature clearings.

The causal organisms were isolated from leaves, stems and hypocotyls of the affected plants. The fungus Sclerotium rolfsii Sacc. (perfect stage Corticium rolfsii Curzi) was identified as the responsible organism for collar rot2 while Thanatephorus cucumeris (Frank) Donk (anamorph: Rhizoctonia solani Kuhn) was proved to be the causative agent of target leaf spot disease3, 4.

Sclerotium rolfsii, a facultative parasite capable of extensive saprophytic growth within the surface layers of soil is widespread in the moist tropics and warm temperate areas5. This fungus has a wide host range including species belonging to many genera of crop plants such as Beta, Brassica, Cinchona, Citrus, Ficus, Musa, Oryza, Solanum, Saccharum and also many ornamental plants and weeds5.

However, this pathogen has not been recorded on Hevea in Sri Lanka, or in any other rubber growing country6, except for one report that S. rolfsii has been observed to attack and kill the young hypocotyl of germinating rubber seeds in Malaysia7. Thanatephorus cucumeris is an ubiquitous fungus that causes many types of diseases on numerous plants including Hevea rubber under diverse environmental conditions8, 9 and it has been identified as a pest of quarantine importance to rubber plantations in South and South East Asia10.

Since the first detection several surveys have been conducted to find the factors contributing to the sudden outbreak of these uncommon diseases in Sri Lanka. It was revealed that sand/soil mixture with organic matter has been used in the preparation of seed germination beds against the Rubber Research Institute recommendation of using pure river sand.

Furthermore, it was found that germinated seeds have been kept for several months in seed germination beds before transplanting in seedling nurseries in all the nurseries where the diseases were detected. The RRI recommendation is to remove the germinated seeds (at the hypocotyl stage) from the seed germination beds and transplant them in beds prepared for seedling nurseries. It was clearly observed that crowded conditions and high humidity in seed germination beds have predisposed the plants to these two new diseases. Furthermore, seedlings in nurseries and field plants in immature clearings with target leaf spot disease history were found covered with creeping weeds.

Subsequently, it was proved that these weed creepers which had overgrown the rubber canopies were harbouring the disease. Therefore, it appears that these new diseases could be kept away merely by adhering to the correct cultural practices, such as the use of pure sand for preparing seed germination beds; removal of seeds just after germination for establishment of seedling nurseries, and the regular weeding of nurseries and field plants.

Nevertheless, several projects were undertaken at the RRISL Laboratories to develop effective chemical control measures with a view to managing sudden outbreaks of these diseases in future. The results of these experiments showed that formaldehyde (0.12%) and tebuconazole (0.06%) are effective against collar rot11 whilst benomyl (0.15%) and pencycuron (0.25%) can be successfully used to manage target leaf spot disease12. Furthermore, it was experimentally shown that the traditional burning of soil in nurseries with disease histories also provides very effective control11, 12.

The author is highly concerned about the spread of a quarantine important pest, Thanatephorus cucumeris, into the field plants of the immature clearings in Sri Lanka merely due to the non-adoption of correct cultural practices. Hence, an Island-wide campaign was launched in 1996 and 1997 to emphasise the importance of adhering to correct cultural practices during the establishment of nurseries and the upkeep of immature clearings. Moreover, a descriptive review on symptomatology, biology and management of Thanatephorus cucumeris together with colour illustrations has been submitted to the Journal of the Rubber Research Institute of Sri Lanka for publication to increase public vigilance in this regard.

References

1. Samaranayake, C. Rootstock seedling nurseries. In Practical guide to rubber planting and processing; ed. A.de S. Liyanage and O.S. Peries. Rubber Research Institute of Sri Lanka, 1984, pp.1 - 3.

2. Jayasinghe, C.K, Liyanage, A.de S, Warnapura, S.S. Collar rot of rubber seedlings caused by Scierotium rolfsii. FAO Plant Protection Bulletin, 1988, 36, 189.

3. Jayasinghe, C.K. The natural occurrence of Thanatephorus cucumeris leaf spot on Hevea brasiliensis in Sri Lanka. Plant Pathology, 1993, 42, 473 - 474.

4. Jayasinghe, C.K. Thanatephorus cucumeris leaf spot disease in Sri Lanka. Iniernational Rubber Research and Development Board Information Quarterly, 1993, part 2: 6-7.

5. Anon. Corticium rolfsii. CMI description on pathogenic fungi and bacteria. No. 410, 1974, Commonwealth Mycological Institute, Kew, Surrey.

6. Anon. Pathology of Hevea brasiliensis. International Rubber Research and Development Board Bibliography, 1988. Secretariat of International Rubber Research and Development Board, Hertford, UK.

7. Weir, J.R. Preliminary studies on some diseases of cover crops under rubber in Malaya. Quarterly Journal of Rubber Research Institute of Malaya, 1929, 1, 29 -40.

8. Langford, M.H. Hevea diseases of the Amazon Valley. Boletim Tecnico do Instituto Agronomico del Norte, 1953, 27, 29.

9. Baker, K.F. Types of Rhizoctonia diseases and their occurrence. In: Parmeter J.R. ed. Rhizoctonia solani: Biology and Pathology. Berkeley: University of California Press, 1970, 127 - 148.

10. Anon. Rubber pests and diseases of quarantine importance to Asian rubber growing countries. Technical document No 11th SALB/96/WP/6, 1996. Association for the Natural Rubber Producing Countries, Kuala Lumpur, Malaysia.

11. Jayasinghe, C.K; Warnapura, S.S. and Fernando, I. Selerotium collar rot of Hevea seedlings and management of the disease. Indian Journal of Natural Rubber Research, 1993, 6: 5 - 9.

12. Jayasinghe, C.K; Fernando, B.B. and Fernando, B.I. In vitro evaluation of fungicides against rubber isolate of Thanatephorus cucumeris and management of foot rot of Hevca seedlings. Journal of the Rubber Research Institule of Sri Lanka, 1997, 80: 45 - 52.