Potential Pathogen of Hevea Rubber

C.K. Jayasinghe

Rubber Research Institute of Sri Lanka

The relative importance of rubber diseases has changed over the years since its domestication. A classic example for this is the recent spread of Corynespora leaf fall to devastate several outstanding rubber clones in the African and Asian continents. Although the fungus Corynespora cassiicola was recorded in South and South East Asian rubber growing countries on various crops more than half a century ago it only came to prominence on rubber during the late 1980s.

In 1988, at a workshop held at the RRIM Experimental Station at Sungai Buloh, Malaysia, three more fungal pathogens namely Fusicoccum, Guignardia and Cylindrocladium were also shown to be potentially threatening leaf pathogens on rubber. Two years later our attention was drawn to these diseases again by Chee1 stating that these diseases have come into prominence in Malaysia, either in the field or on experimental clones. It was further shown that clones of Hevea react differently to these pathogens. Highly susceptible clones for Cylindrocladium quinqueseptatum in budwood nurseries are PB 86, RRIM 607 and RRIC 36; whilst PC 111 and PC 121 showed defoliation in the field due to Cylindrocladium infections 2, 3.

The pathogen C. quinqueseptatum is yet to be reported from Hevea in other rubber growing countries in American Tropics and South and South East Asia, but it is noteworthy that C. quinqueseptatum is endemic in most of these countries, that is Brazil 4, India 5 and Indonesia 6 where it causes severe defoliation during prolonged wet weather on several hosts including clove and eucalyptus, which are grown in the same agroclimatic zones recommended for the cultivation of rubber.

In Sri Lanka the fungus Cylindrocladium quinqueseptatum was first recorded as a pathogen in 1982 on clove 7 from polybag clove seedlings. No attempt was made to study this fungus in Sri Lanka until the early 1990s when investigations were initiated once our attention had been drawn to the importance of this fungus as a foliar pathogen of rubber in Malaysia 1. Further, it is of interest to note that the clone RRIC 36, one of the rubber clones stated to be severely affected in budwood nurseries in Malaysia, was developed in Sri Lanka and sent to Malaysia in 1954 under the multilateral clone exchange programme.

Although the species C. quinqueseptatum was identified8 in the year 1950 and it has a wide host range and geographic distribution within the humid tropics, the biology, epidemiology, pathogenic variations and management of the fungus have received very little attention, except for a few studies in South Africa9, the United States of America 10, Northern Australia 11 and Southern India 12, 13, 14, 15 using mainly eucalyptus isolates.

The present project addressed all of the above issues and was designed to look successively at the cultural and reproductive morphology of Cylindrocladium quinqueseptatum; the morphological, cultural and pathogenic variation among the different isolates; the disease tolerance of Hevea clones grown in the Eastern Hemisphere; the production, germination and viability of spores; the biochemical strategies of infection (that is mycotoxin and enzyme production), and finally the possibility for chemical control as a management strategy. Experiments were commenced in 1993 and project was terminated in 1997, contributing the following information to the world literature 16, 17, 18, 19.

• Cultural characteristics and reproductive morphology of the clove isolate of C. quinqueseptatum (IMI 342173) were established after proving the pathogenicity on rubber and shown to be similar to C. quinqueseptatum isolates studied in different parts of the world from various hosts.

• Some isolates collected from different agroclimatic zones in Sri Lanka showed a great deal of variation in their morphology. Subsequent studies showed that distinct variations exist among C. quinqueseptatum isolates and differences in (a) morphology (b) sporulation on CDA (c) growth at room temperature and (d) pathogenicity are significant and provide a very useful method to separate them. Further, a considerable variation in susceptibility to C. quinqueseptatum existed amongst the Hevea clones grown in the Eastern Hemisphere. Clones PB 28/59, HP 74-181, Tjir 1, RRIC 121, RRIC 45, RRIC 36, RRIC 130, RRIC 110, RRIM 712, RRIM 600 were the most susceptible and IAN 837, AV 1373, RRIC 102 and IAN 717 were amongst the least susceptible clones.

• The fungus sporulated freely when grown on artificial media when incubated between 20EC and 35EC with an optimum at 30EC. The most critical factor which influenced the spore viability and germination was the humidity. Free water, or a film of water (resulting as dew formation at 100% RH) was found to be essential for spore germination. With regard to the lesion production on leaves, lesions with a reasonable size were produced only at 100% humidity.

• The temperature also greatly influenced the spore viability and germination. Spore germination occurred above 10EC and below 35EC. Observations from the epidemiological studies clearly indicated that conducive weather conditions for the pathogen are present in most of the agroclimatic zones where rubber is cultivated, but infections are only likely to reach epidemic proportions during monsoon periods of dull, overcast, rainy weather.

• All isolates were found to be capable of secreting toxic substances to the growing medium and this toxin was proved to be thermostable (up to 100EC) and host specific. The type and the size of the lesions produced on Eugenia and different Hevea clones by the crude toxin of different isolates varied markedly. Three main clusters of clones were distinguished through cluster analysis indicating the marked variation of Hevea clones grown in the Eastern Hemisphere in sensitivity to the crude toxin.
  

• Even though all isolates of C. quinqueseptatum secreted cellulases (cellobiase and b-glucosidase) in culture the pectic enzyme activities were negligible. A marked increase in activity of cellobiase was detected on rubber leaf on the third day following infection and the activity of b-glucosidase, an inherent enzyme of Hevea leaves, increased markedly following infection. Considering the above observations and available literature it is proposed that toxins play a vital role in the initial stages of infection and development of symptoms while latter stages of interaction may be due to the activity of cellulolytic enzymes.
  

• A total of sixteen fungicides were screened against C. quinqueseptatum employing three screening techniques: the conidial germination test (CGT), the poisoned food technique, and the soil fungicide screening test (SFST). Although eleven fungicides were effective in CGT, this number reduced to five in SFST. However, only four that is benomyl, mancozeb, metalaxyl 8% + mancozeb 64%, and oxadixyl 10% + mancozeb 56% were identified as potential fungicides in the management of C. quinqueseptatum. In addition to chemical control, there is a possibility of development of a successful breeding programme to produce disease resistant Hevea clones, using observations reported on clonal susceptibility.

Group of Investigators

Principal Investigator: Dr. C.K. Jayasinghe, Head of Plant Pathology and Microbiology Dept.. RRISL

Collaborating Scientist: Professor R.L.C. Wijesundera, Head of Botany Dept., University of Colombo

Technical Assistance: Misses,T.H.P.S. Fernando & U.M.S. Priyanka,Technical Officers, RRISL,

Miss. C. Ariyaratne, Faculty of Medicine, University of Ruhuna,

Mr. C.A. Nanayakkara, University of Colombo,

Financial Support:

This project was partially funded by Sri Lanka Council for Agricultural Research Policy (12/234/184)

References

1. Chee, K.H. Present status of rubber diseases and their control. Review of Plant Pathology, 1990, 69: 423-431.

2. Anon. A new nursery leaf spot disease of Hevea caused by Cylindrocladium quinqueseptatum. Planters Bulletin,1972, 55-56..

3. Anon. Notes for estate managers. Rubber Research Institute of Malaysia, Malaysia, 1990, 1-2.

4. Figueiredo, M.B. and Namekata, T. Record of Calonectria quinqueseptata, perfect state of Cylindrocladium quinqueseptatum on Anona squamosa and Eucalyptus spp. Arquivos do Instituto Biologico, 1967, 34, 91-96.

5. Sarma, Y.R., and Nambiar, K.K.N. Cylindrocladium leaf rot of clove. Plant Disease Reporter, 1978, 62, 562-564.

6. Reitsma, J. and Slooff, W.C. Leaf diseases of clove seedlings, caused by Gloeosporium piperatum E. & E. and Cylindrocladium quinqueseptatum. Contributions of General Agriculture Research Station, Bogor, 1950, 109, 50-59.

7. Jayasinghe, C.K. and Liyanage, A.de S. Leaf spot in clove. Annual Review of Rubber Research Institute of Sri Lanka, 1982, 70.

8. Boedijn, K.B. and Reitsma, J. Notes on the genus Cylindrocladium (Fungi: Mucedinaceae). Reinwardtia, 1950, 1, 51-60.

9. Crous, P.W. and Wingfield, M.J. A re-evaluation of Cylindrocladiella and a comparison with allied genera. Mycological Research, 1993, 97, 433-448.

10. Hunter, B.B. and Barnett, H.L. Growth and sporulation of species and isolates of Cylindrocladium in culture. Mycologia, 1978, 70, 614-635.

11. Bolland, L., Tierney, J.W. and Tierney, B. Studies on leaf spot and shoot blight of Eucalyptus caused by Cylindrocladium quinqueseptatum. European Journal of Forest Pathology, 1985, 15, 385-397.

12. Anahosur, K.H., Padaganu, G.M. arid Hegde, R.K. Toxic effect of the culture filtrate of Cylindrocladium quinqueseptatum, the causal organism of seedling blight of Eucalyptus hybrid. Indian Journal of Microbiology, 1976, 16, 84-85.

13. Anahosur, K.H., Padaganur, G.M. and Hegde, R.K. Laboratory evaluation of fungicides against Cylindrocladium qiunqueseptatum the causal organism of seedling blight of Eucalyptus hybrid. Pesticides, 1977, 11, 44-45.

14. Sulochana, K.K. and Chandrasekharan, N.M. Induction of the perfect state of Cylindrocladium quinqueseptatum. Current Science, 1981, 50, 999-1000.

15. Sharma, J.K. and Mohanan, C. Pathogenic variation in Cylindrocladium quinqueseptatum causing leaf blight of Eucalyptus European Journal of Forest Pathology, 1991 b, 21,

16. Jayasinghe, C.K. & Wijesundera, R.L.C. In vitro evaluation of fungicides against clove isolate of Cylindrocladium quinqueseptatum in Sri Lanka. International Journal of Pest Management, 1995, 41: 219 - 223.

17. Jayasinghe, C.K. & Wijesundera, R.L.C. Factors affecting the production, germination and viability of Cylindrocladium quinqueseptatum spores. Indian Journal of Natural Rubber Research, 1995, 8: 38-50.

18. Jayasinghe, C.K. and Wijesundera, R.L.C. Morphological, cultural and pathogenic variation among the clove isolates of Cylindrocladium quinqueseptatum. Journal of Plantation Crops, 1996, 24: 34-42.

19. Jayasinghe, C.K. & Wijesundera, R.L.C. Cylindrocladium quinqueseptatum: cultural characteristics and reproductive morphology of the clove isolate IMI 342173 from Sri Lanka. Journal of the Rubber Research Institute of Sri Lanka, 1 996, 77: 28-37.