Isolation of Rigidoporus microporus, the Cause of WRD of Rubber, from Some Forest Associated Plants

Natural rubber while promoting export earnings and livelihood of people supplementthousands of hectares to the forest cover. Over the past decades, the rubber yield hassignificantly increased due to the cultivation of high yielding clones. However, latexproduction still faces serious economic losses due to many biotic constraints which includesignificant losses caused by pathogenic fungi. Among them, White root disease (WRD) isvery destructive in rubber plantations of Sri Lanka and in many other rubber growingcountries. This disease has been identified as one of the major causes for the loss of plantsduring the first five years after planting resulting in low productivity levels. Inspite of the factthat disease management strategies have been clearly outlined by the Rubber ResearchInstitute, the disease incidence is showing an increasing trend. One of the main reasons forthis has been identified as the increment of the host range. In this situation expansion of thehost range of R.micrporus was undertaken. R. microporus was isolated from the symptomatichost plants such as Mucuna bractiacta, Camellia sinensis, Cinnamomum zeylanicum,Artocarpus nobilis, Alstonia macrophyll growing in and around rubber plantations. Some ofthe above crops such as C. sinensis, C. zeylanicum are intercrops recommended by the RRIwhile A. nobilis and A. macrophylla are important forest crop species grown in the vicinity ofrubber cultivations. M. bractiata is also one of the most commonly grown cover cropsrecommended for rubber plantations. The pathogen was isolated, on to Malt Extract Agarafter surface sterilizing in 70% ethanol for 3 minutes. Pure cultures were obtained and thecultural characteristics were recorded for the five isolates separately. For comparativepurpose isolate from H. brasiliensis was employed. Colonies showed white flattened cultures.Isolates from M. bractiacta, C. sinensis, C. zeylanicum, H. brsiliensis and A. nobilis showedpuffy aerial growth while A. macrophylla showed clear concentric zones which are prominentfrom the lower side of the colonies. Among the isolates investigated, the fastest growth ratewas observed in the isolate from C. sinensis showing 1.262cm/day while the lowest growthrate 1.205 cm/day was shown by the rubber isolate. The pathogenecity of the isolatesobtained from the above hosts were proven against the respective hosts by following theKoch’s postulates under green house conditions. The cross infection abilities of the isolateswere also assessed and all isolates infected Hevea brasiliensis showing cross infectionabilities. Pathogenic and genetic variability among the different isolates are beinginvestigated.The information will be valuable in the development of improved managementstrategies against white root disease of rubber.