A comparison of symptom-development by different isolates of Phellinus noxius: the causal agent of brown root disease of rubber

Abstract

Brown root disease caused by the pathogen Phellinus noxius is an emerging disease condition in the Sri Lankan rubber industry. The possibility of the development of variant pathogen isolates with more pathogenic forms can be considered as one of the factors influencing the increased frequency of its occurrence in the country during the recent past especially in certain parts of the country. The study was conducted to evaluate the variability in symptom development ability of 24 Sri Lankan isolates of Phellinus noxius. A pot trial was carried out by artificially inoculating three months-old rubber seedlings with an inoculated mixture of rice bran and saw dust. Forty seedlings were inoculated with each pathogen isolate, and another forty seedlings were kept as controls without inoculation. Starting after two weeks of inoculation, ten destructive samplings were carried out at two weeks intervals to observe the pathogenicity levels of the different isolates. Based on the below-ground signs and symptoms, a pathogenicity score was given to each uprooted plant. Then those ranks were subjected to Kruskal–Wallis analysis and subsequently to the Wilcoxon rank-sum test. A variation of pathogenicity was observed among the 24 Phellinus noxius isolates. As all the isolates showed a stabilized pathogenicity value at three and half months of the inoculation, a cluster analysis was performed for the mean score values of pathogenicity rank of different isolates at three and half months and the developed dendrogram showed that the test isolates were separated into two main clusters at the similarity level 0.8. It denotes that the studied pathogen population consists of variability and these results can be applied at the development of management strategies.