A study of isolates of Rigidiporus lignosus. the causative agent of the white root disease of rubber

Abstract

Rigidporus lignosus (Klotz.) Imazaki cause the whiteroot disease of rubber (Hevea brasiliensis). Five isolates of the fungus, T1, 01, S1, F1 H2, obtained from different rubber growing districts of Sri Lanka, were examined for their growth patterns in different solid media. The secretion of cell wall degrading enzymes was also studied in the isolates T1, S1 H2 when grown in different media with different sources of carbon. The isolate T1, which is known to be the most virulent, grew fast densely on nutrient rich solid media such as potato dextrose agar (PDA) malt extract agar (MEA), but its growth was poor on Czapek Dox agar, Knop-Glucose agar the medium for wood rotting fungi. The other four isolates also grew best on PDA MEA, but the growth was less than that in T1. The growth of S1, O1, F1 H2 on the other media however, was more than that of isolate T1. All isolates except T1 formed branched, strand-like aggregations when growing on solid media. Statistical analysis of the growth studies indicated that a significant difference existed among the isolates also between the isolates the media. The isolate T1 secreted polygalacturonase, pectin lyase, B-glucosidase cellobiase. In T1, the secretion of all the enzymes occurred earlier the amount of enzyme secreted was also higher than in the other two isolates. No polygalacturonase, however, was detected when T1 was grown in the medium with the water insoluble component of rubber roots as the main source of carbon. The pH for optimum activity of polygalacturonase was 6.5 while that for pectin lyase was 10.0.