Field and In-vitro Studies on Corynespora Leaf Fall Disease of Hevea brasiliensis in Nigeria

Aims: This study evaluated the current incidence and severity of Corynespora leaf fall disease (CLFD) in the fields among cultivated rubber clones grown in Nigeria and the growth performance of selected isolates of the pathogen on Potato Dextrose Agar (PDA).
Study Design: The field survey was made in randomized complete block design consisting of three replicates.
Place and Duration of Study: The field study was carried out in the bud wood nursery of Rubber Research Institute of Nigeria (RRIN), Iyanomo, Edo State, Nigeria between April 2013 and August 2013.
Methodology: Ten (10) leaves per replicate were picked at random and assessed for incidence of CLFD based on severity. Disease-scoring rating chart was used in assessment of infection and calculation of Disease Index. The pathogenicity of all the different types of symptoms observed in CLFD was evaluated based on radial growth and biomass of the Corynespora cassiicola. Data collected were subjected to analysis of variance (ANOVA) using SPSS software (version 16.0). Mean separation was done using Duncan’s Multiple Range Test. All tests were carried out at 5% probability level.
Results: The incidence of CLF disease was found to be between 80 to 100% in all the rubber clones screened. When the isolates were grown on PDA, there were significant differences (P = .05) in the growth of the different isolates. The least mycelial diameter (2.08±0.25 cm) was observed in NIG 910 while the highest (6.60±0.13 cm) was recorded in NIG 804. NIG 800 showed the least mycelial biomass of 0.078±0.05 g while the highest mycelial biomass was recorded in NIG 900 (0.35±0.27 g) and NIG 910 (0.35±0.26 g).
Conclusion: In this study, it was observed that C. cassiicola infected all the rubber clones investigated with varying aggressiveness. Newly bred rubber clones should be developed and screened for resistance against C. cassiicola prior to recommendation to growers. Further studies to determine the exact role of cassiicolin in C. cassiicola pathogenicity would prove very useful in Hevea breeding programme.