An attempt to validate molecular and field level screening results for the Corynospora leaf fall disease in rubber (<em>Hevea brasiliensis</em>)

Abstract

Corynespora Leaf Fall (CLF) disease is one of the serious diseases, caused by Corynespora cassiicola, affecting rubber (Hevea brasiliensis L) plantations. Clones rated as resistant to the disease under the polybag nurseries became susceptible at the field level causing major problems in clone recommendations. Therefore, it is of utmost importance to add new CLF resistant genotypes to the breeding pool. Therefore, the present study was carried out to attempt to validate the molecular screening by field screening results. The molecular screening was carried out using 35 genotypes from 2005 hand-pollinated progeny, their grandparents (RRIC 100 and RRIC 103), grate grandparents (RRIC 52 and PB 86), and two check clones (RRISL 201 and RRISL 208). The 2005 hand-pollinated progeny which has comprised with self progenies, raised at 1978 hand pollination by selfing at CLF susceptible clone RRIC 103 and CLF resistant clone RRIC 100. Four SSR Primers (HB 1, HB 11, HB 29, hmct 5) were selected based on polymorphism between the CLF free clone RRIC 100 and susceptible clone RRIC 103 for molecular screening. Field screening was done at polybag nursery, budwood nursery, and at field level in three locations viz., Nivithigalakale, Monaragala, and Gallewatta. Completely randomized design (CRD) was used with five to ten replicates. Disease assessment was carried out allowing plants for the natural infection based on the index developed for scoring of disease severity. Observations were taken three times during peak and off seasons of CLF disease occurrence and were assessed along with control clones. All primers generated two fragments for Hevea and built the genetic distance matrix using a power maker (V 3.0) computer program and a tree diagram was drawn using the Tree view computer program. Cluster analyses revealed four distinct clusters. Two primary clones, PB 86 and RRIC 52, and the clones RRIC 103 and RRIC 201 were grouped and another cluster was again grouped into three main sub-clusters. Around 40% of field screening results obtained agreed with molecular grouping whereas, 57% were not agreed and around 3% of genotypes did not show a clear correlation. However, further screening at the field level and molecular screening is needed.