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.
Highlights
Natural rubber from Hevea brasiliensis is one of the most versatile industrial raw materials
Hand pollinated progeny developed in the year 2005 showed a wide range of Corynespora leaf fall (CLF) disease responses, as complete susceptible to free from the disease, is having second self progenies of the clone RRIC 100 as well as the clone RRIC 103. 2005 hand pollinated progeny which has comprised with self progenies, was raised at 1978 hand pollination by selfing at CLF susceptible clone RRIC 103 and CLF resistant clone RRIC 100
The CLF moderately susceptible clone RRISL208 was grouped with HP 09, HP 05, HP 03, and HP 04 and those genotypes were the products of CLF resistant genotype 1978 HP which showed moderately susceptible CLF response at field screening
Summary
Natural rubber from Hevea brasiliensis is one of the most versatile industrial raw materials. Clones which showed resistance to Corynespora leaf fall during the evaluation were susceptible at field level after some time and disturb the clone recommendations. The development of Corynespora leaf fall-resistant clones along with other performances such as high yield and vigor is a great challenge in Hevea breeding. The laboratory-based in vitro screening methods are not dependable and in vitro screening methods should be used only to obtain preliminary data This consumes considerable time under field experiments. Four SSR Primers (HB 1, HB 11, HB 29, hmct 5) were selected based on polymorphism between the CLF disease free clone RRIC 100 and the complete susceptible clone RRIC 103 (Tharanga et al, 2018). The resistance screening using SSR marker to select high performing CLF resistant rubber clones and correlation between molecular and field screenings have not yet been undertaken. The current study was carried out with the objective of the attempt to validating the results of Microsatellite based molecular markers during molecular screening for CLF resistance with field level screening for the disease resistance
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