Screening small cardamom ( Elettaria cardamomum Maton) field gene bank accessions for phenotypic characters, yield potential and disease resistance

Evaluation of 117 field gene bank accessions of small cardamom for phenotypic characters along with yield potential and disease resistance (rhizome rot and leaf blight) at ICAR-Indian Institute of Spices Research Regional Station, Appangala revealed significant variation with respect to phenotypic characters. The highest plant height (350 cm) and number of bearing tillers (36.4) were recorded in the accessions, field gene bank (FGB) 65 and FGB 16, respectively. The highest fresh weight of capsules (399.61 g) and maximum number of capsules (244.20) plant -1 were recorded in the accession, FGB 13. Further, based on screening for disease resistance, 35 and 15 accessions of cardamom were identified as resistant to leaf blight and highly resistant to rhizome rot, respectively. Based on the multivariate cluster analysis, 117 FGB accessions were classified into 5 clusters viz., clusters 1 and 2 encompassing equal number of accessions (44), cluster 3 with one accession (FGB 10), clusters 4 and 5 with 8 and 20 accessions, respectively.


Introduction
The major challenges in the cultivation of small cardamom are the prevalence of various diseases incited by pathogenic oomycetes, fungi, viruses and nematodes. Among the fungal diseases, leaf blight caused by Colletotrichum species (Govindaraju et al. 1998;Chethana et al. 2016) and rhizome rot incited by Rhizoctonia solani-Pythium vexans complex, are the most widely spread, destructive and economically important. These diseases are cosmopolitan and prevalent throughout the cardamom growing tracts causing significant reduction in the yield by destroying the effective photosynthetic area (leaf blight) and rotting as well as subsequent toppling of the tillers (rhizome rot). Prevalence of the inoculum within the plantation and infective propagules disseminated from adjacent fields and plantations overexposed to sunlight further favours the initial establishment and subsequent spread of leaf blight disease. Whereas, presence of inoculum in the soil as well as in the crop debris, overcrowding of plants and thick shade, favour development of rhizome rot disease. In plantations, leaf blight incidence attains maximum severity during November to February, whereas, severity of rhizome rot is found to be maximum during the month of August (Joseph & Suseela 1995).
Though these diseases could be managed by plant protection chemicals, in view of the increased demand for organically grown spices, health hazards to human beings and other mammals through biomagnifications, environmental pollution due to residual effect, enhanced concerns over the adverse effect of extensive and non-judicious application of plant protection chemicals and development of fungicide resistant variants of pathogens due to mutation, it is highly essential to evolve alternative strategies to vanquish these devastating diseases (Thomas & Bhai 2002).
Characterization of the germplasm is essential to provide information on the desirable traits of accessions, paving way for its utilization in the crop improvement programmes. Extensive exploration for natural resistance in the hotspots of genetic diversity forms the initial stage in identifying resistant sources which is the most reasonable, cost effective and sustainable strategy to manage diseases and thereafter their subsequent characterization. This strategy was successfully adopted in the identification and release of cardamom varieties resistant to rhizome rot, IISR Avinash (Venugopal et al. 2006), IISR Vijetha (Venugopal 2002) and Appangala 2 (Senthil Kumar et al. 2017) against Katte/mosaic disease and several accessions against leaf blight and rhizome rot diseases (Senthil Kumar et al. 2018;Biju et al. 2018). At National Active Germplasm Site (NAGS) of ICAR-IISR Regional Station, Appangala, about 600 cardamom accessions have been maintained which consist of genotypes collected from different hotspots of diversity.
Hence, the present investigation was carried out with the objectives to characterize small cardamom field gene bank accessions based on morphological characters, yield and to identify resistant sources against biotic (leaf blight and rhizome rot) stresses.

Planting materials and location
Small cardamom field gene bank accessions (117 numbers) representing Malabar, Mysore and Vazhukka ecotypes maintained in the Field Gene Bank (FGB) at ICAR-Indian Institute of Spices Research Regional Station, Appangala, Karnataka comprised materials for the study. The FGB accessions were planted (five clumps per accession) during 2011 under uniform shade. Recommended plant protection operations as per package of practices of ICAR-IISR were adopted to manage insect pests and diseases other than leaf blight and rhizome rot.

Screening for disease resistance
Field screening of the cardamom germplasm for leaf blight and rhizome rot diseases was done by visual observation. Per cent disease index (PDI) was calculated by rating the severity of foliar symptoms expressed in a clump. The scoring of leaf blight incidence was based on the expression of the foliar symptoms on the inner tillers (minimum 8-12 tillers). PDI was calculated for all the plants of each accession and mean values were taken to compute PDI for each genotype (Praveena et al. 2013). The incidence of rhizome rot was recorded based on 1 to 5 disease rating scale (Venugopal et al. 2006). The disease rating scale was designed based on the number of infected tillers in a clump. For each accession, disease incidence in five clumps was recorded and PDI was calculated based on the formula of Biju et al. (2018).

Statistical analysis
Observations with respect to vegetative and yield attributes were recorded for three consecutive years (2014-16) and the pooled data were subjected to multivariate cluster analysis using R version 4.0.1 with facto extra package for visualization. Intra and inter cluster distances were estimated using the clv package in R software.

Phenotypic characterization
Morphological characters viz., plant height, number of bearing tillers, number of capsules per plant and fresh weight of capsules exhibited significant variation among the genotypes ( Table  1). Among the 117 FGB accessions evaluated, the plant height was found in the range from 100 to 350 cm. The maximum plant height (350 cm) and highest number of bearing tillers (36.4) were recorded in the accession, FGB 65 and FGB 16, respectively. Interestingly, the accession FGB 32 recorded highest number (40.3) of panicles, longest panicle (120.4 cm) and highest inter nodal length (5.65 cm). The maximum fresh weight of capsules (399.61 g), maximum number of capsules (244.20) plant -1 and highest number of seeds capsule -1 (26.74) were recorded in the accession, FGB 13. It is noteworthy to mention that the capsules of FGB 13 were found to be long (2.20 cm) and wider (1.45 cm) as compared to other accessions under study. High yield obtained in the small cardamom accession, FGB 13 may be attributed to the higher number of capsules plant -1 as positive correlation was obtained between number of capsules plant -1 and yield (Miniraj et al. 2000;Senthil Kumar et al. 2018). Akhila et al. (2017) and Senthil Kumar et al. (2018) also reported significant variation among the genotypes of small cardamom with respect to morphological characters. These characters play an important role in identifying the desirable traits of progenitors to be included in the breeding programmes.
Among the yield characters, the highest coefficient of variation was shown by wet weight of capsules (63.26%) followed by number of capsules (55.52%) ( Table 1). Among the morphological characters, number of panicles recorded highest coefficient of variation (52.05%) followed by inter node length (42.68%) whereas the lowest coefficient of variation was recorded in leaf breadth (9.80 cm). In contrary to the present findings, Akhila et al. (2017) reported highest coefficient of variation in number of vegetative buds (65.84%) followed by number of bearing tillers (20.18%). The significant variations obtained among the accessions for phenotypic characters in the present evaluation implied the presence of significant genotypic difference between the small cardamom accessions.

Clustering and genetic diversity analysis
Based on the multivariate cluster analysis, 117 FGB accessions were classified into 5 clusters at approximate height of 12 (Fig. 1). It was found that clusters 1 and 2 encompassed equal number of accessions (44), whereas clusters 4 and 5 consisted of 8 and 20 accessions, respectively. Interestingly, cluster 3 was solitary with a single accession (FGB 10) indicating its distinctness. Cluster 4 was unique from other clusters as it consisted of accessions (8) collected from one geographical location (Pampadumpara, Idukki, Kerala), while the other clusters consisted of collections representing various cardamom growing regions of Kerala and Karnataka. Even though, 117 FGB accessions represented Malabar, Mysore and Vazhukka ecotypes, no distinct clusters were formed based on ecotypes. The accessions from the same ecotypes were grouped into different clusters which also indicated the close relationship between these ecotypes. Similar inferences were also drawn by Prasath & Venugopal (2004) while categorizing 310 accessions of small cardamom into different clusters.
Trait (morphological and yield parameters) wise cluster means indicated that cluster 4 exhibited highest mean for the traits viz., number of capsules, wet weight, dry weight, plant height and leaf breadth whereas the cluster 5 exhibited highest mean for the traits viz., number of bearing tillers, number of panicles, number of leaves and leaf length (Table 2). These results indicated that the genotypes belonging to the clusters 4 and 5 could be the thrust areas of selection for genetic improvement. Further, cluster 4 exhibited the highest intra cluster distance (194.88), indicating the prevalence of considerable diversity among the genotypes of the cluster whereas clusters 2 and 4 exhibited highest inter cluster distance (312.87) followed by the cluster 1 and 4 (304.28) ( Table 3) indicating potentiality of the genotypes representing these clusters as parental materials for hybridization programmes. This is in line with the findings of Prasath & Venugopal (2004).