PhD

Sigatoka leaf spot disease complex caused by Mycosphaerella spp. is a serious constraint to banana cultivation in Kerala. The present study was undertaken to characterize the associated pathogen and to develop an integrated management package against this disease. The project initiated with purposive sampling surveys conducted in various districts representing different agroclimatic zones of Kerala viz. Malappuram (Northern zone), Palakkad (Northern zone), Thrissur (Central zone), Ernakulam (Central zone), Wayanad (High range zone) and Trivandrum (Southern zone). The percent disease severity (PDS) ranged from 3.33 to 43.90%. The correlation analysis of PDS with weather parameters showed a positive correlation with rainfall however, it was found to be negatively correlated with temperature. The study on symptomatology revealed that there were six types of symptoms noticed on banana var. Nendran (AAB), two types each on Palayankodan (AAB) and Njalipoovan (AB), while only one type of symptom was recorded on Robusta (AAA) and Kadali (AA). Isolation of the pathogen could be achieved on potato dextrose agar medium after 10- 12 days of incubation at very specific conditions. The fungal colony was slightly raised with irregular margin and greyish velvety appearance. Studies on morphological structures revealed that the conidia were hyaline to olivaceous brown in colour which measured about 24.29 - 71.89 µm length x 0.91 - 2.40 µm width with 3-8 septations. The sexual structures were flask shaped perithecia containing long asci bearing eight ascospores. Based on cultural and morphological characters, the pathogen was identified as Mycosphaerella eumusae. The identity of the pathogen was further confirmed by PCR based molecular characterization. In silico analysis of the sequences of the isolates showed 99 -100% homology to Mycosphaerella eumusae. Hence, it is concluded that the pathogen associated with Sigatoka leaf spot disease of banana in Kerala is identified as Mycosphaerella eumusae Crous & Mourichon (anamorph Pseudocercospora eumusae Crous & Mourichon). The screening of accessions maintained in the Germplasm of Banana Research Station, Kannara was done to assess their disease reaction and were grouped into six categories. Five resistant varieties viz., Pisang Lilin, BRS 1, BRS 2, FHIA 01 and FHIA 23 and susceptible varieties viz., Nendran, Grand Naine, Robusta, Moris and Kadali were further selected to investigate the anatomical, biochemical and molecular basis of host plant resistance. Anatomical studies revealed that the resistant varieties were characterized by thicker cuticle, epidermis and mesophyll tissues compared to the susceptible varieties. The number of stomata and the stomatal pore width were more in susceptible varieties compared to the resistant varieties. The biochemical basis of resistance was assessed by quantifying phenols, reducing sugars, non reducing sugars and the activity of defense related enzymes viz., peroxidase, polyphenol oxidase and phenylalanine ammonia lyase in both resistant and susceptible varieties. The results revealed that the phenols, reducing sugars and the defense related enzymes were higher in resistant varieties compared to susceptible ones. The amplification of genomic DNA of resistant and susceptible varieties using Sequence characterized amplified region (SCAR) markers yielded an amplicon of size 644bp in resistant cultivars while the bands were absent in susceptible cultivar. The BLASTn analysis of the sequence of the amplicons showed 90 per cent sequence homology to genomic sequences of Kanthali SCAR marker OPA1363 which is tightly linked to Sigatoka leaf spot disease resistance. Field experiments were conducted to evaluate the efficacy of chemical fungicides and organic / inorganic preparations for disease management. Among the various chemical fungicides, foliar spraying with trifloxystrobin (25%) + tebuconazole (50%), 0.4g/l) was found to be the best followed by copper hydroxide, hexaconazole (5%) + captan (70%), 2g/l) and Bordeaux mixture (1%), without leaving any toxic residues in the harvested fruits. The results of the experiment on disease management using organic / inorganic preparations revealed the effectiveness of PGPR mix II (consortium of Pseudomonas fluorescens and Bacillus subtilis), 2% followed by Pseudomonas fluorescens (2%) given as foliar spray. It is concluded that the present study has enlightened our knowledge on the etiology and management of Sigatoka leaf spot disease of banana cultivation in Kerala.