Cataloguing and documentation of fungal diseases of gerbera
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Date
2016
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College of Horticulture, Vellanikkara
Abstract
Gerbera, a commercial cut flower crop, has widespread acceptance in the international market due to its vibrant colours, shapes, excellent vase life and handling quality. One of the major limiting factors which reduce the commercial value of the crop is the incidence of diseases. Hence, the present investigation was undertaken to identify the fungal diseases of gerbera occurring in Kerala. A purposive sampling survey was conducted in three districts viz., Thrissur, Malappuram and Wayanad during rainy (July-August), winter (November-December) and summer (March-April) seasons for monitoring disease occurrence and sample collection both under protected and open field conditions. Three leaf blights (LB-1, LB-2 and LB-3), two leaf spots (LS-1 and LS-2) and powdery mildew (PM) were the foliage diseases observed during the survey. Petal blight was the only flower disease noticed apart from root rot and wilt disease. Per cent disease incidence (PDI) and per cent disease severity (PDS) assessed during the survey revealed that powdery mildew showed the highest per cent disease incidence and per cent disease severity. Besides, per cent disease severity (PDS) of leaf blight-2 and root rot was also comparatively higher. Correlation studies of weather parameters with disease severity could elucidate the seasonal influence in disease development. Eight pathogens were isolated from diseased samples and pathogenicity of each isolate was proved by Mycelial Bit Inoculation Method (MBIM) for foliage diseases and Micro Droplet Inoculation Technique (MDIT) for foliage, flower and root diseases. Pathogenicity test for each disease also aided in relating symptoms of the disease under artificial conditions. Under natural conditions, leaf blights, LB-1 and LB-2 showed marginal blightening with distinct symptom development and black water soaked lesions noticed for LB-3 disease. Leaf spot-1 (LS-1) appeared as dark brown necrotic spots scattered on the leaf lamina between the major veins whereas leaf spot-2 (LS-2) produced numerous spots with shot holes. Petal blight showed blightening resulting in shot holes whereas, white powdery growth appeared on the leaf lamina in the case of powdery mildew disease. Rotting of roots were noticed for root rot as well as for wilt disease. Cultural and morphological characterisation of each pathogen was carried out for the identification of pathogens and the identity was confirmed by National Center for Fungal Taxonomy (NCFT), New Delhi. The pathogens causing LB-1, LB-2 and LB-3 were identified as Alternaria alternata, Alternaria tenuissima and Myrothecium roridum respectively. The pathogens responsible for leaf spots (LS- 1 and LS-2) were identified as Ulocladium chartarum and Curvularia pallescens and for powdery mildew as Golovinomyces cichoracearum and Podosphaera sp. Curvularia lunata was identified as the causal organism of petal blight. Moreover, root rot and wilt disease observed during the survey were caused due to Phytophthora cryptogea and Fusarium solani respectively. In vitro management studies of each pathogen revealed that tebuconazole 250EC and propineb 70WP showed promising results against foliage diseases. For root diseases, systemic fungicides like hexaconazole 25EC, tebuconazole 250EC, pyraclostrobin 20WG, combination fungicide, carbendazim 12% + mancozeb 63% and contact fungicides like copper hydroxide 77WP and Bordeaux mixture performed exceptionally well by inhibiting the radial growth of the pathogen. Biocontrol agents like Trichoderma viride and Pseudomonas fluorescens tested against each pathogen showed that the fungal antagonist was found more efficient than the bacterial antagonist. Prior to the management studies, the identity of pathogens were reassured by analyzing the sequence of the pathogens in Blastn programme where the sequencing have been carried out at RGCB, Thiruvananthapuram. Management of three major pathogens viz., Alternaria tenuissima, Phytophthora cryptogea and Fusarium solani with chemical fungicides and bioagents were carried out under in vivo conditions. Compared to other systemic and contact fungicides, the combination fungicide, carbendazim 12% + mancozeb 63% (0.2%) recorded 90 per cent reduction of Alternaria leaf blight disease. The per cent disease incidence of Phytophthora root rot was reduced considerably by the application of contact fungicides, propineb 70WP (0.3%) cymoxanil 8% + mancozeb 64% (0.2%) and copper hydroxide 77WP (0.2%). Fungicides like carbendazim 12% + mancozeb 63% (0.2%), difenoconazole 25EC (0.1%), copper hydroxide 77WP (0.2%) and Trichoderma viride (2%) showed noticeable reduction in per cent disease incidence of Fusarium wilt. It is worthwhile to mention that this is the first report of Ulocladium chartarum and Myrothecium roridum on gerbera. It may be concluded that the present study has enlightened our knowledge on the various fungal diseases of gerbera prevailing in Kerala and thrown light on their management aspects
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