Twister disease caused by Colletotrichum gloeosporioides is an emerging threat to onion cultivation particularly during kharif season. Newer fungicides were evaluated for their efficacy against the pathogen. Under in vitro condition, though all the fungicides significantly inhibited the mycelial growth of pathogen, the combi-fungicide carbendazim 12% + mancozeb 63% WP @ 0.1% exhibited maximum inhibition (97.96 %) of mycelial growth of fungus C. gloeosporioides. Similarly, under field condition two foliar sprays of carbendazim 12% + mancozeb 63% @ 0.1% at 15 days interval recorded significant control of onion twister disease with least PDI (17.16). It was closely followed by tebuconazole 50% + trifloxystrobin 25% WG @ 0.2% (20.30 PDI) and metiram 55% + pyraclostrobin 5% WG @ 0.2% (26.80 PDI).

Onion (Allium cepa L.) a biennial bulbous vegetable crop usually grown as an annual; belongs to the family Alliaceae. China and India are the leading onion growing countries. India being the second largest producer of onion, accounts for 22.83 per cent of worlds onion production (Anonymous, 2019). According to all India estimates given by NHB (Anonymous, 2020-21), onion is being grown on an area of 1,654 thousand hectares with total production of 26.92MT. Maharashtra is the leading onion producing state in the country. Onion production is affected by several factors such as diseases, insect pest and competition by the weeds. One of the most important factors responsible for the low productivity of onion is the diseases. In recent years, twister disease which is also called as onion anthracnose or Colletotrichum blight or severe curls disease has become an epidemic in Maharashtra and some parts of Karnataka. The disease is caused by the fungus Colletotrichum gloeosporioides (Penz.) Penz. and Sacc. and it's perfect stage is Glomerella cingulata.

The disease is prevalent in tropical and sub-tropical regions and is severe during rainy season. Temperature of 23-30 C and relative humidity of 80-95 per cent are optimum conditions for the development of onion twister disease. The typical twister disease symptom includes slight twisting and curling of the neck and leaves during nursery stage on young onion seedlings. In general, symptoms of the disease appear on the leaves at about 30 days after transplanting. Initially small, whitish, circular to oval shaped water-soaked lesions develops on the leaves which on gradual elongation coalesce to become large sunken necrotic spots bearing numerous orange or salmon coloured acervuli which later become dark brown or black coloured in a concentric ring or irregular pattern. The necrotic spots spreads throughout the leaf and the leaves get dried turning to brown, showing die-back symptoms (Ebenebe, 1980; Patil et al., 2018). Stems, neck or the entire plant become chlorotic, elongated, slender, twisted and topple down with poorly developed bulbs and roots. The losses are estimated to the tune of 50-100 per cent in several fields which results in lower supply of onion in the market leading to higher prices.

To manage the disease, application of fungicides as spray is a common practice. However, the traditional fungicides are needed to be applied repeatedly. Now day, new fungicide molecules are available in the market which is more efficient against the fungal pathogens even at lower doses. As the twister disease is emerging as an epidemic on onion in recent years, it was felt necessary to evaluate fungicides for its successful management.

A. In vitro evaluation of fungicides against C. gloeosporioides

The efficacy of six fungicides was evaluated in vitro against C. gloeosorioides by poison food technique (Nene and Thapliyal 1979). For each fungicidal treatment, three replications were maintained. The Petri plates poured only with PDA medium served as control. After solidification of the medium, fungal disc of 5 mm diameter was inoculated in the centre of the each plate and allowed to incubate at 27 ± 2°C. 

Observations were recorded on mycelial growth (colony diameter) of the test pathogen till it was fully grown in untreated control plate. The per cent inhibition of the growth of test pathogen was calculated by using the formula given by Vincent (1947).

Where, 

C = Mycelial growth in untreated control, T = Mycelial growth in treatment

B. In vivo evaluation of fungicides against C. gloeosporioides

A field experiment was conducted during Kharif-2021 in order to evaluate the efficacy of various fungicides for the control of twister disease of onion. The experiment was conducted on susceptible onion variety N-2-4-1, laid in a Randomized Block Design (RBD) with three replications having plot size of 3 × 2 meter and 15 × 10 cm plant spacing. 

The fungicides those evaluated in vitro were tested under field condition for their efficacy in controlling the twister disease. The fungicides were sprayed after the initiation of the disease and at 15 days after first spray. The per cent disease index was recorded using five randomly selected plants from each treatment plot before and after each spray in various fungicidal treatments on the basis of leaf area infected and extent of leaf curling using the rating scale given by Patil (2013).

0-5 scale of disease severity

Per cent Disease Index (PDI) was calculated by using the formula 

Per cent disease control (PDC) was calculated by using the following given formula.              

A. In vitro evaluation of fungicides against C. gloeosporioides

Six fungicides were tested for their efficacy against C. gloeosporioides. The obtained results are presented in Table 1 and Plate 1. The data revealed that all the six fungicides tested against C. gloeosporioides exhibited significantly superior inhibitory effect on its mycelial growth as compared to untreated control. Among the various fungicides, carbendazim 12% + mancozeb 63% @ 0.1% showed highest per cent inhibition (97.96) of mycelial growth of C. gloeosporioides followed by tebuconazole 50% + trifloxystrobin 25% WG @ 0.2%, metiram 55% + pyraclostrobin 5% WG @ 0.2%, azoxystrobin 18.2% + difenoconazole 11.4% SC @ 0.1% and difenoconazole 25% EC @ 0.05% with 95.00, 87.18, 79.37 and 77.77 per cent inhibition, respectively. Azoxystrobin 23% SC @ 0.1% was found least (68.51 %) effective.

The results are in accordance with several workers viz., Ekbote et al. (1996) who reported carbendazim + mancozeb as cent per cent inhibitor of mycelial growth at 0.1% concentration. Jayalakshmi et al. (2012) reported that among combi fungicides tested, carbendazim + mancozeb was effective at 0.3% concentration and among systemic fungicides, difenoconazole at 0.05% was more effective than azoxystrobin at 0.1% concentration. Begum et al. (2015) reported greater efficacy of carbendazim + mancozeb and tebuconazole + trifloxystrobin against C. Capsici.  Behera et al. (2019) reported 97 per cent inhibition of C. gleosporiodies with 0.1% carbendazim + mancozeb. Rajashree et al. (2020) reported 97 per cent inhibition of C. truncatum with 0.1% carbendazim + mancozeb and 86.5 per cent inhibition with 0.2% metiram + pyraclostrobin and difenconazole at 0.05% which gave superior results than azoxystrobin 0.1%. In present studies also, the combi fungicide carbendazim 12% + mancozeb 63% WP was found most effective in inhibiting the mycelial growth of C. gloeosporioides. Further, the results revealed that azoxystrobin was comparatively less effective than other fungicides. 

B. In vivo evaluation of fungicides against C. gloeosporioides

The results of the field experiment on evaluation of fungicides against twister disease of onion are presented in Table 2. From the data it is revealed that all the fungicide treatments were significantly superior over control in minimizing the per cent disease index of onion twister disease. Seven days after the first spray, carbendazim 12% + mancozeb 63% WP @ 0.1% recorded least PDI (14.26) followed by tebuconazole 50% + trifloxystrobin 25% WG @ 0.2% (16.90 PDI), metiram 55% + pyraclostrobin 5% WG @ 0.2% (23.80 PDI), azoxystrobin 18.2% + difenoconazole 11.4% SC @ 0.1% (26.53 PDI) and difenoconazole 25% EC @ 0.05% (27.03 PDI). Highest PDI (30.20) among fungicide treatments was reported with azoxystrobin 23% SC @ 0.1%, while maximum PDI (36.7) was reported in untreated control. 

Similar trend of results was recorded after second spray of fungicides. The treatment of carbendazim 12% + mancozeb 63% WP @ 0.1% recorded least PDI (16.10) followed by tebuconazole 50% + trifloxystrobin 25% WG @ 0.2% (19.43 PDI), metiram 55% + pyraclostrobin 5% WG @ 0.2% (25.20 PDI), azoxystrobin 18.2% + difenoconazole 11.4% SC @ 0.1% (28.00 PDI) and difenoconazole 25% EC @ 0.05% (28.96 PDI). Highest PDI (34.23) among fungicide treatments was reported with azoxystrobin 23% SC @ 0.1%, while maximum PDI (40.30) was reported in untreated control.

Observations recorded before harvest indicated that carbendazim 12% + mancozeb 63% WP @ 0.1% recorded least PDI (17.16). It was followed by tebuconazole 50% + trifloxystrobin 25% WG @ 0.2% (20.30 PDI), metiram 55% + pyraclostrobin 5% WG @ 0.2% (26.80 PDI), azoxystrobin 18.2% + difenoconazole 11.4% SC @ 0.1% (29.30 PDI) and difenoconazole 25% EC @ 0.05% (30.16 PDI). Highest PDI (36.63) among fungicide treatments was reported with azoxystrobin 23% SC @ 0.1%, while maximum PDI (45.80) was reported in untreated control.

The results are in accordance with several workers viz., Patel (2009) who reported best results of carbendazim + mancozeb against C. gloeosporioides causing fruit spot of pomegranate. Nargund et al. (2012) reported that carbendazim + mancozeb (0.3%) was effective in reducing PDI of anthracnose of pomegranate. Chauhan et al. (2014) reported 0.05% carbendazim followed by 0.2% mancozeb and 0.2% carbendazim + mancozeb were effective against chilli anthracnose. Madhavan et al. (2017) reported metiram 55% + pyraclostrobin 5% as effective against chilli anthracnose. Chetak and Banyal (2020) reported effectiveness of tebuconazole 50% + trifloxystrobin 25%  against C. truncatum causing urdbean anthracnose. 

Table 1: In vitro efficacy of different fungicides against C. gloeosporioides.

Table 2: Efficacy of fungicides against twister disease of onion under field condition.

*Figures in parenthesis are arc sine transformed values 

T1: Difenoconazole 25% EC @ 0.05%;  T2: Azoxystrobin 23% SC @ 0.1%; T3: Metiram 55% + Pyraclostrobin 5% WG @ 0.2%; T4: Azoxystrobin 18.2% + Difenoconazole 11.4% SC @ 0.1%; T5: Carbendazim 12% + Mancozeb 63% WP @ 0.1%; T6: Tebuconazole 50% + Trifloxystrobin 25% WG @ 0.2%;  T7: Untreated control

Plate 1. Effect of different fungicides on mycelial growth of C. gloeosporioide.

The results of the study suggest that two foliar sprays of carbendazim 12% + mancozeb 63% WP @ 0.1% or tebuconazole 50% + trifloxystrobin 25% WG @ 0.2% or metiram 55% + pyraclostrobin 5% WG @ 0.2% concentration at 15 days interval could aid in effective management of the twister disease particularly in kharif season and subsequently reduces the yield losses.

As the twister disease is emerging a threat to kharif onion the findings of present study after their validation through multi-location trials can play a important role in management of the disease. 

Annapurna Gavireddy, S.B. Latake and V.S. Shinde (2024). Chemical Control of Colletotrichum gloeosporioides causing Twister Disease in Onion. Biological Forum – An International Journal, 16(1): 243-246.