Management of Root Rot (Rhizoctonia solani) of Fenugreek through Newer Combined Formulations of Fungicides

Fenugreek (Trigonella foenum-graecum L.) commonly known as ‘Methi’ has incredible importance in the life of human beings as a food, medicine and belongs to family Fabaceae. It is commercially cultivated for seeds, which are used as condiments and as a flavouring agent for relishing food preparations that stimulates the digestive process and metabolism and also used for manufecturing of pickles, soap and cosmetics (Choudhary et al., 2013). Its seeds have important steroid ‘diosgenin’ which is used in preparation of contraceptives. In industry, seeds are mainly used for extraction of alkaloids and steroids. Fenugreek is consumed in different parts of the world in diverse forms and has been known for treating many ailments recognized to man (Laila et al., 2013) and fenugreek seed oil contains 16 fatty acids including linoleic acid and linolenic acid and lipids which are beneficial for health (Rathore et al., 2017).
       
Rajasthan stands first in the production with more than 80 per cent area under fenugreek cultivation in India. Other major production centres of fenugreek in India are Madhya Pradesh, Gujarat, Haryana, West Bengal and Uttaranchal (Anonymous, 2018).
       
Fenugreek attacked by a number of fungal diseases viz., root rot (Rhizoctonia solani), downy mildew (Peronospora trigonellae), powdery mildew (Erysiphe polygoni and Leveillula taurica), wilt (Fusarium oxysporum), leaf spot (Cercospora traversiana), charcoal rot (Macrophomina phaseolina) and rust (Uromyces anthyllidis) (Godara et al., 2010). Among these, root rot also known as collar rot or foot rot or damping off of fenugreek caused by Rhizoctonia solani is one of the most important diseases which reduce the yield of the plant significantly. The main symptom of this disease is damping off. The freshly emerged seedlings fall over and die and most of the seedlings may die at pre- or post-emergence in severely infected areas. They may also develop foot rot and reddish brown cankers on root and stem near the ground level. The pathogen mainly attacks the root and underground parts, but it is also capable of infecting the other plant parts like the green foliage parts, the seeds and the hypocotyls (Acharya et al., 2014). The roots of infected plants are poorly developed; finer roots are either not formed or rotted. Plants show stunted growth and can easily be pulled out. Heavy losses are incurred due to root rot. Under natural field conditions, the incidence of root rot of fenugreek was recorded up to 20.00 per cent in Jaipur district of Rajasthan. Singh and Rao (2015) reported 34.67 per cent incidence of root rot of fenugreek caused by Rhizoctonia solani with yield loss of 55.26 per cent from Chhattisgarh. Rani and Hedge (2017) recorded 48.35 per cent root rot incidence in fenugreek caused by Rhizoctonia solani Kuhn in Karnataka.
       
In lieu of disease incidence and yield losses, some newer fungicides were tested in comparison to already recommended ones for two consecutive years (2016-17 and 2017-18) under field conditions for managing the disease and increasing crop productivity for the benefit of the end users.

The diseased sample was collected and experiments were conducted in laboratory and field during 2016-17 and 2017-18 at Department of Plant Pathology, S.K.N. College of Agriculture, Jobner, Jaipur (Rajasthan). Jobner, is situated at latitude 26°5² N, longitude of 75°20² E and altitude of 427 meters above MSL (mean sea level). This region falls under semi-arid eastern plain (Agro climatic zone- lll A) of Rajasthan.
       
All the glassware were cleaned with potassium dichromate sulphuric acid solution, washed with sterilized water, sterilized in hot air oven at 160°C for two hours. Potato dextrose agar medium was sterilized by autoclaving at 1.045 kg cm² pressure for 20 minutes. Roots of diseased plants of fenugreek were first washed under the tap water and then cut into small pieces along with healthy portion. These pieces were surface sterilized by dipping in 1 per cent sodium hypochlorite solution for 1 minute. After three consecutive washings with sterilized distilled water, the pieces were transferred to autoclaved PDA medium in Petri plates and incubated at 25+1°C in BOD incubator up to 7 days. Pure culture of the fungus was obtained by hyphal tip technique. The isolated and purified fungus was identified as Rhizoctonia solani on the basis of morphological and colony characters and further got confirmed from ITCC (with I.D. No. 10.836.18) New Delhi.
 
In vitro studies
 
Effect of fungicides on mycelial growth
 
Efficacy of six systemic and non-systemic fungicides viz., hexaconazole 5% SC, tebuconazole 50% WG +trifloxystrobin 25% WG, carbendazim 50% WP, carboxin 37.5% + thiram 37.5%, carbendazim 12% WP + mancozeb 63% WP and captan 70% WP + hexaconazole 5% WP were evaluated for antimycotic effect against Rhizoctonia solani by Poisoned Food Technique at 100, 200 and 500 ppm. Required quantity of each fungicide was added aseptically to 100 ml sterilized PDA medium in 150 ml flask so as to get concentration of 100, 200 and 500 ppm. Just before pouring in sterilized Petri plates, the flasks were shaken several times to ensure proper and uniform distribution of the fungicide. Poisoned medium was poured in sterilized Petri plates and allowed to solidify. Medium without fungicide served as control. Three replications were maintained for each treatment. Each plate was inoculated with 5 mm mycelium bit of the pathogen in the centre of plate. Inoculated plates were incubated at 25+1°C for 7 days. The linear growth of test fungus was recorded and per cent growth inhibition was calculated by Vincent’s (1947) formula:
 
Whereas,
C =Diameter of the colony in check (average of both diagonals), T = Diameter of colony in treatment (average of both diagonals).
 
In vivo studies
 
Effect of fungicides on disease incidence and seed yield
 
 A field experiment was conducted during Rabi 2016-2017 and 2017-18 at Agronomy Farm, SKN College of Agriculture, Jobner (Jaipur) in randomized block design (RBD) with three replications in 2 m × 2 m plots, using RMT-305 as test variety, under artificial inoculation conditions. All the recommended agronomic practices were followed to raise the crop. All the fungicides viz., carbendazim (0.1%), carboxin + thiram (0.2%), hexaconazole (0.2%), tebuconazole + trifloxystrobin (0.2%), carbendazim + mancozeb (0.2%) and captan + hexaconazole (0.2%) were applied as dry seed treatment before sowing. For artificial inoculation, inoculum multiplied on sterilized sorghum grains was mixed at the rate of 20 g/m row at the time of sowing (i.e. first week of Nov.). The healthy and diseased plants were counted at 60 days after sowing. Per cent disease incidence (PDI) and per cent disease control was calculated as per following formulae.
                                                             

Effect of fungicides on growth of the pathogen (In vitro)
 
All the tested six fungicides significantly inhibited the mycelial growth at all the concentrations (100, 200 and 500 ppm) tested (Table 1). Out of these fungicides, tebuconazole + trifloxystrobin had completely inhibited the growth of the pathogen at 200 and 500 ppm concentrations and also gave maximum inhibition at 100 ppm concentration and (93.33%) and found most effective in comparison to older fungicides. Hexaconazole was also rated better in inhibition (89.23%) at 500 ppm. The importance of chemicals cannot be denied in disease management strategies. Our results are in accordance with the findings of Atia et al., (2015) who evaluated nine fungicides against R. solani causing root rot of tomato in vitro and in vivo conditions. The fungicides, propiconazole and tebuconazole + trifloxystrobin were found to be highly effective (100% inhibition) at 250, 500 and 1000 ppm concentrations.
 

 
Effect of fungicides on root rot incidence and seed yield (in vivo)
 
All the fungicides evaluated (Table 2) were found statistically significantly superior over control in reducing disease incidence and in increasing seed yield during both the years (2016-17 and 2017-18) of testing. Among all the fungicides, at pooled level, minimum disease incidence (8.44%) and maximum seed yield (1939.50 kg/ha) were recorded by treating the seeds with tebuconazole + trifloxystrobin (@ 0.2%) and hexaconazole (14.73% and 1796.00 kg) was also found effective. Seed treatment with tebuconazole + trifloxystrobin (0.2%) was proved to be the most effective in reducing disease incidence (83.12%) and in increasing seed yield (84.71%) over control. Seed treatment with tebuconazole + trifloxystrobin and hexaconazole were found statistically at par to each other in increasing seed yield over control. Similar results with other crops have been made by Atia et al., (2015) who tested nine fungicides against R. solani causing root rot of tomato in vivo conditions. Among them, propiconazole and tebuconazole + trifloxystrobin were found to be highly effective in controlling disease. Bag (2009) has also been reported that tebuconazole + trifloxystrobin is the most effective fungicide against sheath blight of rice caused by R. solani  in reducing disease severity and in increasing yield. Tebuconazole alone, has also been reported to be effective on other crops (Meena and Chatopadhyay, 2002; Yadav et al., 2003, Dutta and Kalha, 2011 and Prasad et al., 2017).
 

It was observed that treatment of fenugreek (Trigonella foenum-graecum L.) seeds with tebuconazole + trifloxystrobin (@ 0.2%) before sowing is the most effective in managing root rot disease (Rhizoctonia solani) with increased seed yield and may be the best alternative of older fungicides.