Management of Cercospora Leaf Spot [Cercospora canescens Ellis and Martin] of Mothbean [Vigna aconitifolia (Jacq.) Marechal] through Fungicides

The moth bean, Vigna aconitifolia (Jacq.) Marechal, is a short-day crop and one of the most drought-resistant legumes in India and especially in Rajasthan. It is generally cultivated in hot and dry habitats of Northern-Western parts of India. It is grown for human consumption and also used in many industries. Mothbean is native to India, Myanmar and Pakistan, where it grows as a wild and cultivated plant. Ecologically, mothbean is an annual legume crop of dry and warm habitats and is characterized as the most drought hardy annual legume in arid regions. Mothbean with deep and quick penetrating rooting system, can survive up to 30-40 days in open fields (Kumar, 2002). It has the ability to grow under harsh climate, low rainfall and poor and sandy soil conditions and considered as most important pulse crop of arid Rajasthan (Sharma and Ratnoo, 2014).
 
In India mothbean area was 11.83 lakh hectares and 3.89 lakh tonnes of mothbean production was recorded the 2016-17 (Anonymous, 2016-17). A total of 10.27 lakh hectares and 3.20 lakh tonnes of mothbean production was recorded in the Rajasthan during 2016-17 (Anonymous, 2017-18). Area and production of mothbean highest in Rajasthan contributing (86.81% and 82.26%) followed by Madhya Pradesh (10.99% and 14.65%) respectively. Rajasthan is the largest mothbean growing state contributing about 85 per cent area of the country.
       
Mothbean crop suffers from many diseases, among the diseases Bacterial leaf spot, Yellow mosaic virus, Leaf crinkle virus, Cercospora leaf spot are more prevalent and destructive in nature. Among the fungal diseases, the Cercospora leaf spot occurs frequently in Asia.
       
In India, leaf spot caused by Cercospora canescens Ellis and Martin was first reported by Munjal et al., (1962) from Delhi and has spread to other parts of the humid tropical regions of India (Pandey et al., 2009). The reduction in yield due to the Cercospora leaf spot depends upon how early plant are infected (Poehlman, 1978).
       
In the case of Cercospora leaf spot, infected leaves (especially those more mature) look for brown or rust-colored lesions that vary from circular to angular, are 2-10 mm. Lesions may have a greyish or whitish center with a slightly reddish border. Conidia develop at the center on short conidiophores. Severely affected leaves become chlorotic. Lesions may dry and portions may fall out, giving the leaf a shot-hole appearance. Lesions and blemishes may occur on branches, stems and pods. Maximum loss of 61 percentage was observed in Pakistan in case of grain yield (Iqbal et al., 1995).
       
Has recommended the use of chemicals for more than a decade and the performance of new molecules available in the market should be analyzed with this disease. Information regarding Cercospora leaf spot disease development in relation to other foliar diseases of mothbean is also lacking. Provide necessary information for suitable management of disease and minimize crop losses.

In vitro evaluation of fungicides
 
The bio-efficacy of new molecule fungicides and their combinations at the concentration of 50, 100, 150 and 200 ppm were analyzed against Cercospora canescens under laboratory conditions by following poisoned food technique. The fungicides used were given here under. In the poisoned food technique, the fungus C. canescens was grown on PDA medium in petriplates for 15 days. Fungicidal suspension was prepared by adding required quantity of fungicides in PDA medium to obtain the desired concentration on the basis of active ingredients present in the chemical. Later, 20 ml of poisoned medium was poured in each sterilized petriplates. Mycelial disc of Five mm size from actively growing zone of 15 days old culture was cut by cork borer and placed at the centre of each agar plate, then incubated at 25°C for 16days. Control treatment was maintained without adding any fungicides to the medium. Three replications were maintained for each treatment. After the period of incubation, the colony growth of the C. canescens were recorded. Per cent inhibition of the colony over control was calculated by using following formula given by Vincent (1947) and data were statistically analyzed with CRD design.
Formula;
   
I = Per cent inhibition.
C = Radial growth of C. canescens in control (mm).
T = Radial growth of C. canescens in presence of Fungicides (mm).
       
The following fungicides were tested viz; T₁; T₂; T₃; T₄; T₅; T₆; T₇; T₈; T₉ and T₁₀.
 
In vivo evaluation of fungicides
 
A field experiment was laid out at the Research Farm of College of Agriculture, SKRAU, Bikaner, during Kharif 2019. The efficacy of nine new molecule fungicides and combinations was evaluated. The experiment was conducted in randomized block design with ten treatments and three replications of mothbean crop. Plot size of 3 x 3 m² was maintained per treatment. The sowing was taken up on 19.07.2019. Fertilizers were applied as per package of practices. Spraying was taken up immediately after disease appearance. The observations on per cent disease index was recorded by using scale of 0-9 (Mayee and Datar, 1986). Per cent disease index (PDI) was calculated by using the formula given by Wheeler (1969) formula.
   
 Fungicides used for lab studies and spraying are as follows:-
 

The data of per cent disease incidence in all the experiments were transformed to their Arcsin values (Fisher and Yates, 1963). The statistical analysis of the data of all the laboratory experiments were done following Completely Randomized Design. The data of field experiments were analyzed following Randomized Block Design (Cochran and Cox, 1957).

In vitro evaluation of fungicides
 
The effect of nine systemic, contact and combi fungicides were tested at 50, 100, 150, 200 ppm concentrations by poison food technique on growth of C. canescens under in vitro conditions. Observations on average colony diameter and per cent inhibition of linear growth over control are presented in (Table 1 and Plate 1). The results revealed that all the fungicides were significantly reduced the mycelial growth of C. canescens over control. Among the fungicides, the combi fungicide trifloxystrobin 25% + tebuconazole 50% WG was found more effective and inhibited mycelial growth of C. canescens (76.40%, 87.34%, 94.84% and 100.00%) at 50, 100, 150 and 200 ppm concentrations followed by carbendazim 12% + mancozeb 63% WP (68.28%, 80.46%, 88.28% and 100.00%) respectively over control. The fungicide propiconazole 25% EC also effective against C. canescens and per cent inhibition (67.81%, 76.56%, 85.31% and 96.71%) at 50, 100, 150 and 200 ppm. Similarly, the next fungicide fluopyram 250 SC + trifloxystrobin 250 SC was also found effective and inhibited the growth of C. canescens (50.62%, 63.90%, 75.15% and 85.78%) followed by mancozeb 50% WP (50.46%, 60.15%, 68.59% and 77.25%) respectively at 50, 100, 150 and 200 ppm concentrations. Similarly, hexaconazole 4% + zineb 68% WP, pyraclostrobin 20% WG + azoxystrobin 23% SC and thiophanate methyl 15% + copper oxychloride 40% WP were not effective against mycelial growth of test pathogen at 50 ppm concentration 48.28%, 45.46% and 42.96% respectively but found effective at 100 ppm (58.43%, 54.21% and 50.78%), 150 ppm (68.28%, 61.25% and 61.09%), 200 ppm (74.84%, 71.09% and 70.62%) concentrations respectively. All formulations of fungicides were found highly effective against test pathogen at 200 ppm concentration over control. However, these fungicides were less effective at lower concentration but reversible to increasing concentration of fungicides but chlorothalonil 75% WP fungicides was less effective against the per cent inhibition of mycelial growth of pathogen (14.00%, 21.56%, 31.40 and 42.03%) at 50 and 100, 150 and 200 ppm concentrations.
 

 

       
These findings are conformity with work of Kumar et al., (2016) reported that tebuconazole 50% + trifloxystrobin 25% WG, carbendazim 12% + mancozeb 63% WP and propiconazole 25% EC at 200 ppm concentration showed complete (100%) per cent inhibition of mycelial growth of fungus. Another similar findings were also reported by Kavyashree et al., (2017) under laboratory conditions evaluation of different fungicides against Cercospora canescens of greengram revealed that hexaconazole 5 EC (0.1%), carbendazim (25%) + mancozeb (50%) WS (0.05%) or mancozeb 75 WP (0.1%) and propiconazole 25 EC and trifloxystrobin (25%) + tebuconazole (50%) w/w (0.05%) resulted into 100 per cent inhibition of mycelial growth.
 
In vivo evaluation of fungicides
 
In mothbean due to Cercospora leaf spot disease often causes considerable yield losses. Presently, the cultural practices along with biological control and resistant varieties considerably reduced the incidence of disease. Similarly, the systemic and contact fungicides also prevent the infection of disease but regularly application may lead to resistance against pathogen. Hence, a field trial on fungicidal control of disease was carried out for kharif, 2019.
       
Based on efficacy of different fungicides evaluated under laboratory conditions, nine fungicides were used against Cercospora leaf spot of mothbean under field conditions. The results of experiment presented in (Table 2) revealed that there was significantly reduction in the disease intensity and increase mothbean yield was recorded compared to the untreated control plots. Ten days after the first spray, maximum per cent disease control (72.71 %) with minimum PDI (15.51%) and maximum 8.10 q ha^-1 mothbean yield was recorded by application of trifloxystrobin 25% + tebuconazole 50% WG (0.15%) as foliar spray. This was followed by the application of carbendazim12% + mancozeb 63% WP (0.20%) as foliar spray resulted in 17.68 per cent disease intensity and 68.90 per cent disease control and 7.64 q ha^-1 crop yield. The application of propiconazole 25% EC (0.20%) foliar spray in plots also effective and resulted 20.44 per cent disease intensity and 64.04 per cent disease control and 7.48 q ha^-1. Application of fluopyram 250 SC + trifloxystrobin 250 SC (0.15%) foliar spray showed 21.45 per cent disease intensity that accounted 62.26 per cent efficacy of disease control and 7.32 q ha^-1 followed by mancozeb 50% WP (0.20%) as foliar spray showed 22.28 PDI with 60.80 per cent disease control and 7.18 q ha^-1 followed by application of hexaconazole 4% + zineb 68% WP (0.20%) as foliar spray resulted in 25.40 per cent disease intensity and 55.32 per cent efficacy of disease control and 6.55 q ha^-1 crop yield. Similarly, pyraclostrobin 20% WG + azoxystrobin 23% SC (0.15%) and thiophanate methyl 15% + copper oxychloride 40% WP (0.15%) were no more effective as foliar spray resulted per cent disease inensity (25.76% and 27.63%) and efficacy of disease control (54.68% and 51.39%) with mothbean yield (6.27 q ha^-1 and 6.15 q ha^-1) respectively. Among all the treatments, chlorothalonil 75% (0.20 %) WP showed lowest disease control (36.79%) and 5.85 q ha^-1 yield with highest per cent disease intensity (35.93%).
 

               
These results are comparable with findings of Banyal et al., (2019) reported that two sprays at 10 days interval of trifloxystrobin 25% +tebuconazole 50% (Nativo 75 WG) at 350 g/ha were significantly effective in controlling both the diseases i.e. leaf spot and powdery mildew and gave 77.1 and 67.2 per cent disease control with a maximum average increase in the grain yield i.e. 20.2 and 19.5 per cent, respectively over the check. Also revealed with findings of Yadav et al., (2014) reported that two foliar sprays of two foliar sprays of carbendazim (0.1%) was found most effective with minimum disease intensity (14.45%) followed by topsin-M (15.49%), propiconazole (17.85%), mancozeb (21.67%), copper oxychloride (23.40%) and thiram (28.59%) as compared to control plot (61.57%). These studies are confirmatory to our results.

Among nine fungicides, trifloxystrobin 25% + tebuconazole 50% WG and carbendazim 12% + mancozeb 63% WP found most effective with maximum inhibition of mycelial growth (100%) of Cercospora canescens as compared to propiconazole 25% EC in inhibiting mycelial growth (96.71%) under in vitro condition.
       
In vivo studies, foliar spray of trifloxystrobin 25% + tebuconazole 50% WG (0.15%) and carbendazim 12% + mancozeb 63% WP (0.20%) were found most effective in reducing disease intensity and increasing crop yield.

The authors are very much thankful to Dr D.R. Kumhar, Head of Department, Department of Plant Pathology, Swami Keshwanand Rajasthan Agricultural University, Bikaner for providing all necessary things for our research.

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