​Management of Early Leaf Spot (Cercospora arachidicola) of Groundnut in Rajasthan

Groundnut (Arachis hypogaea L.) also known as peanut or earthnut which belongs to the sub family Papilionaceae of the family Leguminosae. It is an annual legume native to South America. It is grown in most tropical, sub-tropical and warm temperate regions of the world between 40° North and 40° South latitudes (Basu and Singh, 2004). It is an important oilseed and supplementary food crop of the world. It is fourth most important source of edible oil and third most important source of vegetable protein. Groundnut seed contains 9.5 to 19.0% total carbohydrates (Oke,1967; Savage and Keenan, 1994).
       
Globally India ranks first in groundnut acreage and second in production. The total area under groundnut cultivation in India was 39.31 lakh hectares with the total production of 68.63 lakh MT and average productivity of 1,745 kg ha^-1 during the year 2018-19. Eighty per cent of the total groundnut area is confined to five states viz., Gujarat Andhra Pradesh, Tamil Nadu, Karnataka and Maharashtra. The rest of the area and production is distributed mainly in the states of Rajasthan, Uttar Pradesh, Madhya Pradesh, Punjab and Orissa. It is cultivated during kharif season (June- July to September- October) mostly under rainfed conditions. few protective irrigations (Anonymous, 2019). Groundnut cultivated area in Rajasthan was 7.34 lakh hactares, with production of 16.12 MT and productivity 1580 kg ha^-1 during the year 2018-19. Groundnut is mainly cultivated in Bikaner, Jodhpur, Churu, Jaisalmer, Jaipur, Sikar and Nagaur districts of Rajasthan. In Bikaner district, the area under groundnut cultivation increasing day by day due to its high selling price. Bikaner district has grown maximum area of groundnut (2.45 lakh ha), with total production 5.29 lakh tonnes and productivity 2159 kg ha^-1 during 2018-19 (Annonymous, 2019).
       
The crop is suffered from several biotic factors viz., fungal, bacterial and viral diseases that limits its production and productivity. However, only a few are economically important in India, such as fungal diseases like early leaf spot (Cercospora arachidicola), late leaf spot (Cercosporidium personatum) and Collar rot/Crown rot (Aspergillus niger and A. pulverulentus). Among the fungal diseases, the early leaf spot caused by Cercospora arachidicola is the major disease at early stage of groundnut crop in Rajasthan. The yield losses due to this disease could be as high as 30-50 per cent (Subrahmanyam et al., 1980; Damicone et al., 1999; Mohammed, 2004). The disease infects crop directly as well as indirectly and results in huge losses due to leaf defoliation, disruption of photosynthesis and fewer pods that are inferior in quality (Waliyar et al., 2000). Losses are even more when crop is unsprayed. Leaf spot alone reduces 43.01 per cent in pod yield, 15.95 per cent in kernel weight and 32.9 per cent in dry matter weight (Ghuge et al., 1981). The disease is endemic in Rajasthan and causes heavy losses in yield of groundnut crop.
               
The fungicides are the most common tools for controlling disease losses. Recently, Central Insecticide Board (CIB), Faridabade banned 27 pesticides including some important fungicides which are extensively used in plant disease management. These were captan, carbendazim, mancozeb, thiram, zineb, ziram and thiophanate methyl. Therefore, it is an urgent need to find out some new fungicides which are effective against early leaf spot disease. Hence, keeping in view the importance of groundnut crop and potential threat of C. arachidicola in all the groundnut growing areas in the Rajasthan, the present investigation was undertaken to manage of this important disease through new fungicides.

Efficacy of each fungicides were tested against Cercospora arachidicola both in lab as well as in field condition.
 
To evaluate the efficacy of fungicides against Cercospora arachidicola in vitro
 
The efficacy of fungicides against mycelial growth of C. arachidicola was tested at 50, 100, 200 and 500 ppm concentrations. Ten fungicides were evaluated in this study. These were T₁: Hexaconazole 5% EC, T₂: Difenconazole 25% EC, T₃: Propiconazole 25% EC, T₄: Tebuconazole 25.9% EC, T₅: Trifloxystrobin 25% + tebuconazole 50% WG, T₆: Mancozeb 75% WP , T₇: Chlorothalonil 75% WP, T₈: Carbendazim 50% WP, T₉: Carbendazim 12% + mancozeb 63% WP and T₁₀: Captan 70% + hexaconazole 5% WP (Table 1). The efficacy against mycelial growth was tested using Poisoned-Food-Technique (Nene and Thapliyal, 1973). Required quantities of fungicide was thoroughly mixed in melted PDA, just before pouring in sterilized petri dishes and were allowed to solidify for 12 hrs. Each plate was then inoculated with 5 mm disc of mycelial bit taken from the periphery of 10 days colony of C. arachidicola growing on PDA. The inoculated petri dishes were incubated at 25±1^oC. Three plates were used for each treatment serving as three replications. Colony diameter (two diagonals) was measured after 15 days of incubation. Medium without fungicide served as control.  Per cent growth inhibition was calculated by Vincent’s (1947) formula as follows:
Where
I = Mycelial growth inhibition (%).
C =  Diameter of the colony in check (average of both diagonals).
T =   Diameter of the colony in treatment (average of both diagonals).


 
To manage early leaf spot disease of groundnut through fungicides in vivo
   
Calculation and statistical analysis
 
Disease intensity was calculated after spray. Per cent disease control was calculated by following formula:
                                                                 
The data of per cent disease intensity 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 experiment was analyzed following Randomized Block Design (Cochran and Cox, 1957). Economics of each treatment was also computed.

Efficacy of fungicides against mycelial growth
 
The efficacy of ten (systemic, contact and combi) fungicides were tested at 50, 100, 200 and 500 ppm concentrations by poison food technique on mycelial growth of Cercospora arachidicola under in vitro. The data presented in Table 1 and Plate 1 showed that all the ten fungicides caused significantly reduction in mycelial growth as compared to control. Tebuconazole 25.9 % EC was found most effective followed by trifloxystrobin 25% + tebuconazole 50% WG and propiconazole 25% EC resulting in significantly reduction of mycelial growth of C. arachidicola. At 50 ppm concentration, maximum per cent growth inhibition of C. arachidicola was observed in tebuconazole 25.9% EC (84.96%) followed by trifloxystrobin 25% + tebuconazole 50% WG (82.08%) and propiconazole 25% EC (67.04%). At 100 ppm concentration, cent per cent growth inhibition of C. arachidicola was observed in tebuconazole 25.9% EC and trifloxystrobin 25% + tebuconazole 50% WG. Propiconazole 25% EC gave (81.44%) mycelial growth inhibition at 100 ppm followed by carbendazim 12% + mancozeb 63% WP (81.12%) and hexaconazole 5% EC (80.96%). Minimum mycelial growth inhibition was observed in chlorothalonil 75% WP (60%) followed by difenconazole 25% EC (70%) and mancozeb 50% WP (71%). At 200 and 500 ppm concentrations, cent per cent growth inhibition of C. arachidicola was also observed in tebuconazole 25.9% EC and trifloxystrobin 25% + tebuconazole 50% WG. Propiconazole 25% EC gave also cent per cent mycelial growth inhibition at 500 ppm.
 

       
Earlier workers Nath et al., (2013) evaluated different fungicides viz.,tebuconazole (0.05%, 0.10%, 0.15%, 0.20% and 0.40%), folicur (0.10%) and mancozeb (0.30%) in vitro against late leaf spot of groundnut caused by Phaeoisariopsis personata and maximum per cent growth inhibition of pathogen was reported in tebuconazole 0.40% (87.97%) followed by tebuconazole 0.2% (85.24%) and tebuconazole 0.15% (83.50%). Among these, tebuconazole 0.15% was found to be optimum. In leaf spot of husk tomato caused by Cercospora sp. where in 100 per cent inhibition of conidial germination of the pathogen was noticed at 30 ppm of tebucnozole (Ruben et al., 2007). Mushrif et al., (2017b) observed that fungicides tebuconazole at 50 ppm and fungicide carbendazim at 100 ppm could suppress the germination of the spores of Cercospora arachidicola and Cercosporidium personatum completely under in vitro conditions. In the present studies, the fungicide tebuconazole and trifloxystrobin 25% + tebuconazole were effective in inhibiting the mycelial growth of C. arachidicola in vitro. These studies are also in confirmatory to our results.
 
Field efficacy of different fungicides against early leaf spot disease
 
Ten fungicides (systemic, contact and combi products) viz., hexaconazole 5% EC (0.1%), difenconazole 25% EC (0.1%), propiconazole 25% EC (0.1%), tebuconazole 25.9% EC (0.1%), trifloxystrobin 25% + tebuconazole 50% WG (0.2%), mancozeb 75% WP (0.2%), chlorothalonil 75% WP (0.2%), carbendazim 50% WP (0.2%), carbendazim12% + mancozeb 63% WP (0.15%) and captan 70% + hexaconazole 5% WP (0.2%) were tested as a foliar application to assess their efficacy against early leaf spot disease of groundnut under field condition during Kharif 2019. The application of fungicide caused significant reduction in intensity of early leaf spot in groundnut (Table 2). However, amongst the different fungicides tested, application of single spray of tebuconazole 25.9% EC @ 0.1% concentration (treatment T₄) found most effective in controlling early leaf spot disease and had 16.06 % disease intensity during Kharif- 2019. This treatment caused 70.73 per cent reduction in leaf spot intensity as compared to untreated control plots. Treatment T₅, i.e. single spray of trifloxystrobin 25% + tebuconazole 50% WG @ 0.2 % found next best option where it was at par with treatment T₄. It has reduced 68.57 per cent leaf intensity as compare to untreated control plots. It was followed by propiconazole 25% EC (64.20%). Among the tested fungicides, minimum disease control (42.29%) was recorded by chlorothalonil 75% WP at 0.2% concentration (treatment T₇). The maximum disease intensity of 54.86 % was recorded in control plot (T₁₁).
 
Pod yield
 
The fungicides treatments had significant effects on pod yield (Table 2) and all the treatments were found effective in enhancing pod yield as compared to contol. The highest pod yield 32.5 q/ha was recorded in the treatment T₄ i.e., foliar spray of tebuconazole 25.9% EC @ 0.1% concentration. It was followed by trifloxystrobin 25% + tebuconazole 50% WG @ 0.2 % concentration (31.6 q/ha) and treatment T3 i.e. propiconazole 25% EC @ 0.1 % concentration (30.8 q/ha).Treatment T₄ had 12.0 q/ha greater pod yield than control followed by Treatment T₅ (11.1 q/ha). Treatment T₄ and T₅ increased 36.92 per cent and 35.12 per cent pod yield as compare to control plots, respectively.  The minimum pod yield was observed in treatment T₇ i.e. one spray of chlorothalonil 75% WP @ 0.2 % (23.8 q /ha). In control plot where 20.5 q ha^-1 pod yield was obtained.
               
The economics computed on various treatments (Table- 3) revealed that highest net gain Rs 57,500/ha was obtained in the treatment T₄, i.e.  foliar spray of tebuconazole 25.9% EC @ 0.1% concentration followed by T₃, i.e. foliar spray of propiconazole 25% EC @ 0.1 % concentration  (Rs. 49,825/ha). Similarly, Mushrif et al., (2017b) observed that the tebuconazole (0.1%) was effective in registering least disease severity in terms of percent disease intensity, 13.67 and 15.07 and highest pod yield, 2295.92 and 2551.02 kg ha^-1 and haulm yield,2716.84 and 3066.22 kg ha^-1, respectively for two different seasons. Their experimental finding is similar to present finding. The findings of the present research work was in consonance with the findings of Mushrif et al., (2017b) where they had evaluated seven fungicides comprising of triazoles (difenoconazole, propiconazole, tebuconazoleand bitertanol), dithiocarbamate  (mancozeb), benzimidazole (carbendazim) and phthalimide (chlorothalonil) in the field against Cercospora arachidicola and Cercosporidium personatum, the causal agents of tikka disease of groundnut during the kharif 2008 and 2009. They have reported that the tebuconazole (0.1%) was effective in registering least percent disease intensity, 13.67 and 15.07 with maximum pod yield, 2295.92 and 2551.02 kg ha^-1 and haulm yield, 2716.84 and 3066.22 kg ha^-1, respectively for two different seasons. Similarly Khan et al., (2014) evaluated the efficacy of different fungicides on cercospora leaf spot of groundnut and maximum disease control with high pod yield was observed with nativo and triazole fungicides. Nath et al., (2013) studied the efficacy of tebuconazole against late leaf spot disease of groundnut and observed that tebuconazole (0.15%) gave best result and reduced the disease intensity to 52.42% and increased pod yield up to 67%. These findings also corroborated to our investigation.
 

It can be concluded unequivocally considering the results that in groundnut cv HNG-69,  application of single foliar spray of tebuconazole 25.9 % EC @ 0.1% concentration was an effective treatment which reduced early leaf spot intensity and increased pod yield with maximum net return under field conditions followed by foliar spray of trifloxystrobin 25% + tebuconazole 50% WG @ 0.2 % concentration. These fungicides are better option of groundnut cultivators for enhancing the pod yield by managing early leaf spot disease.

The authors are very much thankful to Dr P.S. Shekhawat, Zonal Director Research, Agricultural Research Station, Swami Keshwanand Rajasthan Agricultural University, Bikaner for providing groundnut seeds and land to conduct the experiment.