Effect of different levels of nitrogen and potassium against leaf spots disease of groundnut in different fertility gradient soil in field

Groundnut (Arachis hypogea L.) is one of the principal economic oilseed crops in India next to mustard. It is the most important summer season cash crop as well as oil seed crop in the world (Mensah and Obadoni, 2007). In comparison with other oilseed crops, groundnut stands highest in area, production and productivity in India (7572 ha, 9164 thousand tonnes, 1210 Kg ha^-1 respectively (Anonymous, 2000). Groundnut suffers from different diseases of biotic and abiotic origin. Among the biotic constraints, fungal diseases are one of the major hindrance affecting the qualitative and quantitative yield of the crop. The leaf spot or tikka diease (Cercospora arachidicola and personatum) of groundnut (Arachis hypogea L.) is a predominant, devastating and economically important foliar fungal disease and a major yield reducing factor of groundnut in the world (Hasan et al., 2016). Cercospora leaf spot disease decline the kernel yield from 5.78 to 5.50 g m^-2 plot in every 1% increase in disease severity (Das and Roy, 1995).
        
To achieve increasing demand of oilseed production the farmers traditionally follow staggered and inappropriate application of chemical fertilizers without considering fertility status of the soil. But the increased use of chemical fertilizers in different regions predisposes the cultivated crop towards infection by various diseases in low to severe form. Nutrition plays an important role in the resistance or susceptibility of host against different pathogens. A disease can be eliminated by application of different nutrition but the severity of many diseases is reduced by specific levels of fertility gradient, arranged by soil treatment with different NPK and organic matter like FYM before cultivation. With increasing level of N and decreasing level of K maximum disease severity and poor yield were obtained in wheat infected with Helminthosporium sativum (Devi et al., 2012). Similarly maximum tuber yield and minimum foliar disease were observed in moderate level of N (150kg ha^-1) and maximum potassium (125kg ha^-1) in low, medium and high fertility gradient soil (Mahapatra et al., 2012). The present investigation has been conducted to find out the actual doses of nitrogen and potassium required under different soil fertility status to obtain higher yield of groundnut with less disease.

The experiment was conducted at the Regional Research Station, Gayeshpur, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia, West Bengal during the pre-kharif season for three consecutive years 2012 to 2014. Soil was sandy loam in texture and latitude 22°93N and longitude 88°33E.
        
Four fertility gradients in the experimental field were prepared and maintained by cultivation of maize during rabi season. The field consisted of 4 strips each divided into four equal plots covering a total area of 2000 m² and individual strip of 500 m². Each of the plots was separated by one meter irrigation channel. The fertility gradient strips were made by applying different doses (kg ha^-1) of NPK viz. low (S₁)-(0:0:0), medium (S₂)-(50:31:67), moderate (S₃)- (100:62:134), high (S₄)-(200:124:268). These NPK were applied during maize cultivation. After harvesting of the maize, soil samples were collected from each gradient strips at a depth of 15cm and analysed for pH, organic carbon (Nelsons and Sommers, 1982) and total NPK following standard methods (Jackson,1973). The mean data of available NPK gradient in soils after maize crops were low (S₁) = 278.3: 9.9:80.3; medium (S₂) = 290.1: 22.9: 97.0; moderate (S₃) = 296.0:26.6: 213.5 and high (S₄) = 301.8: 37.1: 282.8 kg ha^-1. The pH levels of the soils were 6.3, 6.2, 6.4 and 6.3, respectively. Every fertility gradient strips were divided into 5×5m² with five treatments including untreated control with four replications of total 64 plots in a split split design. Two nitrogen doses combining with two potassium doses along with untreated control were regarded as treatments. The two different nitrogen doses used were 20 (N₁), 40 (N₂) kg ha^-1, where 50% was used as basal and remaining 50% during earthing up at 35 days after sowing. Two different doses of  K were 20 (K₁), 40 (K₂) kg ha^-1 and recommended single dose of 100 kg P ha^-1 were applied in each plot as basal at the time of sowing. The groundnut variety TAG 17 was shown on 28^th January, 19^th February and 6^th February for 2012, 2013 and 2014 respectively for three consecutive years. The seed rate was 100 kg ha-1 with a spacing of 60 × 30 cm² sowing in furrow method. Hoeing, weeding, earthing up and irrigation were done at both vegetative and reproductive stages of the crop. Earthing up was given at 35 days after flowering of groundnut crop. Insecticides were sprayed as and when required.
        
Severity of leaf spots (tikka) were assessed and was recorded based on a 1-7 scale developed by Noriega-Cantu et al., (2000). Ten plants of each replication were selected randomly for data collection and per cent disease severity (PDI) was calculated as suggested by (Mahapatra et al., 2011).
        
The disease severity data were averaged over the three replications and disease progress curves were calculated as

               
                                               

 
Yi= Severity at 1^st observation,
Xi= Time (days) at first observation,
n= Total number of observation.
 
The crop was harvested on 15^th to 18^th June in 2012 and 2013 and 12^th to 15^th June, 2014 in three consecutive years and converted to quintal per hectare. The number of kernel per plant and weight of kernel of five plants were obtained from randomly selected 5 plants from each replicated plot.
        
The experimental results were statistically interpreted through calculation of ‘Analysis of variance’ by a standard method (Gomez and Gomez, 1984) and the significance of different treatments was tested by Error mean square by Fisher and Snedicor’s ‘F’ test at probability level 0.05. For determination of critical difference (CD) at 5% level of significant Fisher’s and Yate’s tables were consulted.

Two doses of N and K levels in different combinations showed various degree of disease severity (AUDPC) during the study period irrespective of fertility gradients. In 2012, 2013 and 2014 minimum disease severity (35.26, 33.0 and 26.60) and maximum percent reduction over control (10.39%, 9.45% and 14.18%) were observed in N₄₀K₂₀ where as maximum disease severity and minimum reduction was found in N₄₀K₄₀ (35.93, 33.21, 26.26 & 8.56%, 9.08 & 14.5% respectively) which was significantly at par with N₂₀K₂₀. It was also found that high N dose with low K showed minimum disease severity followed by high Nitrogen (N) with high Potassium (K) though their difference in disease severity was statistically significant. The percent reduction over control (N₀K₀) was maximum in N₄₀K₂₀ (9.45%) in 2013 and N₄₀K₄₀ (14.56%) in 2014, where as N₂₀K₄₀ showed minimum disease reduction in both years (Table 1).
 

        
Two levels of N and K showed the difference in severity in four fertility gradient soils. Increasing in fertility gradient the disease severity (AUDPC) was decreased irrespective of NK combinations (Table 1). This severity was also different significantly in between years. In (S₁) the minimum disease severity (39.19) was observed in N₂₀K₄₀ in the year 2012, where as N₄₀K₂₀ showed minimum disease in 2013, 2014 and also in pooled mean. In the year 2013, 2014 and in pooled mean showed maximum disease severity in N₂₀K₄₀ combinations (38.70, 30.60 and 35.98 respectively). In moderate fertility gradient soil (S₂), minimum disease in N₄₀K₄₀ (36.38) and maximum N₂₀K₄₀ (38.49), in medium fertility gradient soil (S₃) maximum disease in N₄₀K₄₀ (36.69) and minimum in N₄₀K₂₀ (33.68) where as in high fertility gradient soil (S₄) maximum disease in N₂₀K₂₀ (30.71) followed by N₄₀K₄₀ (30.29). The percent disease reduction over control was maximum in (S₂)  in comparison to other fertility gradients irrespective of different N and K combinations. It was observed in S₂N₄₀K₄₀ (16.03%) followed by S₂N₂₀K₂₀ (13.54%). The minimum disease reduction was in (S₄) in comparison to other fertility gradient soil irrespective of NK combinations. In 2014, the minimum disease severity was observed in S₄ gradient soil and maximum in S₁ gradient soil irrespective of NK combinations. The minimum disease severity was observed in N₄₀K₂₀ in S₁ (36.23), N₂₀K₂₀ in S₂ (35.85), N₂₀K₄₀ in S₃ (32.46) and N₄₀K₄₀ in S₄ (25.61). The maximum disease reduction was recorded in N₄₀K₂₀ in S₁ (8.79%), N₂₀K₂₀ in S₂ (6.72%), N₂₀K₄₀ in S₃ (17.15%) and N₄₀K₄₀ in S₄ (10.63%) fertility gradient soil condition (Table 1).
        
In the year 2014, the disease severity was comparatively less than other two years and with increased in fertility gradient; the disease severity was significantly decreased as noted in other years. But it is interesting to note that percent disease reduction over control was maximum in S₁ and comparatively less in S₂. The maximum disease reduction was observed in S₁N₄₀K₂₀ (29.08%) followed by S₁N₂₀K₂₀ (20.34%) and S₄N₂₀K₂₀ (20.00%). The minimum disease reduction was recorded in S₄N₂₀K₄₀ (4.51%). Three years pooled mean showed the similar trend, that the minimum disease severity was recorded in S₄N₂₀K₂₀ (25.40) followed by S₄N₄₀K₄₀ (25.56) and the maximum per cent disease reduction was in S₁ gradient soil significantly decrease with increased in fertility gradient irrespective of different NK combinations. The maximum reduction was in N₄₀K₂₀ (16.06%) in S₁, N₄₀K₄₀ (13.68%), in S₂, N₄₀K₂₀ (11.44%) in S₃ and N₂₀K₂₀ (11.58%) in S₄ (Table 2).
 

        
Different NPK nutrition showed, low N level and high K level decreased the disease severity of groundnut. Similarly Tanaka et al., (1993) reported that Cercospora leaf spot of soybean decreased with the increase of K level. Ihejirika et al., (2006) also observed that NPK fertilizer rate was inversely related to groundnut leaf spot disease severity, as nutrient availability to plant induces resistance to diseases.
 
Yield characters
 
The effect of different levels of N K applied in different combinations under different fertility gradient soil resulted different leaf spot severity of groundnut and also effect on the yield and yield characters (Table 2).
 
Number of kernel
 
Different N K combinations showed different in kernel number and with increased in N there was a significant increased in kernel number whereas two K levels showed no significant difference in between them. In 2012 and 2013 the maximum kernel number in N₄₀K₂₀ (36.83 & 34.96 respectively) and minimum in N₂₀K₂₀ (31.90, 30.76 respectively). In 2014, the maximum was obtained in N₂₀K₂₀ (21.45) statistically at par with N₂₀K₄₀ (20.93) whereas minimum in N₄₀K₄₀ (20.20) statistically at par with N₄₀K₂₀ (20.50). Three years pooled mean showed maximum kernel number in N₄₀K₂₀ (30.76) followed by N₄₀K₄₀ (29.92) and minimum in N₂₀K₄₀ (27.89) statistically at par with N₂₀K₂₀ (28.03) (Table 2).
        
The three years pooled mean also showed that with increase in fertility gradient there was a significant increased in kernel number and maximum was noticed in S₃N₂₀K₄₀ (33.74) statistically at par with S₁N₄₀K₄₀ (32.41) where as minimum in S₁N₂₀K₄₀ (24.97) statistically at par with S₁N₂₀K₂₀ (25.08) and S₂N₄₀K₄₀ (26.28).
 
Weight of kernel
 
Two different combinations of N and K levels showed difference in kernel weight in different years and also in pooled mean and maximum was observed in N₄₀K₄₀ in all the years and also in pooled mean except in 2014, where kernel weight was maximum in N₂₀K₂₀ and minimum in N₄₀K₄₀. Minimum kernel weight was observed in N₂₀K₂₀ in 2012, 2013 and pooled mean (Table 2).
        
Three years pooled mean showed the maximum kernel weight in S₁N₄₀K₄₀ (44.22 g plant^-1) followed by S₃N₄₀K₂₀ (40.11 g plant^-1), S₁N₄₀K₂₀ (38.02 g plant^-1) and minimum in S₂N₄₀K₄₀ (33.24 g plant^-1) statistically at par with S₄N₂₀K₂₀ (33.44g plant^-1) and S₂N₄₀K₂₀ (33.53 g plant^-1). All the treatment combinations increased the kernel weight over untreated control and their differences were statistically significant (Table 2).
 
Kernel yield
 
Two different levels of N and K interactions showed no significant difference in kernel yield irrespective of fertility gradient of soil. This trend was observed in the year 2012, 2013 and 2014 and in pooled mean of three years.
        
The three years pooled mean showed that with increase in fertility gradient of soil there was a significance decrease in tikka disease severity irrespective of NK combinations. Maximum reduction and minimum disease severity in different NK combinations in different fertility gradient soil were as in (S¹) N₄₀K₂₀ (AUDPC-34.48 and 16.06% reduction); (S₂) N₄₀K₄₀(AUDPC- 32.88 and 13.68% reduction): (S₃) N₂₀K₄₀ (AUDPC- 31.51 and 11.44% reduction) and (S₄) N₂₀K₂₀ (AUDPC-25.40 and 11.58% reduction). The yield attributes like kernel number per plant, kernel weight per plant (g) and kernel yield (qha^-1) were also high in above N and K combinations in different fertility gradient soil as in (S₁) (30.75; 38.02 g and 22.19 qha^-1); (S₂) (26.28; 33.24 g and 21.91 qha^-1); (S₃) (33.74; 40.11g and 22.35 qha^-1) and (S₄) (30.04; 33.44 g and 24.92 qha^-1) respectively.
 
Relationship between disease severity and yield parameters
 
The correlation co-efficient value (r) and linear relationship between disease severity and yield parameters at different fertility gradient soil showed a negative significant correlation between disease severity and yield parameters. The negative relationships were obtained between kernel yield, number of kernel/ plant, kernel wt./plant and disease severity at all fertility gradient soil except in moderate, high and medium fertility levels and it was confirmed by high R² value irrespective of different N and K levels (Fig 1).
 

 
Different yield and yield parmeters of groundnut showed a positive correlation with N and K nutrition in different fertility gradient soil. Umar-Shahid et al., (1997) reported that groundnut pod yield was affected by K and S nutrition. Smilarly, Dwivedi et al., (1998) reported that pod yield of groundnut was increased with increasing lime rate and P applied as a super phosphate. Dubey (1999) reported that groundnut crop supplied with basal dose of 30 kg ha^-1 K along with 50 kg ha^-1 P₂O₅ and Nitrogen 25 kg ha^-1 gave the maximum pod yield 15 q ha^-1. This result showed some insignificant erratic result in low and high fertility gradient soil in respect to kernel yield in different N and K nutrition. Whereas, in medium and moderate fertility soil, increased N level increased the kernel yield of groundnut, which collaborated the result of Dubey (1999).
               
The findings of the present investigation suggest that fertilizer application in accordance with site specific nutrient status is most important for minimizing leaf spot diseases with subsequent enhancement of yield in groundnut.