Rice (Oryza sativa L.) is an important cereal crop and nearly more than half population of the world subsists on it. It is the third largest crop in Pakistan after wheat and cotton  . Despite higher potential, average yield of rice in Pakistan is 2.38 t∙ha−1 which is lower than most countries in the world. This low production is attributed to delayed sowing, low plant population, imbalance fertilizer use and disease attack especially Rice blast and Bacterial leaf blight  . Rice blast and BLB of rice are more widespread in Pakistan  . The bacterial leaf blight disease is reported to induce 50% or even greater losses in severe cases. Losses in the Tropical Asia varied 2% - 74% depending upon location, weather conditions, crop stage and cultivar  .
Effective control of the disease had not been achieved by chemical methods  . Under prevailing conditions commercial resistant varieties are generally scarce and current varieties are susceptible to these diseases thus reducing their potential due to disease attack. Cultural practices such as fertilizer rate, sowing time and irrigation etc. played an important role for the control of these diseases  . So, there is a need to emphasize over the cultural techniques for the effective management of these diseases. The plant diseases are influenced by specific genotype, fertilizer input and climatic conditions, which vary yearly  . Different plant nutrients and their balanced use played a significant role in reducing pest and disease infestation, which results in higher returns through enhanced yields and better quality  . The cultural control may include the alteration of nitrogen (N) and potash (K) levels and investigating the appropriate doses for the control of these diseases. Optimum fertilization of N and K induces disease tolerance in plants.
It is a fact that excessive N application predisposes plants to disease caused by obligatory parasitic pathogens  . Excessive use of N fertilizers could result in more leaf growth that was over succulent and more susceptible to certain diseases  . It was found that BLB incidence considerably exaggerated with increasing rate of N fertilizer  . On the other hand, it was found that low N fertilizer use increased susceptibility to disease attack. BLB was considerably more severe on the vulnerable cultivars when N fertilizer was added in low rates. Nitrogen deficiency induced modification of many morphological and physiological parameters, limitation of growth, leaf number and leaf area  . It found that narrow brown leaf spot was more severe in low N treatments    ; they also emphasized on the positive role of N fertilization for minimizing disease incidence  .
The role of K is very important regarding the disease control. The potassium aggregate increases the vigor and the resistance of the plant to stress. It fortifies the straw, enhances the quality of grains and helps the transfer of starch, sugars and oils. K strengthens the plant leaf cells and K deficiency in leaf cells makes them weak and susceptible to secondary fungal infection  . Sufficient K nutrition increases cell cuticle thickness, cell wall strength and production of phenols that ultimately implant resistance in crops  . Potassium fertilization greatly affected panicle blast development, the response being significantly linear and negative with increasing levels of potash  . Thus, disease incidence can be lowered by high K levels   . However, in Pakistan K use is only about 2 kg∙ha−1  .
Contrarily,  found that K did not affect the disease severity of Anthracnose crown rot (Colletotrichum fragariae) on strawberry plants. K fertilization did not have a noticeable effect on disease severity scores. Similarly,  investigated the effect of K fertilization on the severity of Narrow brown leaf spot in rice. The present study was therefore planned to evaluate the influence of N and K solely or in combination, on the incidence of BLB of rice.
2. Materials and Methods
A research to study influence of N and K levels on the severity of BLB was conducted at Rice Research Institute (RRI), Kala Shah Kaku, Lahore, during kharif season, 2010. The experiment was laid out in a Randomized Complete Block Design (RCBD) with factorial arrangement comprising three replications using net plot size of 2.25 m × 6.0 m. The treatments of an experiment comprised; 0 kg N ha−1 (control), 75 kg N ha−1, 100 kg N ha−1, 125 kg N ha−1 and 0 kg K ha−1 (control), 50 kg K ha−1, 75 kg K ha−1, 100 kg K ha−1. Thirty days old nursery of Super Basmati was transplanted manually in puddled field on 16th July, 2010. Phosphorous fertilizer was added @ 75 kg∙ha−1. Whole of the phosphorus and zinc was applied at the time of soil preparation. Full dose of K were added to the plots just before transplanting of rice nursery according to respective treatments. N was added in two splits i.e. 1/2 of N was applied just after transplantation of nursery and remaining 1/2 half dose was applied 30 days after transplanting (DAT). Machete 60EC was applied @ 2 t/ha to control weeds just after four days of transplanting. The remaining (fallow) weeds were controlled manually. ZnSO4 (35%) was applied @ 12.5 kg∙ha−1 at 12 DAT. Rice was inoculated with Xanthomonas oryzae pv. oryzae following clipping method at tillering and heading stage. Pathogen was inoculated by piercing method. The leaves were inoculated at booting stage. Paddy yield was recorded after harvesting each plot and was converted into t∙ha−1. Data regarding BLB incidence was analyzed following disease rating scales (Table 1) developed by IRRI  . Percent disease incidence was calculated according to the formula as follows:
Disease Incidence (%) = (Total lesion area)/(Total leaf length) × 100
The data obtained were analyzed statistically by using Fisher analysis of variance technique and difference among treatment means was compared by using least
Table 1. Disease rating scale for assessment of BLB in rice crop.
significant difference test (LSD) at 5% probability level. The differences were only considered when significant at p < 0.05  .
3. Results and Discussion
The analysis of variance confirmed the statistically significant variation among the different rates of nitrogen (N) application as well as among the different rates of potassium (K) application. Moreover, interactive effects of N and K application were also found to be statistically significant.
The results showed that disease incidence of bacterial leaf blight (BLB) in rice decreased as K application increased at 0 kg ha−1 N but this decreased was not statistically significant over control (no application of N and K). However, at 75 kg ha−1 N, K application decreased the disease incidence significantly. The minimum disease incidence of BLB (15.76%) in rice was observed when 75 kg ha−1 N was applied with no application of K and these results were followed by the application of 75 kg ha−1 N with 100 kg ha−1 K where disease incidence was 21.22%. However, application of 75 kg ha−1 N with 0 kg ha−1 K was statistically at par with the application of 75 kg ha−1 N with 100 kg ha−1 K but statistically significant from all other combinations of N and K in terms of disease incidence in rice. Maximum disease incidence in rice was observed when 125 kg ha−1 N was applied with 50 kg ha−1 K followed by control (no application of N and K) (Table 2). The percent diseased leaf area showed a parabolic trend to various N fertilization levels at 0 kg K ha−1 i.e. more where no N was applied, decreased by 55% when N application was made at the rate of 75 kg N ha−1, and again increased by 11.9% to 26.6% at 100 kg N ha−1 to 125 kg N ha−1 against control (0 kg N ha−1). At 50 kg K ha−1 level, the percent diseased leaf area remained unaffected with N fertilization from 0 kg N ha−1 to 100 kg N ha−1, however percent diseased leaf area (% DLA) reaches at its maximum at 125 kg N/ha. On the other hand, percent diseased leaf area (% DLA) remained unaffected with any rate of N at 75 kg K ha−1 level. The percent diseased leaf area (% DLA) once again showed a parabolic trend at 100 kg K level where comparatively high percent diseased leaf area (% DLA) was recorded in treatment without N fertilization and it decreased by 24.48% at 75 kg N ha−1 level. A further increase of N fertilization by 25 kg N ha−1 (i.e. 100 kg N ha−1) did not contributed any way to percent diseased leaf area (% DLA). However, when N fertilization was increased up to 125 kg N ha−1 the percent diseased leaf area (% DLA) again increased by 33%. Percent diseased leaf area (% DLA) varied differently in relation to N fertilization at different K rates. Irregular trend was seen at different K levels however, 75 kg N ha−1 to 100 kg N ha−1 gave lower percent diseased leaf area (% DLA) as compared to control and 125 kg N ha−1. Based on results, it was deduced that BLB could be minimized by a better combination of N and K rather to apply these nutrients in a haphazard manner. So for better management of disease a balanced combination of N and K should be applied. It is suggested that K:N ratio is more important than the effect of each individual nutrient in blast development  .
At no N application, non-significant increase in paddy yield was observed by
Table 2. Effect of nitrogen and potassium on the diseases incidence of bacterial leaf blight and paddy yield of rice.
Note: Values sharing the different letters differ significantly at 0.05 probability; NS = Non-Significant.
K application. However, at 75 kg N ha−1, K application increased paddy yield significantly. Further increase in N dose along with K didn’t influence paddy yield significantly. Maximum paddy yield (4.32 t∙ha−1) was recorded with the application of 75 kg ha−1 N with 100 kg ha−1 K followed by 75 kg ha−1 N with 75 kg ha−1 K. Application of 75 kg ha−1 N with 100 kg ha−1 K was statistically at par with the application of 75 kg ha−1 N with 75 kg ha−1 K but statistically significant from all other combinations of N and K in terms of paddy yield. Minimum paddy yield was recorded in control (no application of N and K). K application increased the paddy yield but this increase was statistically not significant at 0 kg ha−1 N (Table 2). Results indicated a marked increase in yield from 0 kg N ha−1 to 100 kg N ha−1. This increase in yield was attributed to more number of panicle bearing tillers, more 1000 kernel weight and more number of kernels per panicle at 75 kg N ha−1 and 100 kg N ha−1. This increase in yield was supported by  ; they found an increase in yield with increased N rates. An increase in yield with increasing N fertilization was observed. However, there was a significant decrease in yield at 125 kg N ha−1 irrespective of different K levels. This decrease in yield may be due to high disease percentage   . Similarly, an increase in yield up to 85 kg N ha−1 was observed beyond this level decrease in yield was attributed to accumulative effect of N toxicity and disease severity. The yield also displayed a positive increase with increase in K fertilization  . These results were advocated by  they explained that potassium fertilization increased the yield over the control.
Economic analysis revealed that application of 75 kg N ha−1 with 100 kg K ha−1 gave maximum gross income of Rs. 154,652 ha−1 (Table 3). The highest gross income at this level is attributed to highest yield at the fertilization rate. Application of 75 kg N ha−1 with 100 kg K ha−1 gave maximum net returns amounting Rs. 37159. The highest cost benefit ratio of 1.32 was obtained by 75 kg N ha−1 with 100 kg K ha−1 and 100 kg N ha−1 and 0 kg K ha−1. The production level at T8 is favored for lower disease severity.
Table 3. Economic analysis of different application rates of N and K in rice.
Fixed cost Rs. ha−1 = 85,311.0, Straw price Rs. per ton = 850.0, Paddy price Rs. per 40 kg = 1300.0, BCR = Benefit cost ratio, N = Nitrogen, K = Potassium.
Rice blast responded significantly to N fertilization. Both deficiency and excessive use of N promoted disease severity. BLB also responded significantly to K fertilization. Both deficiency and excessive use of N promoted BLB disease severity. Lower diseased leaf area (% DLA) was observed where N was applied at the rate of 75 - 100 kg∙ha−1. Results revealed that balanced nutrition is required to BLB. As the rate of N fertilizer was increased the demand for potassium also increased. Super Basmati gave best paddy yield, highest harvest index value, higher gross income, maximum net returns, higher benefit cost ratio and more resistance against BLB when fertilized @ 75 kg N ha−1 & 100 kg K ha−1.