Nitrogen and potassium fertilization on the yield and intensity of the maize white spot

Submitted on 04/23/2014 and approved on 07/08/2015. 1 This paper is part of the first author ’s master dissertation. 2 Universidade Federal de Lavras, Departamento de Fitopatologia, Lavras, Minas Gerais, Brazil.gabrieldornelas@yahoo.com.br 3 Universidade Federal de Lavras, Departamento de Fitopatologia, Lavras, Minas Gerais, Brazil.eapozza@dfp.ufla.br; pauleste@dfp.ufla.br 4 Embrapa Milho e Sor go, Gurupi, Tocantins, Brazil. rodrigo.veras@embrapa.br 5 Universidade Federal de Lavras, Departamento de Ciência do Solo, Minas Gerais, Brazil. adelia.pozza@dcs.ufla.br 6 Universidade Estadual do Centro Oeste, Departamento de Agronomia, Guarapuava, Paraná, Brazil. leandro.alvarenga.s@hotmail.com * Corresponding author: gabrieldornelas@yahoo.com.br Nitrogen and potassium fertilization on the yield and intensity of the maize white spot 1


INTRODUCTION
The white spot -Pantoea ananatis (Paccola-Meirelles et al., 2001;Bomfeti et al., 2008), is one of the most important diseases in corn crop.Historically, it only occurred at the end of the cycle of the crop; however, high severity was observed in young plants, during the vegetative phase, resulting in premature wilt of plants in susceptible genotypes (Mendes & Tebaldi, 2011).The disease occurs in temperature and relative humidity higher than or equal to 14° C and 60%, respectively (Rolim et al., 2007) in regions above 700 m.a.s.l.(altitude) (Silva & Menten, 1997), due to the longer duration of leaf wetness period, provided by dew.In these conditions, damages can cause losses of up to 60% of corn yield (Rolim et al., 2007).
Among environmental factors that influence the occurrence of the disease, some studies have highlighted the important role of temperature and water (Fernandes et al., 1995;Fantin et al., 2005).However, few studies have described the interaction of mineral nutrition with this disease.The nutritional status of plants is an environmental factor that can be manipulated with relative ease, via fertilization.The supply of nutrients in a balanced way assists in building of several horizontal resistance barriers, including the wax layer (Pozza et al., 2004) and the cell wall, responsible for slowing the penetration and colonization of the pathogen, reducing the epidemic progress rate (Pozza & Pozza, 2012).
In the literature, there are several reports about how the effect of macro and micro nutrients promote reduction of the intensity of diseases in different crops, such as cercosporiosis (Pozza et al., 2001;2004;Garcia Júnior et al., 2003) and the stain of Phoma in coffee trees (Lima et al., 2010), anthracnose in strawberries (Tanaka et al., 2002), Phoma in Brassicas (Sochting &Verret, 2004) and the perforations in plum leaves (Tutida et al., 2007) among others.
When it comes to corns, nutritional imbalances between N and K predisposed plants of two cultivars to infection by Colletotrichum graminicola (Ces.)Wils.(SensuArx, 1957).In both cultivars, the amount of damaged leaf area depended on the interaction between the nutrients.The lowest severity levels were observed at the lowest dose of N, combined to the highest dose of K, and nitrogen fertilization influenced the K content negatively in the upper canopy.Caldwell et al. (2002) also report an increase in the severity of cercosporiosis (Cercospora zeae maydis Tehon & Daniels) of corn with increased doses of N (0, 60 and 120 Kg ha -1 ) and K (0, 25, 50 and 150 Kg ha -1 ) in the soil.However, there is still no information about the influence of N and K have upon the maize white spot.
Therefore, this study was performed in order to assess the effect of nitrogen and potassium fertilization on the yield and intensity of the white spot infection of corn.

MATERIAL AND METHODS
The experiments were installed in two field areas.The first one was implemented on 11/25/2011, in an area located on Palmital farm of the Universidade Federal de Lavras (UFLA), in the city of Ijaci, Minas Gerais, located at 21º09'50"S, 44º55'09''W and 837 m.a.s.l.(altitude).The second experiment was implemented on 11/18/2011, in EMBRAPA Milho e Sorgo's experimental area, in the city of Sete Lagoas, Minas Gerais, 19º26'00"S, 44º10'59''W and 732 m.a.s.l.(altitude).In both experiments, the Pioneer® 30P70 corn was cultivated, a simple hybrid likely to develop the white spot.The experiments were conducted under natural infection.The soil preparation was done by minimum cultivation with scarification and harrowing on the sowing eve.
The experimental design was installed in randomized blocks, in 5 x 5 factorial design, and four repetitions.The treatments corresponded to five doses of N and five doses of K combined with each other, in each area.The N doses were 20, 40, 80, 150 and 190 Kg ha -1 .The doses of K were calculated individually in each experimental area, according to the results of soil analysis (Table 1) and progresso da doença.O efeito foi observado apenas para o N. O K não influenciou a produtividade e a severidade da doença nesses experimentos.Observaram-se maiores áreas abaixo da curva de progresso da severidade da mancha branca e maior produtividade com o aumento das doses de N. Sendo assim, com o aumento das doses de N houve aumento da mancha branca do milho e também da produtividade.
following the recommendations for the crop in Minas Gerais (Ribeiro et al., 1999).The doses of K were 15, 30, 60, 120 and 180 Kg ha -1 of K 2 O for experiment 1 and 8. 75, 17.5, 35, 50 and 100 Kg ha -1 of K 2 O for experiment 2. The parts consisted of four rows of 5m long and the useful area by two central rows of 3 m each.The sources of N and K were urea and potassium chloride, respectively.Twenty-five percent and 50% of doses of N and K respectively were applied at sowing, and the rest was divided in two side dressings, one 35 days after sowing and the other 15 days after the first one.
In addition to N and K, 120 and 70 Kg ha -1 of P 2 O 5 were applied in experiments 1 and 2, respectively and, 2 Kg ha -1 of Zn in the form of zinc sulphate and 30 Kg ha -1 of calcium sulfate (14% of S) in the form of phosphogypsum, according to the results of soil analysis and recommendations of Ribeiro et al., (1999), at sowing date.Two micronutrient sprays of 2.0 L h -1 containing 5% Zn, 3% Mn, 0.50% of B, 0.60% Cu, 0.06% Mo and 4% S in 200 L ha -1 of water were also applied, in stages V3 and V6, (Ritchie et al., 2003), that is, 30 and 45 days after planting.
Planting fertilization and zinc sulfate and phosphogypsum were applied in the furrow.Sowing was performed manually, at a spacing of 0.70 x 0.20 m.For weed control, applications of glyphosate (2.0 L ha -1 ) were used at pre-emergence and nicosulfuron (1.5 L ha -1 ) mixed with atrazine (5.0 L ha -1 ) as post-emergence herbicides.
The evaluation of the disease severity was initiated at 43 and 56 days after emergence (DAE) in experiment 1 and 2, respectively, with the appearance of the first symptoms.From then on, evaluations were carried out every 10 days, totaling six and seven evaluations, in areas 1 and 2, respectively.These severity evaluations were performed with the help of diagrammatic scale (Agroceres, 1996).The data was used to calculate the area under the curve of progress of disease severity (AUCPS), according to Shanner & Finney (1977).
After the manual harvest the grains were threshed, weighted and the humidity content was determined.Grain yield data were refined to 13% of humidity content and expressed in t ha -1 .Climatic variables were monitored by minimum, maximum and average temperatures, average relative humidity and precipitation, in both places with Campbell Scientific ® weather stations implemented near the experiments.
Data of the AUCPS and of grain yield, from each experiment, were submitted to analysis of variance.Then, the significant variables in the F test were submitted to the adjustment of linear regression models.Pearson's correlation analysis were performed between yield and the AUCPS of both experiments.Analysis were conducted in the SAS ® program (SAS Institute Inc., 1996).

RESULTS AND DISCUSSION
The average of the maximum, average and minimum daily temperatures, during the conduction period of the experiments were 28, 22 and 18 °C.Average relative humidity was of 76% and precipitation of 1078 mm in experiment 1 (Figure 1A).In experiment 2, the maximum, average and minimum daily temperatures were 29, 22 and 18° C, and the average relative humidity was 74% and the precipitation was 1225 mm (Figure 1B).According to Oliveira et al. (2004) and Reis et al. (2004) the weather conditions favorable to the occurrence of white spot are relative humidity above 60%, high precipitation and moderate temperatures between 20 to 25 °C.

Area under the curve of progress of disease severity (AUCPS)
In both experiments there was no significant interaction (P > 0,05) between N and K to the AUCPS of the white spot (Table 2).However, the increasing doses of N influenced the AUCPS.
With the increase of N doses, a linear rise of AUCPS was observed.There was an increase of 23.8 (Figure 2A) and 51.7% (Figure 2B) in disease severity from 20 to 190 Kg ha -1 of N in experiments 1 and 2, respectively.The contribution of nitrogen fertilizer on plants' resistance to diseases varies, among other factors, depending on the pathogen, genotype, dose and source of the nutrient used and on the interaction between nutrients (Pozza & Pozza, 2012).Appropriate doses of Soil Analysis Laboratory of the Department of Soil Science (UFLA). ( Soil Analysis Laboratory of the Department of Soil (UFV).

Soil pH
Rev. Ceres, Viçosa, v. 62, n.4, p. 351-359, jul-ago, 2015 N contributes to the synthesis of lignin, phytoalexins and tannins, but in excess, N reduces the production of these compounds, due to the demand of carbon in photosynthesis via cycle of Krebs.Thus, the synthesis of the secondary metabolites via shikimic acid is compromised and the turgor cell also raises, providing greater amounts of water and also the production of carbohydrates, such as glucose (Huber & Thompson, 2007;Taiz & Zieger, 2013), essential to the infection process of the pathogen.In addition, horizontal resistance barriers as the layer of wax and also the cell wall may be compromised due to the rapid cell growth, making them thinner and less resistant to fungal penetration (Pozza & Pozza, 2012).Similarly, Fidelis et al. (2003), relate the increased severity of the white spot of corn in early stages of growth with high doses of N compared to lower doses.The reaction of 23 cultivars of corn, submitted to low N (32 Kg ha -1 of N at sowing) and to high N supply (32 Kg ha -1 at sowing + 90 Kg ha -1 of N on side dressing), was 6 and 30% of the severity of white spot, respectively, at 30 days after flowering.However, other authors, studying the isolated effect of N on the severity of the white spot on the corn, found no significant effects of nitrogen fertilization and the intensity of the disease.Pegoraro et al. (2001) relate no substantial differences among six corn hybrids to the reaction of the white spot with two doses of nitrogen fertilization (42.5 and 85 Kg ha -1 of N).According to the authors, these N doses were high enough to ensure the nutrition of plants not favoring high intensities of the white spot on the corn.Meanwhile, these doses were lower than the last two used in experiments of this study and, also lower than the highest dose used in the experiment of Fidelis et al. (2003).The results obtained by Pegoraro et al. (2001) also agree with Duarte et al. (1999) and Souza & Duarte (2002) in experiments with lower doses of N (8 and 32 Kg ha -1 ), in which changes in the susceptibility of cultivars to the white spot were not observed.But these authors also did not study how much the dose can increase without causing increment in the intensity of the disease, as studied in this work.
In other pathosystems, N can both reduce and increase diseases.The tropical rust (Physopella zeae (Mains) Cummins & Ramachar) of the corn (Tomazela et al., 2006), for example, increased with the addition of 200 Kg ha -1 of N, also followed by increased yield.This study reports the importance of nitrogen fertilization favoring foliar diseases of the corn, although it did not study the interaction with other nutrients and did not quantify its increment in relation to higher doses over time.
Still in relation to the corn, Carvalho et al. (2013), studying the interaction of doses of N (75, 150, 300, 600 and 1200 mg dm -3 ) and K (63, 125, 250, 500 and 1000 mg dm -3 ) in pots, and two cultivars, one moderately resistant and the other highly susceptible, report an interaction between N and K.According to these authors, for both cultivars, the largest leaf area injured (LAI) was obtained in the treatment with the lowest doses of N and K (75 mg dm -3 de N and 63 mg dm -3 of K, respectively.The increasing of the doses of N-NH 4 + reduced the K content in the upper canopy, for both cultivars. In this paper, the amount of K that already present in both soils (Table 1), may have contributed to supply the plants without needing to provide the doses via fertilization.This can be the cause of the lack of interaction between the two nutrients on the severity of the disease.For future researches, it is necessary to find soils with low levels of K.

Grain Yield
For grain yield, the interaction between the doses of N and K was not significant (P > 0,05) for both experiments, but the doses of N (P < 0,01) influenced this variable (Table 2).
In experiment 1, the quadratic effect of the N dose on grain yield was observed.The increment of nitrogen fertilization to dose 170.5 Kg ha -1 (Figure 3A) increased yield.The response of the N doses compared to the lowest dose (20 Kg ha -1 ) was 18.8, 48.2, 73.7, 73.4% in the doses 40, 80, 150 and 190 Kg ha -1 , respectively.For each Kg of N applied to the soil, an increase of 47.0, 40.2, 28.4,21.6 Kg ha -1 of grain was obtained, in the doses 40, 80, 150 and 190 Kg ha -1 , respectively, compared to the lowest dose.In the treatment with the highest dose of N (190 Kg ha -1 ), yield was 3,677 Kg ha -1 greater than the lower dose (40 Kg ha -1 ).
In experiment 2, a rising linear effect to the N (Figure 3B) was observed.The increase in grain yield compared to the lowest dose (20 Kg ha -1 ) was 0.56% per Kg of N applied.That is, there was an increase of 11.3, 34.0, 73.7, 96.4% in the doses 40, 80, 150 and 190 Kg ha -1 , respectively.For each Kg of N applied, an increase of 24.6 Kg ha -1 of grain was obtained.In the treatment with the highest dose of N (190 Kg ha -1 ) the yield was 4,200 Kg ha -1 more grains than the lowest dose (20 Kg ha -1 ).
The use of high doses of N has contributed to the increment in the yield of grain in the majority of the researches done, being fundamental in the composition of the yield of the corn crop.An example of this was the increased yield of corn with 147 Kg ha -1 of N, in direct planting, without irrigation, in the Brazilian "cerrado", obtained by Fernandes et al. (1999).Also Silva et al. (2005), in the region of Ilha Solteira, São Paulo, obtained the maximum yield of rain-fed corn, achieved with the dose of 166 Kg ha -1 of N. In another experiment in the same region, the maximum efficiency of N was achieved with doses between 144 and 174 Kg ha -1 and with quadratic response to doses of N (Silva et al., 2006).
According to Sousa & Lobato (2004,) corn plants require around 20 Kg ha -1 of N per ton of grains produced.Thus, in the present experiments, income below the expected to the largest dose was obtained, that is, in experiment 1, 21.90 Kg per ton of corn was spent, which is equivalent to a loss of 9%, and in experiment 2, 22.20 Kg per ton of corn were spent, equivalent to a 10% of loss.Nevertheless, this loss can be related to the severity of the white spot, which contributed to the reduction of yield.
Once K did not contribute to an increase in yield, Pavinato et al. (2008) conducted an experiment with six doses of N (0, 80, 120, 160,200 and 240 Kg ha -1 ) in the form of urea and four doses of K 2 O (0, 40, 80 and 120 Kg ha -1 ), allocated in bi-factorial design, and also did not observe an increase in yield of corn when higher doses of K were applied.They relate that yield was not affected by the application of K and justified this by the high levels of K available in the soil.However, there was an increment with the doses of N and there was no significant interaction observed between N and K fertilizers.These results are similar to those found in experiments 1 and 2 described above, in which doses of K did not affect the yield of grain, which is only dependent of increased doses of N. The lack of interaction between the two nutrients (Table 2) can also be justified by "medium and good" rates of the levels of K in the soil (47 and 107 mg dm -3 ), in experiments 1 and 2 respectively (Table 1) according to the tables of interpretation of results of soil analysis presented by Ribeiro et al., (1999).
Positive and significant values were observed for correlations between the severity of the disease (AUCPS) and grain yield (t ha -1 ), that is, with both N or K increasing (Table 3).
In the biggest AUCPS, grain yield was higher, this could be explained by the fact that N is the main nutrient for increasing yield and it is required in greater amounts by the crop, but it also encourages the severity of the pathogen.In this pathossystem, other studies showed different correlations, perhaps due to the low doses of N used in these researches.For example, Souza & Duarte (2002) with doses of N (8 Kg ha -1 and 32 Kg ha -1 ) found no correlation between white spot and corn yield.On the other hand, Brito et al. (2012), in an experiment with chemical control of white spot, with the same fertilization (36 Kg ha -1 at sowing + 90 Kg ha -1 of N as side dressing), found a negative correlation between the severity of white spot and yield, -0.41, -0.37, -0.46 in the cities of Lavras, Passos and Patos de Minas, respectively.
In another pathossystem, however, Vaz-de-Melo et al. (2010) observed the reaction of corn hybrids to Curvularia ssp on two levels of fertilization with N (high -80 Kg ha -1 and low -40 Kg ha -1 ) and found a significant correlation between the severity of the Curvularia spot with yield.Nevertheless, the correlation between Curvalaria severity and yield was weak and negative (-0,178) under high N conditions but negative and stronger under low N supply.Such results points out the effects of the disease severity on yield under limiting N conditions.
Therefore, the use of balanced mineral nutrition can help establish integrated management program allowing greater efficiency to control the white spot on the corn, in addition to strategy based on evasion and protection, in order to reduce the damage of this disease in plantations.Although N has raised the severity of the disease, yield also increased in response to the doses applied in the soil, requiring recommendations of additional management practices for better efficiency.

CONCLUSIONS
Under the conditions that the experiments were conducted, the increase of dose of nitrogen favored the yield and the severity of the white spot on the corn.

Figure 2 :
Figure 2: Area under the curve of progress of disease severity (AUCPS) of the white spot in the corn crop depending on the N doses, (A) experiment 1 (Ijaci) and (B) experiment 2 (Sete Lagoas).

Table 1 :
Results of chemical analyses of the soils in Sete Lagoas -MG (Soil 1) and Ijaci-MG (Soil 2) prior to the establishment of the experiment

Table 2 :
Summary of the analysis of variance for severity of white spot and yield of corn, according to the doses of N and K in experiment 1 (Ijaci) and in experiment 2 (Sete Lagoas) ns non-significant ** and * significant, probability of 1 e 5%, by F test, respectively.