INTRODUCTION
The growth in cultivated area and corn (Zea mays L.) productivity can favor the increase of the severity of foliar diseases and contribute to the fact that diseases considered of little importance become a cause of concern for the producers. Foliar diseases are among the main factors of losses in productivity and grain quality in all corn producing regions, being the Bipolaris spot (Bipolaris maydis (Nisik. & Miyake) Shoemaker) and polysorbite rust (Puccinia polysora Underw), those of greater occurrence in Tocantins, as observed by Santos et al. (2013) and Chagas et al. (2015).
These diseases are favored by high temperature and high humidity, planting in more than one season, adoption of no-tillage and susceptible cultivars (Reis et al., 2011). These practices contribute to the survival of pathogens in cultural remains, which corresponds to new sources of contamination in subsequent plantations (Parreira et al., 2014).
The control of foliar diseases is increasingly done through the use of pesticides (Costa et al., 2012) due to more effective responses in reducing the severity of the diseases and to provide greater increases in productivity. However, the use of foliar phosphites as an alternative or complementary control method should be tested in the corn crop, as they may improve the nutritional aspects of the crop, especially in the stages of increased metabolic activity, where the application of the product represents a supplementation of nutrients and quick absorption of phosphorus and potassium (Nojosa et al., 2009).
Phosphites are capable of directly or indirectly activating the natural defense mechanism of plants by the production of phytoalexins, which confer resistance against phytopathogens microorganisms. According to Daniel and Guest (2005), this activation can occur due to the high mobility of the phosphites, which are rapidly absorbed, have a systemic action, act to reduce mycelial growth, sporangia formation and release of zoospores of phytopathogens, such as rust. Countless studies report stimulating the formation of natural substances of self-defense of the plant, protecting it from the attack of fungi, as well as presenting fungicidal effect, acting directly on the fungus (Meneghetti et al., 2010; Silva et al., 2013; Neves and Blum, 2014; Ventura et al., 2018).
Studies related to the effect of phosphites on disease reduction and yield increase on corn crop under “cerrado” conditions are still scarce, so that products that activate the defense mechanisms of corn plants and favor increased efficiency of fungicides should be tested, since they may add to the more efficient and economical control of foliar fungal diseases. Thus, the objective of this work was to evaluate the control of foliar fungal diseases and corn productivity as a function of the application of phosphite alone or in mixture with fungicides.
MATERIAL AND METHODS
Plant material
Seeds of two hybrids (30F53YH, simple hybrid by Pionner and AG7088 PROX MAX, simple hybrid by Agroceres) from Zea mays L., with a cycle of 120 to 130 days, flowering at 65 days were used for field experiments.
Experimental conditions
The experiments were conducted in the “cerrado” field in the rainy season in 2014/2015 and in 2015/2016 in Gurupi (TO), in the experimental area of the Federal University of Tocantins (UFT), 11° 43’ S e 49° 04 N’ at 280 m altitude. The climate is kind of B1wA’ with moderate water deficiency according to the climatic classification of Koppen (1948).
Seeding was done manually in conventionally prepared soil in order to obtain a population of approximately 60 000 plants in both experiments. The basic fertilization consisted of the application of 450 kg ha-1 of the formulation NPK 05-25-15 in the planting groove and in the cover was applied the dose of 150 kg ha-1 of urea in the phase between V4 (four expanded leaves) and V6 (six expanded leaves) in an open groove next to the planting lines. For the control of invasive plants, a mixture of atrazine + nicosulfuron herbicides at a dose of 5 + 1.5 l ha-1 respectively and with manual weeding was applied in emergent powders.
Pest control was carried out through the application of the insecticide methomyl at a dose of 0.6 l ha-1. Harvesting was done manually, in the useful area of the plots. In order to obtain the data of productivity (kg ha-1) and mass of one thousand grains expressed (kg ha-1), the ears were threshed in a harvester.
The climatic variables of the period of conduction of the experiments were: average precipitation (mm dia-1), mean, maximum and minimum temperatures (°C) and average relative humidity (%), obtained at the meteorological station of the Federal University of Tocantins.
A randomized complete block design was used in a 2 x 4 factorial scheme, with three replications, two hybrids and four leaf treatments. Each experimental unit consisted of four lines, spacing 0.7 m between plants and a length of five meters, with the two central lines as the useful area for evaluations. The hybrids used were the 30F53YH (simple hybrid of the Pionner company) and the AG7088 PROX MAX (simple hybrid of the company Agroceres). The treatments used to control the diseases were: 1 (Phosphite), 2 (Fungicide), 3 (Fungicide + Phosphite) and 4 (Water - control), applied in eight - leaf stages and in pre - a constant pressure costal CO2 based spray with fan-like tips spaced 0.5 cm across a 3-meter wide bar.
The fungicide used was PRIORI XTRA (chemical group: Azoxystrobin-Strobilurin; Ciproconazole-Triazole) and phosphite was UBYPHÓS + K, whose composition is 30% Phosphorus (P2O5) plus 20% Potassium (K2O) at the recommended doses for corn crop.
The severity of the foliar diseases with incidence began at 60 days after planting in the first year and at 58 days in the second year. It were repeated every seven days until the beginning of the formation of pasty grains. For this, a scale of visual scores ranging from 1 to 9 grades proposed by Agroceres (1996) was used. The severity notes were used to calculate the Area under Disease Progression Curve (AUDPC), as indicating for Campbell and Madden (1990).
Stadistical analysis
The test of normality and homogeneity of variances was performed. When significant, the average data obtained from AUDPC, mass of thousand grains and productivity were submitted to analysis of variance and the means were compared by means of the Tukey test at 5% of probability, with the aid of the software ASSISTAT 7.7.
RESULTS AND DISCUSSION
A variation in the total precipitation during the months of December to April was observed in the 2014/2015 harvest, with a total precipitation of 595 mm, average temperatures of 26 °C, maximum of 32 °C and minimum of 22 °C; relative humidity: mean ± 83%, maximum 95% and minimum 61%. In places where this climatic condition prevails, the emergence of foliar fungal diseases is usually very favorable (Chagas et al., 2015)
In the 2015/2016 harvest, a total precipitation of 577 mm was observed, accumulated from the planting until the last day of April, and of this total, 302 mm occurred in the month of January, where there was intense precipitation. During the period of evaluation of the diseases a daily precipitation average of 4.5 mm was observed, maximum temperature of 34 °C, minimum of 23 °C and average of 28 °C and average relative humidity of 75%.
In the agricultural year 2014/2015, the AG7088 PROX MAX hybrid had lower AUDPC for the Bipolaris spot (B. maydis) disease where the mixture of phosphite and fungicide was used, differing from the other treatments (Table 1). There was no significant difference between the treatments isolated from fungicide and phosphite and both did not differ from the control. In the 2015/2016 crop, the statistical difference between treatments for this hybrid was also verified. The application of the fungicide or the phosphite provided better control on Bipolaris spot in comparison with the two product mix and the control, which presented the highest AUDPC.
In the hybrid 30F53YH the highest AUDPC of the Bipolaris spot occurred with the application of the fungicide mixture with the phosphite in the first crop (2014/15). In the second harvest (2015/16), there was an increase of AUDPC in the control and with application of phosphite alone. Better control occurred where the fungicide was used alone or in mixture with the phosphite. The use of fungicides in the aerial part of the plant provides better physiological conditions for the translocation of photoassimilates and greater productivity (Cunha et al., 2010). In the works by Oliveira et al. (2011) and Wise and Mueller (2011) observed satisfactory results for the control of leaf diseases in the corn crop associated with a higher yield of productivity. Positive results in the control of leaf diseases in corn through the application of fungicides have been found in Brazil (Donato and Bonaldo, 2013; Manfroi et al., 2016).
In the 2014/2015 crop, the AG7088 PROX hybrid had a low leaf area affected by polysorbite rust (P. polysora). According to Belusso et al. (2012), the AG7088 PROX hybrid has good resistance to foliar diseases. The severity of the Bipolaris spot in the 30F53YH hybrid was also low than in the control.
In the hybrid AG7088 PROX no difference was verified among the treatments for the AUDPC of polysorbite rust (P. polysora) in the harvests evaluated. It was observed low disease severity in this hybrid, a fact that may be related to hybrid resistance, in addition to the climatic conditions already mentioned above. Rust develops best at temperatures around 23 to 28 °C, high relative humidity and water available on the leaf surface (Casela and Ferreira, 2002). Conditions not verified in the 2015/2016 harvest, where, in general, high temperatures and low rainfall were recorded, mainly between the periods from flowering to grain maturation.
In the hybrid 30F53YH, in the 2014/15 crop, the phosphite + fungicide and fungicide mixture applied provided lower AUDPC (Table 1). Thus, the use of the fungicide in mixture with the phosphite was the most effective in the control of the polysorbite rust, since the application of the phosphite did not maintain the same average of AUDPC as that observed when only the fungicide. In the following harvest (2015/16) there was no difference among the treatments, due to the low incidence of rust in all disease evaluation periods.
Normally, higher levels of disease severity occur when corn reaches flowering, so, in order to have a lower incidence of diseases in plantations, management should preferably be done in a preventive way. The time of application of the treatments in the vegetative phase (eight leaves) and in the pre-plowing, occurred induced the resistance to pollinating rust disease, mainly in the susceptible hybrid 30F53YH. Neves and Blum (2014) studying the influence of fungicides and potassium phosphite on the control of Asian rust and soybean (Glycine max L.) yield verified that all the treatments reduced the severity of the disease, but the fungicides were more efficient.
Regarding the Curvularia spot (sp.), in the harvest (2014/15) there was no difference for the hybrid AG7088, probably due to the low severity. In the later harvest (2015/16), there was an increase in AUDPC values in all treatments. The highest AUDPC was observed with the application of the fungicide in this hybrid (Table 1).
The increase of Curvularia spot in plants where a control effect was expected is a notable fact (Table 1), since treatments with fungicide and phosphite alone or in mixture were not determinant to reduce the levels of disease severity when compared to the control in the AG7088 PROX hybrid. In this sense, Mourão (2015) evaluated the mycelial growth of Curvularia sp. under the effect of the systemic fungicide thiophanate-methyl in in vitro tests and observed that even using twice the recommended dosage there was no inhibition of colony growth when compared to the control. This species of Curvularia sp. was isolated from leaf lesions of corn plants in experimental field. It was thus shown that the fungus Curvularia sp. presented resistance to the fungicide molecule under these conditions. The same may have occurred in the field, where the fungicide used in these tests did not demonstrate efficient control when applied in the hybrid AG 7088 PROX considered more susceptible.
In the 30F53YH hybrid, the treatment Phosphite + Fungicide and Phosphite differed from the others, presenting higher AUDPC (Table 1). In the second crop, the fungicide presented greater efficiency in the control of the Curvularia spot. The greater control response with fungicide used may be due to the increased incidence of Curvularia spot in this crop, which allowed to verify with more clarity the effects of the treatments.
Vaz-De-Melo et al. (2010) evaluated the severity of the Curvularia spot in 23 corn hybrids and reported that at that time, although the damages were low when compared to those caused by other diseases, the increase of this disease in the southern region of Tocantins was already verified. It is worth mentioning that according to the scientific literature consulted it can be affirmed that there is a lack of further studies in relation to the Curvularia spot in the Brazilian conditions.
According to Sobrinho and Cardoso (1997), the highest temperatures and leaf wetness were the factors that had higher levels of Curvularia spot in corn in Parnaíba in the state of Piauí, a fact also observed in this last experiment, since high temperature occurred during almost throughout the long evaluation period.
The results found in this study corroborate with those found in a study by Chagas et al. (2015) in municipalities of the state of Tocantins. Although polysorbite rust has been verified with lower severity, these authors emphasize the importance of these diseases in more susceptible cultivars in climatic conditions favorable for their development.
Regarding productivity, there was a difference among the treatments tested in the hybrids AG7088 PRO MAX and 30F53YH in the year 2014/2015, which was not detected in the year 2015/2016. However, hybrids productivity in this crop (2015/2016) was lower than the previous test (Table 2). As mentioned before, the severity of foliar diseases in both crops was low, probably due to the genetic resistance of the hybrids and the irregularity of the rains, which also affected the productivity of the hybrids in the second harvest.
In general, applications in plants with phosphite in admixture with the fungicide or alone provided the highest yields (Table 2). It is known that plants respond differently to the presence of threats to their tissues and that the responses expressed by them are directly related to their nutritional status. According to Carmona and Sautua (2011), phosphite can contribute to the plant having a greater amount of important nutrients, such as P (phosphorus), K (potassium) and micronutrients and in a more readily absorbable way, acting beneficially in the plants. However, these authors reports that the effect of phosphite on the fertilization of plants is still quite contradictory, since several authors argue that phosphite is a poor source of P available, especially in plantations where phosphate fertilization is inadequate. Responses vary from culture to culture and should not be related to phosphorus alone. Considering that corn is a fast growing plant and requires nutrients in greater quantity and speed, the use of phosphite should be better studied to verify this interaction (Borin et al., 2017).
Generally, corn productivity is indirectly related to susceptibility levels of the cultivar to certain diseases, which in fact may be true when a plant is attacked early with high severity. In this case, the application of the fungicide has a greater effect on the control of diseases in these plants, contributing to increase productivity (Cunha et al., 2010). In resistant cultivars, the application of fungicides does not result in higher yields (Costa et al., 2012), as observed in the present work with hybrid AG 7088 PROX.
In relation to the mass of a thousand grains, for the AG7088 hybrid there was a difference between the fungicide treatment and the others, which presented a larger mass of one thousand grains. For the hybrid 30F53YH there was no difference among the treatments tested for this variable (Table 2). In the later harvest there was no difference treatments.
In general, the results obtained only with the application of phosphite suggest that there has been a benefit of the use of phosphite in the reduction of diseases in the corn crop when applied preferentially in a preventive way, being able to become an in the reduction of diseases and to improve the aspects nutritional characteristics of the crop. In this sense, Meneghetti (2009) reports many advantages of using phosphite in soybean (Glycine max L.), such as: high mobility and rapid absorption of nutrients, easy application through agricultural equipment, high systemicity, low inherent toxicity, improved nutritional status of plants and high efficiency against various diseases and all of this together with the low cost. Besides, according to Nojosa et al. (2009), these positive effects are greater especially in the stages of higher metabolic activity of the plants, where the application of the product represents a supplement of nutrients and a faster absorption of phosphorus and potassium, influencing increases in productivity in crops as well conducted.
According to Costa et al. (2012), in view of the enormous diversity of diseases in corn, fungicide use should be used in crops where there are favorable conditions for disease development (climate, management and production system), prevalence of disease history in the area, use of susceptible cultivars and greater cost benefit of the application.
In cases where planting occurs under these conditions, mixing the phosphite with the fungicide may be a viable alternative in preventing or reducing disease progression and increasing corn productivity, due to the factors previously observed in this study. According to Scott et al. (2015), phosphorus- and silicon-based fertilizers are cited in the literature as inducers of resistance, however few results are found on the use of phosphites in disease control (Borin et al., 2017).
CONCLUSIONS
The application of phosphite and fungicide (PRIORI XTRA) alone or in combination do not control the incidence of Bipolaris spot (Bipolaris maydis) and polysorbite rust (Puccinia polysora) in rice cultivation (hybrid AG7088 and hybrid 30F53YH). Nevertheless, is possible the control of Curvularia spot (Curvularia sp.), using fungicide in hybrid AG7088. In the hybrid 30F53YH both treatment, phosphite alone or combined with fungicide permit it. Besides, the phosphite alone and combined with fungicide produce an increase in productivity in the first harvest, with no effect in the second harvest.