Effectiveness of different fungicide formulations and number of applications in controlling wheat leaf rust

Mestrando no programa de Pós Graduação em Fitopatologia da Universidade Federal de Viçosa. Viçosa, MG, Brasil; Doutorando no programa de Pós Graduação em Agronomia da Universidade Federal de Santa Maria. Santa Maria, RS, Brasil; Bacharel em Agronomia, Erechim, RS, Brasil; Doutorando no programa de Pós Graduação em Fitotecnia da Universidade Federal do Rio Grande do Sul. Porto Alegre, RS, Brasil; Professora Adjunta de Fitopatologia da Universidade Federal da Fronteira Sul, Erechim, RS, Brasil. Autor para correspondência: Jhonatan Paulo Barro ( jhonatanbarro@gmail.com) Data de chegada: 14/01/2017. Aceito para publicação em: 18/04/2017. 10.1590/0100-5405/174240

with four applications, fungicides were applied at the phenological stages of: tillering, elongation, booting and flowering.The efficiency of fungicides on leaf rust control was analyzed based on the disease severity assessment by calculating the Area Under Disease Progress Curve (AUDPC).Severity data were obtained based on percentage scores of leaf area with visible disease symptoms/signs according to Cobb's diagrammatic scale.Fungicides with azoxystrobin + tebuconazole and trifloxystrobin + prothioconazole showed efficient control of wheat leaf rust (higher than 85%).For yield components, there was no statistical difference between numbers of applications, but there was a statistical difference between fungicide formulations, except for grain yield.Furthermore, for yield components, all fungicide treatments significantly differed from control.
Palavras-chave:Puccinia triticina, AACPD, controle químico, estrobilurinas, triazóis Wheat (Triticum aestivum L.) is an annual grass crop grown during the winter.This crop is important in the composition of sustainable agricultural production systems as an alternative to winter rotation in production systems, contributing to integrated management of pests, diseases and weeds.In addition, this crop has great importance in the diet because of its quality and quantity of protein (De Mori (11)).Flour is the major form of wheat consumption and is widely used in the human diet as bread, pasta, biscuits and other food.It is also used in animal diet as hay, grain or bran in the feed composition.
Several factors can reduce wheat yield, including losses due to diseases.For susceptible cultivars, chemical control through fungicide application has been the strategy most widely used by producers since it has offered security to farmers when applied early in the appearance of the first symptoms.
Wheat leaf rust (Puccinia triticina Ericks.) is one of the most common diseases found in wheat crops.This disease is characterized by the appearance of pustules with yellow-dark to brown spores in the uredia, from seedling emergence to the maturity stage.Temperature range for the disease development is between 15 and 20°C under high humidity, and the fungus requires at least three hours of continuous wetness to infect the host plant (Embrapa (13)).It should be emphasized that the causal agent of wheat leaf rust is a biotrophic parasite, i.e. in the absence of wheat, the pathogen survives only in volunteer plants present in the fields, roadsides and highways (Reis & Casa ( 16)).
For effective disease control, resistant cultivars are recommended.Another form of control is through the reduction of primary inoculum, eliminating volunteer plants at the beginning of their development (Embrapa (13)).As to chemical control, leaf treatment is preferred by means of application of mixtures of triazole and strobilurin fungicides at recommended doses (Reis et al. ( 17)).Combination of these strategies or integration of cultural and genetic control, as well as the use of fungicides, would be the most effective ways to control this disease (Xue et al. ( 18)).
The aim of this study was to evaluate the effectiveness of two fungicide formulations and two numbers of applications in controlling wheat leaf rust.

MATERIAL AND METHODS
This study was conducted in the experimental area of Federal University of Fronteira Sul, Erechim Campus, in the 2015 growing season.Local climate is Cfa according to Koppen's classification, and rainfall is well distributed throughout the year.The soil is classified as Oxisol Alumino Humic (Embrapa ( 14)) and has medium organic matter content (3.7%).The area where the experiment was installed was in winter fallow for five seasons.
Experimental design was completely randomized, 2x2 factorial arrangement, consisting of two fungicide formulations and two numbers of application, with four replicates.Each experimental unit consisted of a plot, 3m wide and 4m long, totaling 12m².Treatments are described in Table 1.
Before sowing, an application of glyphosate herbicide was performed to control weeds existing in the area.TBIO Pioneiro 2010 wheat cultivar was used in this study; it is considered moderately resistant to wheat leaf rust (Biotrigo (4)).Seeds were treated with imidacloprid (penetrant mobile insecticide) + thiodicarb (contact and ingestion insecticide) and carboxin + thiram (penetrant mobile and non-penetrant fungicide).Sowing was performed on 06/25/2015 at 17cm-spaced rows and plant density of 250/m2 (Biotrigo (4)).
At sowing, the crop was fertilized with formula 05-30-15 (NPK) at the rate of 160 kg/ha.In addition, cover fertilization was made with 150 kg/ha nitrogen as urea (45% N), divided into two parts: 40% at early tillering and 60% at flowering beginning.
Regarding the number of fungicide applications, there were treatments with four applications, at tillering (preventive), elongation, booting and flowering, and there were treatments with three applications, at elongation, booting and flowering stages.The fungicides azoxystrobin + tebuconazole and trifloxystrobin + prothioconazole were applied at 0.5 L/ha, considering 150 L/ha spray volume.Furthermore, for better fungicide effectiveness, the adjuvant Aureo ® was added at the recommended dose of 0.5 L/ha.Fungicides were applied with a manual precision sprayer under constant CO 2 pressure.Leaf rust severity data were obtained based on percentage scores of leaf area with symptoms/signs.Fully formed pustules were considered those with abundant sporulation.Following the appearance of the first symptoms, five infected wheat leaves were sampled from each plot, at weekly interval, up to kernel hard dough stage.Then, these leaves were scanned and compared according to diagrammatic Cobb's scale (Peterson et al. ( 15)).
To determine the Area Under Disease Progress Curve (AUDPC), severity percentages obtained in each evaluation were used.The AUDPC values were calculated according to the equation proposed by Campbell & Madden (6) (Equation 1): (1) where: Y i and Y i+1 are the values of two consecutive severity assessments, and t i and t i+1 are the dates of the two assessments .
Harvest was done by hand cutting the plants in a 3m 2 area.After harvest, grain moisture, hectoliter weight (HW), total yield (kg/ha1 ) and thousand grain weight (g) were determined.Moisture was determined by adopting the oven method at 105°C; HW was determined by using a hectoliter scale of 250mL capacity; the weight values obtained for the plots were extrapolated to yield per hectare, and thousand grain weight was also determined according to the Rules for Seed Analysis (Brasil (5)).
Data were subjected to analysis of variance according to F test at 5% probability, and means were compared according to Tukey's test at 5% probability.Similarly, treatments were compared with control according to Dunnett's test at 5% probability.Statistical analysis was performed by using ASSISTAT 7.6 beta software.

RESULTS AND DISCUSSION
Wheat leaf rust severity (%), according to the evaluation time (days), is found in Figure 1.During the 21 days of evaluation, the disease progression in the control treatment (T5) reached 100% severity.For T3 and T4, the severity value was the same (5.6%);thus, their lines were superposed, showing no difference between the numbers of applications.T1 and T2 showed similar performance, but the severity value in T1 was relatively higher (7.8%), compared to T2 (5.6%), which could be justified since the preventive application in T2 (four applications) helped control the disease.
The results obtained in the severity analysis were used to calculate the daily increase (Table 2).
As shown in Table 2, for the sampling interval between 7 and 14 days, there was no daily increase in the disease severity for T1, T3, T4 and T2.This may be explained due to a fungicide application exactly in this interval, reducing the disease severity.Control treatment had a progressive increase in disease severity due to the absence of fungicide treatment.
Results of AUDPC and percentage of control are shown in Table 3.The tested fungicide formulations showed a good control of wheat leaf rust, all above 85%.The use of mixtures formulated with strobilurins and triazoles proved to be more efficient than the same compounds applied alone to control wheat leaf rust.In a study by Assunção & Torres (2), a mixture with strobilurin (trifloxystrobin) and triazole (tebuconazole) was more efficient than the same compounds applied alone.Rust control was 96.4% for the formulated mixture (trifloxystrobin + tebuconazole), 90.4% for treatment with tebuconazole and 86.4% for treatment with trifloxystrobin.
It is important to emphasize the low control effectiveness, especially  (3) noted that application of strobilurin fungicide group can prolong the period of healthy green leaf area maintenance in the crop and, consequently, may have a positive impact on the yield.To determine yield components, it was evaluated HW (kg hL - 1 ), grain yield (kg/ha) and thousand grain weight (g).There was no statistical difference between the numbers of applications for any of the analyzed components.However, there was statistical difference between the tested fungicide formulations for HW and thousand grain weight, but not for grain yield (Table 4).
For HW and thousand grain weight, the fungicide formulation trifloxystrobin + prothioconazole differed significantly from azoxystrobin + tebuconazole.This fungicide formulation presents preventive and curative action, with unique combination of active ingredients, which is ideal for modes of action rotation.
Weather conditions once again were not good for the wheat crop in 2015 season.Heavy rainfall at harvest time and occurrence of frost out of season affected productivity in Rio Grande do Sul State.As shown in Table 4, HW was below 78, a parameter which is related to wheat quality Type 1 (Embrapa Trigo (12)).
Thousand grain weight was also below average for this cultivar, which is 35 g (Biotrigo (4)).In addition, all fungicide formulations and numbers of applications were superior to control (Table 5).Barros et al. (3) obtained the same results in a chemical control test, noting that the thousand grain weight was higher in plots treated with fungicides, thus showing the effect of chemical treatment in the thousand grain weight.According to Cunha & Bonaldo (9), the increase in seed weight is not only a result of disease control, but also of greater transport of nutrients and increased maturation period provided by fungicides.
The yield obtained in this experiment was below yield average reported for the producing regions in the state of Rio Grande do Sul for the 2015 agricultural season, which was 1700 kg/ha (Conab (7)).
Dallagnol et al. (10) reported that the influence of leaf diseases on wheat can reduce grain yield by up to 63.52% when the control of wheat leaf rust is not performed.It is important to emphasize that leaf diseases must be controlled to ensure high yields in wheat crop.
As to the number of applications for leaf rust control, the fungicide formulation azoxystrobin + tebuconazole shows higher severity with three applications, compared to the treatment with four applications.
For yield components, there was no statistical difference between the numbers of applications, but there was statistical difference between fungicide formulations, except for yield.Also considering yield components, all fungicide treatments differed significantly from the control.

Figure 1 .
Figure 1.Severity (%) of wheat leaf rust (Puccinia triticina) observed in leaf samples collected after treatments with different fungicide formulations and numbers of applications at seven-day intervals.

Table 1 .
Treatments according to fungicide formulations (active ingredients) and number of applications for wheat leaf rust control.

Table 2 .
Daily increase (%) in wheat leaf rust severity (Puccinia triticina) after treatments with different fungicides and numbers of applications, at seven-day intervals.Erechim, RS. 2015 by some triazoles like tebuconazole, which can be explained by the reduction in the sensitivity of the causal agent of leaf rust to the fungicide, as reported in some studies in the 2005 growing season (Arduim et al. (1)), recommending the mixture of triazoles with strobilurins for more effective leaf rust control.Furthermore,Barros et  al.

Table 3 .
Area Under Disease Progress Curve (AUDPC) and control percentage for the tested treatments.Erechim, RS, 2015

Table 4 .
Yield Components (HW, Yield and thousand grain weight) after treatment with different fungicide formulations for wheat leaf rust control.Erechim, RS. 2015.Means followed by the same letter in the columns do not differ statistically according to Tukey's test (p ≤ 0.05). *

Table 5 .
Yield Components (HW, Yield and thousand grain weight) after treatment with different fungicide formulations and numbers of applications for wheat leaf rust control.Erechim, RS. 2015.
*Means are statistically different from control according to Dunnett's test (p ≤ 0.05).