How to deal with soybean diseases in the 2020/21 harvest

In unfavorable years, such as the 2019/20 summer harvest, the importance of the quality of the direct planting system becomes clearer, considering soil profile, crop rotation, quantity of straw available and its effects on soybean productivity . Areas with a good soil profile had lower losses due to drought compared to areas where there was limited root development in depth. In years of high rainfall associated with sowing, this is also evident. In the last two harvests, it is estimated that 20% to 30% of soybean areas in Rio Grande do Sul needed to be reseeded due to problems with crop establishment, especially caused by soil pathogens, such as Phytophthora infestans, more common in compacted soils.

However, in years with lower rainfall, in addition to direct damage to soybean crops, disease pressure is lower. In the 2019/20 harvest, in the CCGL experimental area, in Cruz Alta, Rio Grande do Sul, rainfall was 365mm between the months of November and March, 460mm less than that observed in the 2018/19 harvest (825mm in total ). Temperatures were also higher, on average, approximately 1,4°C higher than in the previous harvest, especially in the months of December, February and March, in which they frequently exceeded 30°C. As a result, the severity of rust, caused by the fungus Phakopsora pachyrhizi, was low, being the lowest pressure observed in recent years in the region.

Figure 1 shows the evolution of soybean rust in the last five harvests, evaluated on the BMX Lance IPRO cultivar, sown in the first half of November and without the application of fungicides. There is a variation in the disease progress curve between different harvests, directly related to the environmental conditions in each year. However, in the 2019/20 harvest the final rust severity was, on average, just 6%.

With the dry climate, the predominant disease in the 2019/20 harvest was powdery mildew. However, with the occurrence of very high temperatures, even powdery mildew, a disease favored by drier environments, had a pressure considered average (between 15% and 20% final severity), as temperatures above 29°C tend to limit its growth. The incidence of leaf spots was low, with emphasis on the occurrence of brown spot (Septoria glycones) and for cercospora blight (Cercospora kikuchii).

In relation to fungicide applications, in the 2019/20 harvest there was no significant response in productivity with the application of fungicides at stage V4, in the so-called weeding application. However, this application resulted in an average increase in leaf spot control of 33%. Some products resulted in increases of up to 45% in stain control, when compared to treatment without application in V4. For powdery mildew control, this application was also efficient, accounting, on average, for an 88% increase in control. Some treatments provided 100% powdery mildew control compared to the treatment without application in V4. The treatments that showed superior control of spots and powdery mildew were those carried out with carbendazim 500g/L + flutriafol 84g/L, tetraconazole 80g/L + azoxystrobin 100g/L, propiconazole 250g/L + difenoconazole 250g/L, difeconazole 250g/L + cyproconazole 150g/L and tebuconazole 250g/L + trifloxystrobin 100g/L. In the 2018/19 harvest, with greater pressure from rust and other diseases, the application in V4 resulted in up to six bags/ha of increased production. 

The research data generated in the last harvest (2019/20) highlighted and reinforced the importance of the first application of fungicides for maintaining the productive potential of soybeans. In a study to evaluate the first application, carried out 45 days after emergence, at the V8 stage, it was observed that it accounts for a variation in grain yield from four bags/ha to up to 12 bags/ha, depending on which fungicide was used. in management. To this end, applications were fixed from R1, only the fungicide used in the first application varied. The commercial fungicides evaluated are presented in Table 1. This variation was mainly due to the better control of powdery mildew, septoria and cercospora.

Fungicide variations in other applications resulted in differences of one to eight bags/ha (data not shown). In addition to the importance of taking into account the choice of active ingredients, always focusing on diseases that are most likely to occur depending on the characteristics of the cultivar used and the climate, the moment at which applications begin is extremely important. Delayed applications, which occur after the vegetative stage and the closure of the lines, usually result in average losses of four bags/ha to five bags/ha, when compared to applications started in the vegetative stage, before the closure of the lines. In years of intense rust pressure, this difference can be even greater. In the 2015/16 harvest, the one with the greatest pressure in the last five years (as shown in Figure 1), the average difference between starting at the Vn or R1 stage, with the same number of applications, was 15 bags/ha.

The first application of fungicides is essential for assertiveness in disease management. Many diseases are transmitted via seeds or are present in crop residues, such as leaf spots and anthracnose. Rust and powdery mildew can also occur already in the vegetative stage. Therefore, applications starting at around 35 DAE to 45 DAE favor preventive disease control and guarantee better foliage coverage. As current cultivars have a greater share of the middle and lower third in productivity and diseases generally begin in the lower reaches, efficient coverage from the beginning of the cycle results in a lower rate of disease progression. As a result, control is more effective.

Climate forecasts for the 2020/21 summer harvest are for the occurrence of La niña, with lower rainfall to be observed until the month of February in the South region. In drier years, rust tends to establish itself later. However, powdery mildew usually appears earlier. Therefore, in the first applications it is important to consider active ingredients that have control efficiency against powdery mildew, but also against stains (since the inoculum can be in the seed or in the straw), such as tebuconazole, tetraconazole, epoxiconazole, difenoconazole, chlorothalonil and copper oxychloride. In work varying the reinforcement used in the first two fungicide applications, it was observed that the use of chlorothalonil or copper oxychloride resulted in superior control of powdery mildew, when compared to mancozeb reinforcement in the first applications. 

Since the 2018/19 harvest, CCGL has monitored the availability of rust spores in the air through spore collectors installed in its experimental area. Although there is variation in the amount of spores available during the year, mainly due to climate conditions, it is possible to detect spores even in the off-season, probably due to the possibility of inoculum survival in guax soybean plants, as Rio Grande do Sul does not carry out the sanitary void. Last year the quantity of spores in the off-season was low due to the lower rainfall observed throughout the year.

Even with the lowest precipitation that occurred in 2020, collectors show that there is the presence of spores in the air. Data on the number of spores observed per cm2 of slide since September 2020 are presented in Figure 2. Generally, the greater number of spores is associated with periods of greater precipitation.

Even if spores are present, what will determine whether or not rust occurs in the field is the climate. For penetration into the leaves to occur, the fungus does not need rain. Six hours of wetness, guaranteed by dew, are enough. The latency period (time required for spore production) is most influenced by temperature. At an average temperature of 26°C the latency period is approximately six days. At a temperature of 15°C, the latency period ranges from 12 days to 16 days. After infection, for the disease to progress throughout the crop cycle, rainfall is important. Although rust spores can travel long distances through wind currents, rain prolongs leaf wetness, facilitates the release, survival and deposition of spores, aiding in the spread of inoculum and contributing to the increase in severity observed in the field.

Years of La niña are not synonymous with the absence of rust. The 2016/17 harvest was influenced by the phenomenon La niña. In this case, there was a delay in the start of the rust epidemic due to the lower amount of rain at the beginning of the cycle. However, with the return of the rains, the final severity of rust without the application of fungicides reached 80% (Figure 1).

It is also important to highlight that fungicide applications under water stress conditions require some special care, especially from the reproductive stage onwards, when the risks of phytotoxicity are greater. The ideal conditions for fungicide applications are humidity above 60%, temperature below 30°C and winds between 3km/h and 10km/h, since in the absence of wind the phenomenon of thermal inversion can occur, in which the fungicide It is not deposited on the foliage, due to drift. Therefore, in periods of water deficit, nighttime applications generally have an advantage. Ideally, when deciding when to apply (day or night) it is always to check the environmental conditions with devices that measure these variables. The most recommended are thermo-hygroanemometers.

Still, it is not always that the farmer will find ideal conditions for applications. In these cases, to continue respecting the application intervals, adjustments to the droplet size and spray volume will be necessary. It is important to avoid very fine or very thick drops, giving preference to a spectrum of fine to medium drops, to reduce the risks of drift and evaporation. Larger volumes of spray (close to 100L/ha) also reduce the risk of phytotoxicity under water stress conditions.

Another point that can favor the occurrence of phytotoxicity in conditions of water deficit is the use of fungicides considered hotter, such as triazoles and triazontilione. Therefore, in drier years, the association of fungicides with proven action on the plant's physiology can mitigate the possible effects caused by fungicide applications under water stress conditions.

Regarding application intervals, in drier years these can be a little more flexible. However, decisions must be made technically, based on crop monitoring, plant health and, of course, the physiological condition of the plants. In work carried out at CCGL in the 2016/17 harvest, also La niña, a reduction in productivity of 48kg/ha was observed for each day of delay in fungicide applications, when comparing intervals of 15 days or 20 days, with the same number of applications. This was due to better rust control when shorter intervals were used.

As for the weather, we don't know exactly what to expect from the upcoming harvest. However, starting with good choices, decisions regarding the number and intervals of applications will certainly be easier and safer. Assertiveness in disease management starts early. Nothing that isn't already public knowledge, but it's always good to remember!

Caroline Wesp Guterres, Gilmar Seidel, Elaine Deuner, Andiara Marchezan and Leonardo Graminho Tassi, Cooperativa Central Gaúcha, CCGL 

Cultivating Great Crops December/January 2021

With each new edition, Cultivar Grandes Culturas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in the cultivation of large crops, the Magazine helps farmers in the search for management solutions that increase their profitability. 

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