Monitoring and control

According to Embrapa Soja, Asian rust is currently one of the most important diseases affecting the crop. In Brazil, its first occurrence was detected in 2001 and, since the 2007/2008 harvest, damage to productivity has remained relatively lower as a result of the application of fungicides.

In order to preserve the efficiency of fungicides and monitor the sensitivity behavior of Asian rust to the control mechanisms existing on the market, great efforts are invested in studies monitoring the sensitivity of fungicides. Phakopsora pachyrhizi (fungus that causes soybean rust) to fungicides from the chemical groups of DMIs (fungicides that act on the demethylation of sterol synthesis in fungi, such as the active ingredient tebuconazole), QoIs (such as the active ingredients azoxystrobin, pyraclostrobin and trifloxystrobin) – initiated in 2005/2006 harvest – and SDHIs (carboxamides) – more recently.

In the case of Bayer CropScience, such research is being conducted through a monitoring methodology developed by researchers from the Bayer Research and Innovation Center in partnership with experts from the company's Fungicide Institute, in Monheim, Germany, recognized and adopted by Frac International (Fungicide Resistance Action Committee) and also validated by the national and international scientific community.

Since the beginning of this work, the tests have been conducted following the same methodology and procedures. The proposal is to establish a basis for comparison between the EC 50 values ​​(product concentration necessary to control 50% of individuals) of a fungus population that has not suffered selection pressure from fungicides with the values ​​found in monitoring studies for each ingredient. asset in question, relating them to results of the same products in test fields – conducted in parallel. The main objectives are: to identify in advance a possible situation of imminent resistance (when the resistance mechanism is quantitative), to verify whether resistance management strategies present positive results and, finally, to verify whether resistance is the real cause of the absence of resistance. disease control. In this case, it is important to check the variation in the sensitivity of the fungus P.pachyrhizi to fungicides.

The results obtained help in making decisions for the control of certain phytopathogenic fungi in cultivated plants, such as defining the application interval, doses, combination and rotation between different products with different mode and/or site of action; as well as defining the demand for new molecules that act in the control of this fungus.

Observations from the 2007/2008 harvest demonstrated that the samples collected in the main soybean producing regions of Brazil – in the month of March – predominated populations less sensitive to first generation DMIs (triazoles such as the active ingredient tebuconazole), mainly in some states of the Midwest.

In the 2008/2009 harvest, samples collected in the same month and the locations of the previous harvest showed that the predominance of populations less sensitive to first-generation DMIs extended to other locations in the country, in addition to states in the Central-West and Southwest regions (São Paulo and Minas Gerais) and South (Paraná and Rio Grande do Sul).

Between the 2009/2010 and 2013/2014 harvests, it was observed that populations less sensitive to first-generation DMIs were detected in practically all Brazilian soybean-producing states. The fluctuation in the sensitivity of the fungus to fungicides from this chemical group has clearly occurred since the 2007/2008 harvest - a period in which triazoles were officially no longer recommended for applications in isolation in the crop.

For fungicides from the strobilurin group (QoIs), no changes in performance were observed during this period, in any region, showing that the active ingredients belonging to this chemical group remain important and indispensable in the control of Asian soybean rust.

Innovation in defense against resistance

From the beginning of monitoring until its launch on the market, prothioconazole has presented the lowest values ​​when determining the EC50 in the rust monitoring program. This is the result of thousands of experiments in demonstration areas in different soybean producing regions in Brazil, which received the trifloxystrobin + prothioconazole treatment in the first applications. The comparison is made with fungicides recently launched on the market, such as combinations of strobilurins (QoI) and carboxamides (SDHI).

As it is a fungicide composed of an innovative active ingredient and differentiated binding at the fungus' site of action, prothioconazole is the new generation in the chemical group of DMIs, being chemically classified as triazolinthione (Frac classification on mode of action 2014 www.frac.info).

In this sense, it is important to emphasize that combinations of two or more modes of action for a fungicide must be truly complementary, that is, acting at completely different sites of action during the fungus' development phase, such as, for example, inhibiting the biosynthesis of ergosterol. – important substance for maintaining the integrity of the fungal cell membrane, in addition to the inhibition of mitochondrial respiration (complex III), which blocks the transfer of electrons between cytochrome b and cytochrome c1, at the QoI site, interfering with the production of ATP.

The combination trifloxystrobin + prothioconazole acts in two ways: to control Asian soybean rust and in the complex of diseases (such as target spot, powdery mildew, honeydew, anthracnose and end-of-cycle diseases). Therefore, it is recommended to use preventively in the first application or in the first two when the plan for using foliar fungicides is more than two applications. In this way, it is possible to fully explore the spectrum of action offered, robustly initiate the prevention and control of soybean rust and, consequently, further improve the performance of the subsequent fungicide.