August Technical Notebook Part 2: More than essential

Soybean cultivation (Glycinemax) is currently the most important for Brazilian agriculture, with a cultivated area close to 35,8 million hectares in the 2018/19 harvest, representing 57% of the area destined for grain production (CONAB, 2019).

           

Among the various limitations to achieving the maximum productivity potential of soybean crops are diseases. In Brazil, more than 40 diseases caused by fungi, bacteria, nematodes and viruses have been identified (Embrapa Soja, 2013).

Prior to the 2001/02 harvest, soybean farmers generally carried out one to two applications of fungicides to control foliar diseases, which consisted mainly of powdery mildew (Erysiphe diffusa) and DFCs (end-of-cycle diseases - Septoria glycones e Cercospora kikuchii). In May 2001, the occurrence of Asian soybean rust was detected for the first time (Phakopsora pachyrhizi) in Brazil (Jaccoud Filho et al. 2001, and Yorinori, 2001), which drastically changed the cultivation and management of soybeans in Brazilian territory.

Due to the high potential for reducing productivity of this pathosystem and because it produces spores in high quantities, which are disseminated by the wind, it was and is still necessary to adopt different control methods jointly throughout the country and in neighboring countries. . Among these methods are the sanitary vacuum, to avoid the presence of soybeans during the winter, reducing inoculum for the following harvest, the sowing schedule, the use of early varieties sown at the beginning of the ideal period for soybean cultivation, the use of with resistance genes and chemical control.

Triazoles or demethylation inhibitors (IDM) were the first fungicides to be used to control this disease. These specifically bind to and inhibit the activity of a single enzyme (cytochrome P450 14a-demethylase - cyp51), which is necessary in the ergosterol biosynthesis pathway in fungi, which performs essential functions for the organization and structure of the fungal cell membrane. .

As they are fungicides with a specific site of action (only one enzyme in a single metabolic route), lower sensitivity of P.pachyrhizi to IDM products was officially reported in 2008 in Brazil, confirming that this reduction in sensitivity is due to the presence of six mutations in the cyp51 enzyme gene (F120L, Y131F/H, K142R, I145F and I475T), in addition to an overexpression of the enzyme -target (Schmitz et al. 2013).        

These mutations, when present in the fungal population, have an adaptive cost, that is, the population with the mutation tends to have disadvantages in relation to the fungal population without the mutations (Klosowski et al, 2016). Therefore, the longer the period of non-use of triazole-based fungicides during the year (sanitary void and sowing schedule), the lower the increase in this frequency of mutations.

The second group of fungicides to be used to control Asian rust was external Quinone Inhibitors (IQe), or strobilurins. These products, by binding to a single site (Qe cytochrome b site), inhibit the mitochondrial respiration of the fungus, which prevents the production of energy for the development of the pathogen.

Strobilurins also have a specific site of action and, after ten years of use of these active ingredients, alone or in mixtures with triazoles that already showed reduced efficacy, reduced sensitivity was reported. P.pachyrhizi to this group of fungicides (Frac, 2014). In a study published in 2015, the presence of the F129L mutation in the cytochrome b gene was confirmed (Klosowski et al, 2015). There is no adaptive cost for this mutation, according to Klosowski et al (2016), where the population with the presence of the F129L mutation in the cytochrome b gene developed equally to the population without the presence of the mutation.

More recently, starting in 2013, when Brazil already had more than 30 million hectares cultivated with soybeans, the first carboxamide-based fungicides, or Succinate Dehydrogenase Inhibitor (ISDH), were launched. These fungicides also act to inhibit respiration, but they act on complex II, binding to the enzyme succinate dehydrogenase, interrupting the process.

These active ingredients from the ISDH group were first released onto the market in mixture with strobilurins. As already discussed, since 2014, populations of P.pachyrhizi had less sensitivity to strobilurins, which led to very high selection pressure on carboxamides, resulting in, in just three years of use, the appearance of Asian rust populations with less sensitivity to these active ingredients, with a mutation in the C subunit of the enzyme (C-I86F) (Frac, 2017).

In addition to Asian rust, other diseases have received attention, especially those that survive in crop residues and seeds from one harvest to another. With the intensification of Brazilian agriculture, with at least two harvests per year in most of Brazil's agricultural areas, crop rotation has progressively decreased, benefiting necrotrophic fungi, such as Septoria glycones (brown spot), Corynespora cassiicola (target spot), Colletotrichum dematium there. truncata (anthracnose), among others.

According to data published by different research institutions, for areas with initial symptoms or a history of necrotrophic diseases, spraying fungicides on vegetation has shown an advantage. As the lower third is responsible for at least 30% of the production of each soybean plant in current varieties, the conservation and protection of these leaves before closing the rows is extremely important for maintaining high productivity.

Among foliar diseases caused by necrotrophic fungi, target spot has gained prominence. This disease is currently present in soybean crops in the Brazilian Center-West, mainly in the state of Mato Grosso. It is difficult to control chemically, as only some active ingredients within the group of triazoles and carboxamides present good control, whereas for strobilurins and MBCs or benzimidazoles (carbendazin) losses of sensitivity of this pathogen have already been reported (G143A mutation in the cytochrome gene b for strobilurins and E198A and F200Y mutations in the b-tubulin gene for benzimidazoles - Frac 2016).

One of the only carboxamides that promoted good control of target spot in soybeans, it is also widely used in cotton to control Ramularia areola. As the target spot affects soybeans and cotton, the same active ingredient is being used on soybeans and cotton in five or more applications throughout the year, promoting selection pressure for resistant populations of C. cassiicola. Due to this intense use, in 2018, Frac reported the detection of two mutations (B-H278Y and C-N75S) in the succinate dehydrogenase enzyme gene, leading to a reduction in the fungus' sensitivity to carboxamides.

Therefore, it is noted that for Asian rust, all groups of site-specific fungicides registered until July 2019 already have reduced efficiency in controlling the disease, with the exception of morpholines. For the target spot, the fungus already shows loss of sensitivity to strobilurins, benzimidazoles (carbendazin) and carboxamides, leaving only triazoles as effective site-specific fungicides registered until July 2019, according to Table 1.

Due to the fact that there was an extensive area under soybean cultivation in recent harvests and the use of at least two sprays of site-specific fungicides throughout the area, populations of fungi resistant to fungicides were selected. According to the diseases occurring in each region, different fungicides were used on a larger scale. This culminated in the selection of resistant populations, with different percentages of mutation frequency in each region of the country, mainly in the case of P.pachyrhizi. Therefore, the use of the same fungicide application program, even alternating different site-specific modes of action, showed different control results in different regions of the country, according to Figure 1A.

The use of multisite fungicides, one of the anti-resistance measures, is currently more than essential. The association of mancozeb (Unizeb Gold), a multisite fungicide, with site-specific fungicides brings greater security in the final result of controlling Asian rust (Figure 1B) and other diseases. With the current resistance scenario and the difficulty of predicting what the response of each site-specific fungicide will be in subsequent harvests, the use of multisite fungicides helps to guarantee higher levels of control, guaranteeing the maintenance of high productivity.

With projections published in 2019 by the Ministry of Agriculture, Livestock and Supply (Mapa), soybean cultivation will continue to grow in area and production over the next ten years, reaching 45 million hectares in the 2028/29 harvest. With such a large area, in a tropical environment and in a scenario of fungal resistance to fungicides, it is recommended that every soybean farmer strictly follows the recommendations of the Fungicide Resistance Action Committee (Frac), so that the risks of productivity losses due to diseases can be reduced as much as possible and so that the effectiveness of fungicides can also be maintained, an essential technology for soybean cultivation in Brazil.

With the predominance of short-cycle materials planted in 76% of the soybean area in Brazil (source: Bip Spark Soja Harvest 2018/19) and fungicide applications being anticipated, it is necessary to pay attention not only to rust, but above all to the complex of diseases that attack soybeans, which can reduce productivity in some situations by between six bags/hectare and ten bags/hectare, without the farmer noticing.

UPL innovates once again, through an intelligent management platform, composed of Tridium in the pre-closing of the rows, followed by two sequential applications of Unizeb Gold, together with the site-specific fungicide chosen by the farmer in R1 and R1 + 15 days. In this way, the producer begins his protection journey from the beginning, with adequate resistance management, ensuring high productivity at harvest time.

Frac recommendations for disease management in soybeans

• Pay attention to constant monitoring of soybean diseases, especially rust, and carry out applications at appropriate intervals following the manufacturer's recommendations;

• Apply fungicides preventively, always in association with multisite fungicides;

• Always use commercial mixtures made up of two or more fungicides with different mechanisms of action;

• Rotate fungicides with different mechanisms of action (triazoles, strobilurins, carboxamides, morpholines and multisites);

• Do not exceed the maximum number of two applications of fungicides with a specific mechanism of action in the same cultivation cycle;

• Use appropriate application technology;

• Do not plant “off season” soybeans;

• Respect the sanitary void and eliminate remaining voluntary plants in washes and roadsides (guaxas);

• Try to plant at the recommended time, using varieties with a shorter cycle and, if possible, with genetic tolerance to the disease;

• Carry out crop rotation.

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Ayrton Berger Neto

Coord. from the UPL Experimental Station