Seed treatment with fungicides in soybeans

An important tool to protect the germination and emergence of soybean seedlings, seed treatment with fungicides must be adopted carefully and appropriately, with special attention to aspects such as need, spray volume, efficiency and product compatibility.

Seed treatment with fungicides offers a guarantee of better establishment of the plant population by controlling important pathogens transmitted by seeds and reducing the chance of their introduction into unaffected areas.

Unfavorable conditions for seed germination and soybean seedling emergence, especially water deficiency, slow down this process, exposing the seeds for longer to soil fungi, such as Rhizoctonia solani, Pythium spp., Fusarium spp. And Aspergillus spp. (A. flavus), among others, which can cause seedling deterioration or death.

The main pathogens transmitted by soybean seeds are: Cercospora kikuchii, Fusarium pallidoroseum (syn. F. semitectum), Phomopsis spp. anamorph of Diaporthe spp. And Colletotrichum truncatum. The best control of the first four pathogens mentioned is provided by fungicides from the benzimidazole group. Among the products evaluated and indicated for the treatment of soybean seeds, carbendazin, methyl thiophanate and thiabendazole are the most efficient in controlling Phomopsis spp., Fusarium pallidoroseum (Syn. F. semitectum) Cercospora kikuchii and mainly Sclerotinia sclerotiorum, when present in the seed, in the form of internal, dormant mycelium. Traditionally known contact fungicides (Captan, Thiram and Tolylfluanid), which have good emergence performance in the field, do not completely control these fungi, especially Phomopsis spp. And Fusarium pallidoroseum (Syn. F. semitectum) in seeds that present high levels of these pathogens (>40%).

Contact and systemic fungicides, already formulated and most suitable for treating soybean seeds, are presented in Table 1.

How to carry out the treatment

The function of contact fungicides is to protect the seed against soil fungi and that of systemic fungicides is to control phytopathogens present in the seeds. Therefore, it is important that fungicides are in direct contact with the seed.

Seed treatment with recommended products such as fungicides, insecticides, micronutrients and inoculants can be done sequentially, with specific seed treatment machines as long as they have separate tanks for the products, since the mixing of agrochemicals in tanks has not been regulated. (Normative Instruction 46/2002, of July 24, 2002, which revokes SDA Ordinance No. 67 of May 30, 1995). On small properties, seed treatment can also be carried out using a rotating drum or concrete mixer.

Treatment using machines 

Advantages over conventional treatment (drum)

a) lower risk of operator poisoning, since fungicides are used in liquid form;

b) better coverage and adherence of fungicides, micronutrients and inoculant to the seeds;

c) yield of around 60 to 70 bags per hour;

d) greater operational ease, as the equipment can be taken to the field, as it has a hitch for the tractor's power take-off.

Industrial Seed Treatment (TIS)

In many companies, industrial seed treatment (ITS) is already part of the seed processing stages, being carried out using special and highly sophisticated equipment, which combines the application of fungicides, insecticides, micronutrients, nematicides, among other products. . This type of treatment is gaining ground in the soybean seed market (in Brazil, in the 2015/16 harvest, around 30% of seeds were treated in this system), where a large part of the companies that sell seeds already carry out pre-treatment. -bagging, before storage or upon delivery of the seeds to the producer.

This treatment carried out at the Seed Processing Unit (UBS) presents a series of advantages compared to conventional treatment (drum or concrete mixer). Highlights include precision in the volume of syrup and quantity of seeds to be used; better coverage of the seed with the chemical product; lower risk of operator poisoning and higher yield per hour (there are machines for industrial treatment on the market, with the capacity to treat up to 30 tons of seeds per hour).

However, care must be taken with seed treatment packages, as a wide range of products are often used on the same seed, such as a combination of fungicides, insecticides, inoculants, micronutrients, nematicides, growth regulators and polymers, which can cause phytotoxicity to seeds, in addition to the environmental impact, due to the excess of products used, which are often not necessary in certain agricultural realities or situations.

The phytotoxic effect can affect the physiological quality of seeds, reducing germination and seedling emergence. This effect causes thickening, shortening, rigidity and longitudinal fissures in hypocotyls, especially in deep sowings; atrophy of the root system; delay in the vegetative development of the aerial part of the plants, associated with the shortening of the inter-node distance and in some situations the presence of multi-budding in the cotyledonary node, thus reducing the establishment and productivity of the crop.

Given this, it is essential that farmers pay attention to the way in which the market imposes these seed treatment packages, and some aspects must be taken into consideration before carrying them out.

Need for treatment

Before carrying out the treatment, the farmer must know the needs of his crop, because what is the point of treating soybean seeds with certain insecticides, nematicides, among other products with specific action if there is no presence of these insect pests or nematodes in his area? ?

Product efficiency

A very important aspect lies in knowing the efficiency of the fungicides (Table 2) that are being applied to the seeds. To this end, the technician who will recommend the treatment must be constantly informed, through research data or technical information, in order to avoid a less efficient application that will only increase the cost of the crop's final production.

Product compatibility

It is necessary to always use the products that are recommended (and registered with the Ministry of Agriculture, Livestock and Supply) for the crop, and know the compatibility between the formulations applied, such as when inoculants are applied, as in some works it is possible to verify the reduction of efficiency of some inoculants through the influence of products (fungicides).

Volume of syrup

This is a very important aspect, as with the wide variety of products and packages for treating soybean seeds on the market, several formulations are often applied, which may exceed the recommended volume of syrup.

In the past, 600 mL/100 kg of seeds were used, when the products were dry powders (mostly) and water was used as the vehicle for applying fungicides. Currently, most products (mixtures of contact + systemic fungicides) are already formulated with other vehicles, including dyes, polymers, etc. For this reason, depending on the products (formulations), volumes of up to 1.100 mL/ 100 kg of seeds have already been used without compromising the quality of the seeds. However, it is worth highlighting that the seeds must contain high physiological quality (germination and especially vigor) and sowing must be carried out immediately after treatment. Seeds with mechanical damage and low vigor tend to loosen the seed coat when high volumes of syrup are used, damaging seed quality,

Storage after treatment 

Ideally, sowing should be carried out immediately after seed treatment and inoculation. However, studies carried out with braided, laminated polypropylene packaging for storing industrially treated seeds have not revealed problems as long as the seeds have high physiological quality (vigor and germination) and the final volume of syrup does not exceed 0,6 L/100 kg of seeds. On the other hand, if the seeds are treated and inoculated industrially, follow the manufacturers' recommendations regarding transport, storage and shelf life of the inoculant.

Ademir Assis Henning, Embrapa Soja 

Article published in issue 210 of Cultivar Grandes Culturas.