Biological seed treatment

Learn the importance of knowing the role of biological tools for seed treatment against initial diseases in soybean and corn

17.06.2022 | 14:18 (UTC -3)

Ally more and more different methods of combating diseases become essential in the Brazilian agricultural scenario. If the aspects were not enough environmental factors, the loss of sensitivity of pathogens to active ingredients and the reduced number of new fungicides require the association of biological and chemical control. Per This is why it is important to know the role of biological tools for seed treatment against initial diseases in soybean and corn.

Soybeans, commodity of greatest importance worldwide, has broken records of production each year. According to CONAB (2016), the production of this legume, in 2015/2016 harvest in Brazil, it was around 95 million tons. Already in 2016/2017 harvest, the IBGE (2017) estimate is that they will be harvested in the country at around 110 million tons, a number that would represent an increase of approximately 15% in production, with a small increase in the cultivated area, in relation to the harvest last. Your production was 16.669 million tons in the current harvest in Rio Grande do South, totaling 5.499.742 hectares of planted area. (IBGE, 2017). What has been observed in recent crops, especially soybeans, is the fact that most diseases come together with the seed, making them the source of initial inoculum for main diseases that cause damage to this crop, as well as to corn. In these cases, the damage is not only caused by the inoculum, but also by those caused in the crop stand, which depending on the magnitude, result in in many cases, the replanting of large areas.

Corn is the cereal largest production volume in the world, with Brazil being the third largest producer worldwide, the area occupied by culture already exceeds 15 million hectares with production around 90 million tons in the 2016 harvest (CONAB, 2016).

Purple spot or leaf blight of cercospora, easily identified by the purple color of the seeds
Purple spot or leaf blight of cercospora, easily identified by the purple color of the seeds

Despite the development of production techniques for these crops has been carried out for decades, still presents several problems that prevent the obtaining of productivity close to their productive potential. Despite this, in the last harvest, the Brazilian average productivity was greater than 5,4 thousand kilos per hectare. Even though it is relatively low compared to other countries, the Crop productivity has been growing at a rate of 1,6% per year.

Soybean anthracnose is a disease that, once in the seeds, causes death before or shortly after emergence.
Soybean anthracnose is a disease that, once in the seeds, causes death before or shortly after emergence.

The longer the time between physiological maturity and harvest, the greater the chances of seed infestation. The main ones incident pathogens are Fusarium sp., which causes damage to roots, stems and grains, Colletotrichum graminicola, responsible for leaf lesions and stem rot, Stenocarpella maydis, most frequently rot found in corn, Aspergillus flavus, which causes the popular burning grain with its mycotoxins, Penicillium sp., causing a reduction in germination, among other fungi that cause losses in seed quality.

White mold is another disease that deserves attention from producers as it is highly destructive.
White mold is another disease that deserves attention from producers as it is highly destructive.

Seed treatment becomes an effective measure to increase the quality of genetic material and germination power, reduce inoculum potential in seeds, provide greater protection of seedlings to fungi that inhabit the soil and standardize the initial establishment of the crop. In this context, it is highlighted that the seed treatment fungicide market has more than doubled in recent years 10 years, with soybean cultivation being responsible for this increase expressive. It is estimated that the cost of seed treatment in soybean is around 0,8%-2,0% of the total production cost of the crop, being, therefore, a low-cost investment that, in addition to avoiding the introduction of pathogens in new areas will reduce sources of primary inoculum in the crop, as well as the incidence and severity of some diseases.    

Sanity testing as a tool in choice of TS

The seed in general constitutes the most efficient way of disseminating necrotrophic pathogens over long distances and, therefore, it is of great importance to know which species are present in the lot. Just like human beings, present symptoms of an illness, undergo a series of tests to direct the treatment and health analysis of the seeds that will go to the field will determine which pathogens and in what quantities are present, so that possible to conclude which fungicide in seed treatment should be used.

In soybean cultivation, several diseases can be transmitted by seeds, being the most relevant for discussion, between soil pathogens Fusarium spp., Cercospora kikuchii, Colletotrichum truncatum, Sclerotinia sclerotiorum, Diaporthe/Phomopsis, Phytophthora soyae, Rhizoctonia solani, Pythium spp. e Stenocarpella maydis.

Fusarium spp.

Causes sudden death syndrome in soybeans responsible for seed rot, seedling tipping and plant death from the middle to the end of the crop cycle, causing greater losses when manifests between V5 and R1, due to the abortion of flowers and pods. That genus also causes damage to corn crops, where it causes stalk diseases (Fusarium graminearum) and rot of the cob (F. moniliforme).

Cercospora kikuchii

Disease known as purple spot or Cercospora leaf blight, easily identified by its purple color of the seeds. However, it should be noted that not every purple seed will manifest symptoms of the disease. The seeds infected cells cause primary lesions in the cotyledons and hypocotyls, initiating thus the primary cycle of the disease. Some data demonstrate that the Transmissibility of the pathogen via seed is around 30%.

Colletotrichum truncatum

Soybean anthracnose is a disease that, Once in the seeds, it causes death before or shortly after emergence. In addition Furthermore, when transmitted to the cotyledon, the primary cycle of disease, where the plants that emerge present cankers with fruiting of the pathogen (arrows) clearly visible, from which the infection of stems and leaves, with the transmissibility rate of this disease being approximately 10%.

Sclerotinia sclerotiorum (mold white)

It is another disease that deserves attention because part of the producer because it is highly destructive and transmissible via 40%-70% seeds. It is one of the diseases capable of interfering from germination until and, mainly, the flowering of soybeans. The treatment of seeds in the case of this disease is of fundamental importance, as the pathogen is found in the integument, where the infectious process begins. Once introduced into the crop and having favorable environmental conditions, the fungus increases the quantity of sclerotia for the following harvests.

Diaporthe/Phomopsis

It is a pathogenic complex that has presented increasing importance in last harvests, causing productivity losses, associated with several diseases: stem and pod dryness, stem canker and seed rot. Despite the The main form of control is the use of resistant cultivars, which seed treatment with fungicides from the Benzimidazole group (thiophanate methyl, carbendazim and thiabendazole) reduces the transmission of the disease, being recommended for control.

Diaporthe/Phomopsis is a complete pathogen that has shown increasing importance in recent harvests
Diaporthe/Phomopsis is a complete pathogen that has shown increasing importance in recent harvests

Phytophthora soyae, Rhizoctonia solani and Pythium spp

These are pathogens that also have the seed as a vehicle for dissemination. They are known to cause root rot and occur more often in moist soils. Pythium sp. tends to have a high incidence in conditions of lower temperatures than ground. Seed treatment is essential, especially in the case of Phytophthora soyae that has been causing considerable losses. Damages due to P. sojae are rotting of seeds, death of seedlings and adult plants in any development phase. Fungicides containing metalaxyl and mefenoxam present control efficacy in seed treatments on this pathogen.

Rhizoctonia solani é a polyphagous fungus that can cause seedling damping, stem and root rot in both soybeans and corn. However, its damage tends to be more severe in soybeans. Normally the pathogen is identified by reddish-brown lesions on the roots just below from ground level, causing strangulation of the affected tissue and death of the plants.

Damage caused by P.sojae is seed rot, death of seedlings and adult plants at any stage of development.
Damage caused by P.sojae is seed rot, death of seedlings and adult plants at any stage of development.

Stenocarpella maydis

 The main inoculum sources of Stenocarpella macrospora e Stenocarpella maydis are infected seeds (MCGEE, 1988; CASA, 1997) and crop residues. You classic symptoms are panicle discoloration, leaf wilting, reduced root system, stem and grain rot.

Biological control as a tool in TS

Seed treatment is a growing market continuous growth, given the large number of necrotrophic pathogens that affect major cultures. At the current panorama, where there is an intensive system of   monoculture, there is no break in the vicious cycle of survival of pathogens, and, as there is no satisfactory control of these fungi in the seeds, the diseases will be transmitted to the aerial part, consequently increasing its severity. If there is no program correct control, infection will probably occur during flowering, resulting in new seeds infected by these diseases. In addition to the problem health, it is possible to highlight the importance of TS for adverse conditions of sowing where water deficit occurs, soils with low temperature and high content humidity, compacted soil, deep seeding, history of tipping over and crops in new areas. In addition to chemical tools, it gains more and more importance of using biological products for seed treatment, For this purpose, a control agent containing both fungi and bacteria.

R. solani is a polyphagous fungus that can cause seedling damping, stem and root rot
R. solani is a polyphagous fungus that can cause seedling damping, stem and root rot

Biological control of diseases is a practice that has been developed for over 90 years and is gaining more and more highlighted in the current market, in the increasing search for agricultural sustainable. In the Brazilian scenario, biological control was inserted in the second half of the 90s and since then it has gained more space. According to ABCBio (2015) currently 107 commercial products are available registered, 78 of which are microbiological (fungi, bacteria and viruses), 7 pheromones and 22 macrobiologicals. In the exclusive case of seed treatment for disease control, there are currently a much smaller number of products registered compared to other application modalities. However, much if has advanced research in this direction, seeking new alternatives and formulations for these products. Strictly speaking, seed treatment It exercises two principles of disease control: eradication and protection. O first focuses on the already established presence of the pathogen(s) on the seed carrying out its effective control; the second establishes the protection of healthy seed against pathogens that are present in the soil and can act in a deleterious to its viability.

When talking about adoption of biological control, it is not just about replacing products chemicals, but to move towards the development of more sustainable, with the integrated use of control measures (chemical, biological, cultural, etc.).

When it's about biological products there are several mechanisms used by microorganisms to provide control of pathogenic organisms. Per definition biological control is the control of one organism by another microorganism. To promote this control, these microorganisms have various mechanisms of action, such as: antibiosis, competition, parasitism, predation, hypovirulence, and induction of host defense (Bettiol, 1991). Although, other concepts are also accepted, such as the one cited by Cook & Baker (1983) which says that biological control is the reduction, the sum of inoculum or activities that determine the disease caused by a pathogen, carried out by one or more organisms other than man. Unfortunately, there are many factors that interfere in the success of biological control, since, to be successful, it is necessary have extensive knowledge about the edaphoclimatic conditions of the region, as well as, of ecology.

Soil is a system living environment in which plants, microorganisms and fauna interact (Verhoef, 2004). In that such a complex environment, it is possible to find both beneficial organisms and phytopathogens, being that, in edaphically balanced environments, there will be a quantity sufficient number of microorganisms that promote natural biological control and also soil suppressiveness to disease (Baker & Cook, 1974; Cook & Baker, 1983; Bettiol & Ghini, 2005).

In general, in antagonists of the genera are present in the soil microbial community: Trichoderma sp., Pseudomonas sp., Bacillus spp. and others. Of these, Trichoderma spp. And Bacillus spp. are the most studied to date and, in the case of Bacillus spp., it is known that despite being affected by changes climate, it is one of the genres capable of developing well in a wide temperature range. However, for both Bacillus spp., as for Trichoderma sp., the effect of soil moisture is important.

Due to the various advantages that these microorganisms offer for biological control, many companies have dedicated himself to research, both for products with foliar application and for products with formulation for application in the furrow and seed treatment. These last two, mainly for the control of soil fungi such as rhizoctonia sp. Fusarium sp. sclerotinia sp., among others.

At Figure 1 presents data relating to the control provided by different products applied via seed treatment in soybean crops and corn on some pathogens of economic importance.

 Figure 1- Efficiency of fungicides via seed treatment in Corn. Itaara, 2017
 Figure 1- Efficiency of fungicides via seed treatment in Corn. Itaara, 2017

Figure 1 shows that they were differences in control were verified for the two diseases evaluated in seeds and corn seedlings. All fungicides provided superior control to inoculated control and observing the control percentages presented by the treatments containing both species of Bacillus spp. (sp1 and sp2). In case of Colletotrichum graminicola species 2 of Bacillus sp. provided slightly greater control than species 1. Regarding the use of the two species combined there was a positive synergistic effect, with an increase control compared to isolated products. The use of fungicides microbiological tests, in the case of this disease, provided superior control to fungicides Methyl Thiophanate and Piraclostrobin, however, inferior to Fludioxonil+ Metalaxyl- M. In the case of Rhizoctonia solani, O use of both types of Bacillus spp. combined was not as effective in terms of increasing disease control. Despite this, control values ​​between 40% and 70% are considered excellent when it comes to products of biological origin. At the case of bacteria of the genus Bacillus sp. its mechanism of action is related to the ability to parasitize and degrade fungal spores and hyphae, widely described in the literature (Whipps, 2001). In this way, the growth of the fungus is inhibited, meaning that, This adhesion of bacterial cells to the hyphae occurs largely due to the formation of a biofilm.

In case of other two important pathogens that cause diseases in corn, it is observed that, species 2 (62,5%) of Bacillus sp. provided better control than that observed for species 1 (43,75%). Although, when both species were used there was a synergistic effect and, in this case, providing control greater than 80%, even superior to fungicides Fludioxonil+Metalaxyl-M and methyl thiophanate (Figure 2). Now for Diplodia maydis, it is observed contrary behavior, while, Bacillus sp.1 provided greater control when compared to Bacillus sp.2. In this case, the synergistic effect was slightly smaller, when the two species are used together (Figure 2).

Figure 2- Efficiency of fungicides via seed treatment in Corn. Itaara, 2017
Figure 2- Efficiency of fungicides via seed treatment in Corn. Itaara, 2017

In the case of soybean cultivation, the same products were tested to control the main fungi which are transmitted by seeds: Colletotrichum truncatum, Pythium sp. Fusarium pallidorroseum e Macrophomina phaseolina.

in a way general, in the case of Colletotrichum truncatum e Pythium sp., was observed similar behavior in relation to response to products biological. In this case, the second type of Bacillus sp. tested, provided better control for both fungi and, contrary to what occurred with corn diseases, the combination of two species did not show better control over these two pathogens (Figure 3). Despite this, both presented satisfactory control, with reductions in incidence of diseases similar to fungicides Thiophanate methyl and Pyraclostrobin.  

Among the different genera of bacteria and fungi capable of carrying out biological control stand out Bacillus spp. and the gender Trichoderma sp. Bacillus sp. has the ability to form endospore, being a microorganism with varied mechanisms antagonistic. This makes it possible for Bacillus spp. settle for long periods of time in specific ecological niches, in addition to allowing versatility of action mechanisms to break down barriers imposed by the phytopathogen defense system (LANNA FILHO et all., 2010).

Figure 3- Efficiency of fungicides via seed treatment in soybeans. Itaara, 2017
Figure 3- Efficiency of fungicides via seed treatment in soybeans. Itaara, 2017

For Fusarium pallidoroseum e Macrophomina sp. both species of Bacillus sp. tested showed control in the range of 50% to 62%, and the use of these combinations promoted increase of around 6% to 10% (Figure 4). Already registered products use the Bacillus subtilis for biocontrol of pests and diseases, or even to increase plant productivity (NGUGIA et al., 2005; YAO et al., 2006). One of the apparent reasons this organism presenting the biocontrol capacity may be the in situ effect due to exposure of living cells from B. subtilis (HAMMAMI et al., 2009).

Figure 4- Efficiency of fungicides via seed treatment in soybeans. Itaara, 2017
Figure 4- Efficiency of fungicides via seed treatment in soybeans. Itaara, 2017

In view of the above and taking into account the importance of increasingly combining different integrated control methods of diseases, the role of biological control in management is reiterated. Is required think about this type of control beyond the environmental aspects, which are undoubtedly extremely important. In addition to this factor, it is necessary to think about the issue that involves the loss of sensitivity of pathogens to the active ingredients used, therefore, given the great population diversity of pathogens and the reduced number of new fungicides for control, it becomes essential combine biological control with chemical control, seeking to increase not only product efficiency, such as greater longevity of existing products for seed treatment.

Article published in issue 223 of Cultivar Grandes Culturas, December 2017/January 2018. 

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