Solutions to face La Niña

According to the bulletin issued on August 20, 2010 by the Weather Forecasting and Climate Affairs Center (CPTEC) of the National Institute for Space Research (INPE), the La Niña phenomenon appears in the equatorial Pacific, with the possibility of persisting for the less until the beginning of 2011. For the months of September, October and November 2010, the consensus climate forecast suggests a greater probability of below-average rainfall in the South Region, especially in Rio Grande do Sul, while temperatures are forecast within the climatological normality. Therefore, we will possibly have the 2010/2011 soybean harvest drier than normal.

Drier climate can favor some soybean diseases, such as powdery mildew, nematodes and gray root rot, in addition to affecting seedling emergence. On the other hand, diseases directly related to high humidity, such as phytophthora root rot, red root rot and rust, should not present high incidence and/or severity.

According to researcher Ademir Henning, from Embrapa Soja, in 2010 the risk is very high of below-average and poorly distributed rainfall during the soybean sowing period. In this situation, it is very important to raise awareness among soybean farmers regarding the use of seeds of high physiological (with high vigor and germination) and sanitary (pathogen-free) quality, in addition to the need to treat seeds with mixtures of contact fungicides. + systemic, to guarantee protection to the seed in the soil, avoiding reseeding due to failures in the stand establishment. Furthermore, in areas subject to attack by elasmo caterpillars or corós, the application of products containing insecticides may be a good alternative.

If soybean sowing is not carried out under ideal soil moisture conditions, serious problems may occur during emergence. In such circumstances, seed treatment with fungicides offers additional guarantee for crop establishment at reduced costs (estimated at less than 0,6% of the crop installation cost). The adoption of this technique allows the farmer to save seeds and avoid reseeding operations, which are extremely harmful to the profitability of the crop, as the ideal sowing period is often missed and cultivars may be changed (which is why, normally, there is a surplus on the market), in addition to problems related to the application of herbicides and fertilizers, as well as phytosanitary problems, mainly soybean rust, which is known to be more severe in late plantings.

The efficiency of several fungicides in controlling the main pathogens transmitted by soybean seeds, such as: Cercospora kikuchii, Cercospora sojina, Fusarium semitectum, Phomopsis spp. and Colletotrichum truncatum, is annually evaluated at Embrapa Soja. Control of the first four pathogens is provided by systemic fungicides, especially from the benzimidazole group. Among the products tested and currently recommended for treating soybean seeds, fungicides from the chemical groups thiabendazole, carbendazim and methyl thiophanate have been the most efficient. Traditionally known contact fungicides (captan, thiram and tolylfluanid), which perform well in the field in terms of emergence, do not fully control Phomopsis spp. and F. semitectum in seeds that present high levels of these pathogens (above 40%). For this reason, such products should always be used in mixtures with systemic fungicide (Table 1).

In addition to controlling these pathogens, seed treatment is an efficient practice to ensure adequate plant populations when soil and climate conditions during sowing are unfavorable to germination and rapid emergence of soybeans, leaving the seed exposed for longer to pathogens that inhabit the soil. soil, such as: Rhizoctonia solani, Phytophthora soyae, Pythium spp., Sclerotium rolfsii; Fusarium spp. (mainly F. solani) and Aspergillus spp. (A. flavus) which, among others, can cause seed deterioration in the soil or seedling death.

According to researchers Álvaro Almeida and Leila Costamilan, this disease is most favored by periods of drought and high temperatures. It will probably be very frequent during the 2010/11 harvest. There have been several harvests in southern Brazil, which means the presence of a high amount of inoculum in the field. Its most visible symptom is the anticipation of the cycle of plants distributed randomly or grouped in areas with drier soil, resulting in grains of smaller size and lower quality. These plants have rotted roots, easily peeling off the bark, and the wood is grayish in color. Work carried out at Embrapa Soja showed the correlation between rain and the occurrence of this disease in Paraná. In a year of water deficiency, more than 50% of soybean plants showed the disease 105 days after sowing. In the following two years, when there was no water restriction, this rate reached 30% at the end of the crop cycle.

It is a difficult disease to control, as there are no resistant soybean cultivars, crop rotation is not efficient in the short term and treatment with fungicides has no effect. To minimize its impacts, the farmer must invest in ways to maintain an adequate supply of water to the plants, such as irrigation, direct planting (which increases the amount of organic matter) and improving the physical conditions of the soil, avoiding compaction and favoring the development of soil. root system.

According to researcher Leila Costamilan, this disease is not expected to occur significantly in this harvest, mainly because it depends on high soil humidity to infect soybean plants. Furthermore, farmers have preferred to use resistant cultivars in areas where the disease has already been detected in previous years, reducing the risk of the problem developing. It will only cause damage such as rotting of seeds and death of seedlings in areas where heavy rains or accumulation of moisture occur during the period of germination and emergence of seedlings of susceptible cultivars, even leading to the need for reseeding. The disease can also cause symptoms in adult plants, such as root rot and darkening of the lower part of the stem and lateral branches, associated with sporadic rains during the harvest.

Researcher Cláudia Godoy highlights that for the occurrence of soybean rust, the environment plays a relevant role in the development of epidemics during the harvest, as most cultivars are susceptible and the fungus survives from one year to the next on volunteer plants or other hosts. . Work published abroad and in Brazil shows that the dispersion and development of rust are favored by the better distribution of precipitation during the harvest. Rain directly influences the period of leaf wetness (necessary for infection), dissemination (acting on the release, dispersion and deposition of fungal spores) and also indirectly, on the survival of the inoculum (due to less exposure of spores to UV radiation during periods cloudy). According to data from the Antirust Consortium, the evolution of the disease has shown different patterns in harvests, associated with precipitation patterns in different producing regions (Figure 1). In the 2006/07 and 2009/10 harvests, under the influence of El Niño, rust started earlier and the total number of outbreaks was greater than in the 2008/09 harvest, under the influence of La Niña. Although the irregular rainfall forecast for the 2010/11 harvest may adversely affect the development of rust, the guidelines for managing the disease have not changed, and crops must be monitored and the occurrence of the disease in the region monitored. Rust control must be carried out after the initial symptoms of the disease in the crop, or preventively, taking into account the phenological stage of the crop, the presence of rust in the region, climatic conditions, application logistics, the presence of other diseases and the cost of control.

Low humidity in winter, especially in the central region of Brazil, has hindered the sprouting of volunteer plants and the survival of the fungus, contributing to the sanitary void, whose main objective is to reduce the fungus inoculum during the off-season. Therefore, a delay in the occurrence of the first rust outbreaks in the next harvest is expected, further emphasizing the need for monitoring to avoid unnecessary fungicide applications.

Unlike rust, soybean powdery mildew is favored by conditions of low relative humidity and may predominate mainly in regions with mild temperatures, if climate forecasts are confirmed. Powdery mildew has the capacity to cause damage of up to 35% of productivity; however, the cultivars present different levels of resistance and their control can be carried out through the application of fungicides. Red root rot (Fusarium spp.) – Researchers Claudine Seixas and Álvaro Almeida point out that this will probably not be a very common disease. important in this La Niña harvest, as its development is related to a good supply of water in the soil. Four species of Fusarium may be associated with the symptoms of the disease, F. tucumaniae is the prevalent one in Brazil. This fungus reduces root volume and nodulation. The neck region of the plant has a reddish stain. The leaves of infected plants present the symptom known as “carijó” leaf, with chlorotic spots between the veins of the leaves, which later become necrotic. Generally, plants with these symptoms occur in reboleiras.

Compacted soils with high humidity favor attack by this fungus. Controlling this disease can be helped with scarification or subsoiling, which allows the “break” of the compacted layer, favoring root development and increased water infiltration.

Root-knot nematodes – Meloidogyne javanica, M. incógnita and Heterodera glycines – Researcher Waldir Dias reports that in the case of root-knot nematodes, soybean roots are thickened, and the plants can wither in the hottest hours of the day, present low development and reduced grain yield. With cyst nematode, the plants turn light green, with reduced development and yield.

Everything that is favorable or negative for soybeans is also good or bad for nematodes. In years of drought, when there is a lack of water in the soil, soybeans are more vulnerable to damage by these organisms, due to the difficulty in absorbing water by the root system, caused by nematodes. The lack of free water in the soil, however, has the advantage of hindering the spread of these organisms. Furthermore, more stressed soybean plants will not favor the multiplication of nematodes.

Therefore, knowing that some soybean diseases can be favored by periods of drought, measures are recommended to reduce possible losses in La Niña years. Seed treatment, selection of cultivars resistant to powdery mildew and nematodes, chemical control of powdery mildew and improvement of water maintenance conditions in the soil, mainly with the use of direct planting, are factors that have a positive effect on the control of diseases favored by drought and , consequently, in the yield of soybeans.

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