Integrated management of white mold in soybeans

Practices integrated approaches, with a special focus on preventive measures, are essential for correctly and sustainably tackle white mold in soybean crops, fungal disease with high destructive potential, whose pathogen once introduced in the cultivation area it is almost impossible to eradicate.

White mold or white stem rot, caused by fungus Sclerotinia sclerotiorum(Lib.) de Bary, is one of the oldest diseases that affect soybean crops, with occurrence in different producing regions of Brazil. It has the ability to infect more than 500 species of plants, mostly dicotyledons.

White soybean mold is widely distributed in all continents, with the capacity to cause losses in the productivity of approximately 30%. However, when control measures are not efficient, Total loss of production may occur. Furthermore, it is important to highlight that the fungus has the potential to cause disease in numerous crops of agricultural importance, such as lettuce, peanuts, potatoes, eggplant, canola, carrots, sunflowers, beans, tomatoes and various species of flowers.

Spread in cultivation areas is favored by use of contaminated seeds, lack of crop rotation, combined with the adoption of direct planting, introduction of sensitive species in areas infested such as forage turnip, sunn hemp, cotton, sunflower and beans and, in soybeans grown under a sprinkler irrigation system. The fungus infects the plant at any stage of development. However, the most vulnerable period comprises the stages of full flowering (R2) until the beginning of pod formation (R3/R4).

Climatic conditions are the main modulating factors infection, growth and development of the fungus, as well as the formation of their resistance structures (sclerotia). The favorable conditions for pathogen are high relative humidity and temperatures between 10ºC and 21ºC.

The infection by S. sclerotiorum begins when sclerotia germinate and produce mycelium, which penetrates directly into plant tissues. Upon infecting the host plant, the disease causes rot and drought, mainly in the soybean stem, due to colonization of green fabrics. The first visible symptoms are water-soaked lesions, which evolve into a light brown color. It is also possible to observe a profuse white mycelial growth, a characteristic that gave rise to the name of the illness. In a few days the mycelia transform into sclerotia.

the fungus S. sclerotiorum can persist in the area from one cropping cycle to another, due to colonization of cultural remains present in the soil, through the formation of resistance structures, popularly known as sclerotia. Such structures enable the fungus to remain viable for up to 11 years and are responsible for storing reserve substances, which allow survival of the pathogen until environmental conditions are favorable to its development. The sclerotia formed by the fungal mycelium present dark coloring and variable size, from a few millimeters to a few centimeters.

Symptoms presented by infected plants include the yellowing, withering and drying of branches and leaves. In more stages advanced stages of disease development, there is the formation of sclerotia. O tangle of mycelia, therefore, transforms into a black, rigid mass giving rise to sclerotia, which can be present both on the surface such as inside stems and pods colonized by the fungus.

Management of this disease should preferably be carried out through the adoption of several integrated control measures, both with objective of reducing the initial inoculum. The first step to mold control white is to prevent the entry of the pathogen into the area, by cleaning agricultural implements used in crops with a history of the disease and acquisition of certified seeds with certified phytosanitary quality. In addition In addition, it is recommended to treat seeds with fungicide. At the moment there are three products registered for use in treating soybean seeds for the control of S. sclerotiorum: thiophanatomethyl + fluazinam, carboxin + thiram and fludioxonil + metalaxyl-M + thiabendazole.

Crop rotation and/or succession with species not hosts is strongly recommended, especially with the use of grass species such as corn, wheat and white oats. In conventional cropping systems, As the soil is disturbed, the sclerotia are exposed to light and high temperatures and, consequently, degraded. Efficient weed management is also important for reducing the initial inoculum of S. sclerotiorum, since these plants can be hosts of the pathogen responsible for causing the disease.

For the chemical control of white mold on soybeans, fungicides registered with the Ministry of Agriculture, Livestock and Supply (Mapa) are fluazinam, procymidone, carbendazin, boscalid, methamsodium, iprodione, procymidone, picoxystrobin, boscalid+ dimoxystrobin, in addition to thiophanate mixtures methyl with fluazinam, chlorothalonil, mancozeb and procymidone. However, it is It is important to highlight that the efficiency of chemical control will depend, above all, the time, quality and method of application. One of the main difficulties encountered in this control measure is to provide conditions for that the product can reach the lower parts of the plants and the surface of the soil at a time when the plant canopy has already completely closed.

Given the difficulties faced in the chemical control of white mold on soybeans, the use of biological control has become a recurring practice. Several studies have demonstrated the efficiency of using fungus Trichoderma spp. in control biological S. sclerotiorum. In areas with a history of the disease, the Trichoderma spp. is generally applied to the soil before or at the beginning of the development of culture.

Soybean genotypes with some degree of mold resistance white have already been reported in the literature. However, there is currently no record of soybean cultivars that present an acceptable level of genetic resistance to white mold in the Brazilian market.

For efficient management of white mold in the soybean, practices that make up integrated disease management must be used. When combined, these techniques contribute to a more effective measure of control. sustainable and rational. Preventive measures will always be recommended, due to the fact that once the pathogen is introduced into the cultivation area, its Eradication becomes a task of great difficulty.

Box – Soy in Brazil

Soy (GlycinemaxL. Merril) is among the most agriculturally important leguminous crops. Your world cultivation area in the 2016/17 harvest was 120,3 million hectares, with production of 351,3 million tons, according to information from the Department of Agriculture of the United States (USDA).

Brazil has the largest cultivated area with soybeans in the world, with 33,9 million hectares. However, when observing the Brazilian production, which in the last harvest was 113,9 million tons, the country is the second largest producer, behind only the United States, which produced approximately 117,21 million tons. The USA has the second largest soybean growing area in the world. Despite having a larger area planted, Brazil has lower average productivity (3.36 kg/ha) compared to to the USA (3.50 kg/ha). It is important to highlight that, together, these two countries correspond to 65,8% of world soybean production.

According to the National Company of Supply (CONAB), the three Brazilian states with the highest soybean production are Mato Grosso (30,51 million tons), Paraná (19,53 million tons) and Rio Grande do Sul (18,71 million tons), in an area of 9,32 million hectares, 5,25 million hectares and 5,57 million hectares, respectively. When looking at the average productivity of each state, Paraná stands out in first place with approximately 3.72 kg/ha, followed by Rio Grande do Sul with 3.36 kg/ha and Mato Grosso do Sul with 3.27 kg/ha.

The genetic productive potential of soybean is influenced by several environmental factors, called biotic stresses and abiotic. Diseases are among the main limiting biotic stresses the yield of this crop. Fungi are pathogenic agents responsible for cause diseases more frequently in soybeans grown commercially in Brazil. The greater frequency of fungal diseases is due to climatic conditions favorable to the development of fungi, together with the practice of monoculture and the use, in some cases, of contaminated seeds.

Article published in issue 227 of Cultivar Grandes Culturas, April 2018.