White mold, caused by Sclerotinia sclerotiorum (Lib.) de Bary, is one of the oldest diseases of soybeans. Its occurrence and damage levels have increased significantly in Brazil, both in the highest areas of the Cerrado and in the most traditional cultivation areas in the South and Southeast, potentially reducing productivity by up to 70%. It is estimated that approximately 23% of the Brazilian soybean production area is infested by the pathogen, comprising 6,8 million hectares that require the adoption of measures to control the disease.
The fungus that causes white mold affects more than 400 species of plants and the damage is more severe in regions with a rainy climate, mild temperatures and high relative humidity. Maintaining soil moisture is essential for the development of the disease, as the germination of sclerotia (fungus survival structures) depends on soil moisture, temperatures between 15°C and 25°C, and little sunlight. . Due to the dependence on these conditions, the occurrence of white mold on soybeans varies in intensity between harvests.
One of the distinguishing characteristics of the fungus S. sclerotiorum is the formation of sclerotia. The sclerotium is a structure composed of a cluster of hyphae, commonly rounded or elongated, black in color and firm in consistency, which plays an important role in the survival of the fungus from one harvest to the next. Sclerotium germination has a carpogenic or myceliogenic origin, triggering new cycles of the disease. The production of apothecia from sclerotia is called carpogenic germination and is the main source of infection in soybean crops. Mycelogenic germination is characterized by the growth of hyaline, septate, multinucleated and branched hyphae, originating from micropores of the sclerotium.
The fungus is capable of infecting any part of the soybean plant, however, infections most frequently begin in the inflorescences, petiole axils and lateral branches. The pathogen can attack the entire aerial part of the plant, affecting leaves, stems and pods. The infected soybean plant initially presents watery lesions, from which hyphae grow, forming abundant white mycelium, which characterizes the name of the disease. The attacked tissues rot as a result of the action of various toxins produced by S. sclerotiorum. At this stage, rotting of lateral stems, pods and leaves can be observed, or even the main stem with death of the entire plant. Sclerotia are formed both on the surface and inside the stem and infected pods, and can detach on their own or be thrown to the ground during harvest, increasing the inoculum in the area.
Disease management has as strategic objectives the reduction of inoculum (sclerotia in the soil) and the reduction of incidence and its rate of progress. The reduction of inoculum in the soil is achieved by making sclerotia in the soil unviable and by reducing the production of sclerotia in diseased plants, through measures such as: formation of straw for uniform soil coverage, preferably from grasses; rotation and/or succession with non-host crops; use of biological control through soil infestation with antagonistic agents; use of good quality seeds treated with suitable fungicides; use of chemical control, through foliar sprays of fungicides mainly during the plant's greatest vulnerability period (R1 to R4). To reduce the incidence of white mold and its rate of progress, the following measures are important: choosing cultivars with plant architecture that favors good aeration between plants (few branched and with small leaves) and with a shorter period of flowering, and the use of plant population and row spacing appropriate to the cultivars. Another measure that significantly contributes to reducing the spread of the fungus S. sclerotiorum is the cleaning of machines and equipment after use in an infested area to prevent the spread of sclerotia to new areas.
The effectiveness of controlling white mold in soybeans is only achieved with the integration of these measures, not presenting satisfactory results in isolation. Research has been carried out seeking to develop cultivars resistant to the disease in several crops, both through classical breeding and transgenics. However, no soybean cultivar is completely resistant to white mold, but some of these materials present partial resistance. Obviously, cultivars with partial resistance are less affected by the disease than susceptible cultivars. There are still no commercial soybean cultivars originating from these programs.
In 2008, activities began that gave rise to cooperative trials for the chemical and biological control of white mold on soybeans, the result of discussions at the Soybean Research Meetings organized by Embrapa. These test networks are carried out by phytopathologists from various research and teaching institutions in the country, with experiments being conducted in the states of Goiás, Mato Grosso do Sul, Mato Grosso, Minas Gerais, Bahia, São Paulo, Paraná and Santa Catarina.
The fungicides that showed the best levels of control were fluazinam and procymidone, sprayed alone or in association with methyl thiophanate or carbendazim, ranging from two to four sprays at intervals of ten days, starting at the R1 stage of plant development (beginning of flowering). . Two other fungicides, still in the registration phase, also showed a high level of control. The results of these tests are contained in the publication “Cooperative tests for chemical control of white mold in soybean crops – harvests 2009 to 2012”, series Documents 345 (available in Embrapa Soja online publications at www.cnpso.embrapa.br).
The efficiency of chemical control of white mold in soybeans depends on several factors, such as dose, moment of application, number and interval between applications, in addition to the application technology used. As the main form of infection of soybean plants by S. sclerotiorum occurs through the colonization of flowers and pods at the beginning of development by ascospores of the fungus (carpogenic germination), the plants need to be protected by fungicides between the R1 (beginning of flowering) and final R4 (pod formation) stages, if there is presence of apothecia in the crop.
Under field conditions, sclerotia can be attacked and degraded by microparasites such as fungi and bacteria. Among these microorganisms, some species of the fungus Trichoderma and bacteria Bacillus stand out among the most efficient antagonists of soil-dwelling pathogens, with some commercial formulations already registered with the Ministry of Agriculture, Livestock and Supply (Mapa). The application of antagonists must be done before the sclerotia germinate, that is, when the sclerotium is resting on the soil surface, as it is more vulnerable to attack.
For biological control to work properly, environmental conditions similar to those that favor the germination of sclerotia are necessary for the establishment of biocontrol agents, whose reproductive structures are more sensitive and dependent on soil humidity and shading, as well as mild temperatures. . For this reason, the direct sowing system on grass straw has proven to be a prerequisite for the success of this control measure.
In cooperative biological control trials, formulations of fungi and bacteria potentially antagonistic to the fungus were evaluated. S. sclerotiorum and a plant extract based on lignosulfonate, in two applications at stages V2 and V4, respectively. All biofungicide treatments outperformed the control without control and did not differ from each other, presenting control rates that varied from 29% to 39%. Reductions in carpogenic germination ranging from 19% to 24% were also observed, representing an important contribution to the management of the disease.
Main Brazilian soybean producing regions infested with Sclerotinia sclerotiorum, pathogen that causes white mold
Evolution of the area cultivated with soybeans in Brazil (data from Conab) and the area infested with white mold (data estimated from information from the productive sector and agricultural research), in the last seven harvests
General appearance of a soybean crop heavily attacked by white mold (Photo: M.C. Meyer)
Mycelial growth of Sclerotinia sclerotiorum in a soybean plant (Photo: M.C. Meyer)
Soybean plant with white mold showing abundant formation of sclerotia (Photo: M.C. Meyer)
Sclerotia of Sclerotinia sclerotiorum in pre-cleaning residue of soybeans from crops attacked by white mold (Photo: M.C. Meyer)
Carpogenic germination of sclerotia Sclerotinia sclerotiorum. A) apothecia formed under laboratory conditions; B) Apothecium formed in the field, originating from sclerotia buried approximately 2cm deep; C) Apothecia formed in the field, from sclerotia on the soil surface (Photos: M.C. Meyer)
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