Measures to control white mold on soybeans

The fungus Sclerotinia sclerotiorum, which causes white mold, is difficult to control and finds soybean cultivation conditions in Brazil a very favorable environment to survive and spread. Preventive measures, which include the use of fungicides before the disease becomes established, are the most effective in tackling the problem.

Soybeans are affected by several fungal diseases. In Brazil, number one is Asian soybean rust, followed by leaf spots. However, hiddenly, root and stem rot end up surreptitiously leading to the profitability of the crop. And these diseases are distributed across the country in different ways, benefiting from the particularities of each growing region, which ends up favoring them.

In this context, white mold, caused by the fungus Sclerotinia sclerotiorum, became increasingly important for Brazilian soybean farming with each harvest, more due to its difficulty in controlling than due to the real potential for damage to the crop, compared to rust. In the 2005/2006 harvest, some commercial soybean crops in Jataí showed an incidence of the disease with losses of up to 33%. However, in the 2007/2008 harvest, the presence of the disease was observed in almost all cultivation areas, and several severe cases presented reductions of up to 70% in the crop's productive potential.

The first reports from farmers in that municipality date back to the 2001/2002 harvest. Currently, it is known that soybean growing regions that offer milder conditions and better wet conditions (free water) have a high chance of developing greater productivity losses due to the aggressiveness of white mold. On the other hand, some regions, despite having plants with symptoms, do not offer conditions, during the soybean cultivation cycle, for the disease to gain economic importance. In other words, it compromises profitability, but does not pose an economic risk of losses in productivity.

The major problem surrounding this disease in Brazil is the ease with which this fungus survives in the conditions in which soybeans are grown in the country. The fungus spreads mainly in the form of sclerotia, ascospores and dormant mycelium inside and outside seeds. Sclerotia can be spread by implements and agricultural machinery, but mainly spread between seeds. In this case, in the south of the country, especially forage turnip, where the seed is similar to sclerotium, and if the farmer is not careful with the quality of the seed, he may end up unintentionally inoculating his crop with Sclerotinia sclerotiorum. The ascospores are ejected from the shells and launched with the help of the wind to nearby plants and crops.

Wind-borne ascospores normally adhere to all surfaces of the plant, however, depending on where they land, for example on the upper part of leaves at the top of the plant, the conditions for their germination and infection are severely poor, and the likelihood of occurrence the disease is minimal. However, the main site of infection, the senesced petals attached, generally over the armpits, incredibly facilitate the infectious process, as the fungus finds in these petals, organic matter (necrotrophic, saprophytic fungus) without resistance to infection, normally observed in living tissues ( leaves and stems). And in these locations, in general, the period of shading and wetness after closing the soybean lines can reach up to 16 hours a day or more. In other words, the longer the period of wetting, for example, if at 10 am there is still the possibility of wetting the hem of your pants while walking in a soybean field, the greater the probability of establishing the pathogenesis. If there is a region that provides (higher temperature, lower rainfall) a shorter period of wetness (12 hours, for example), the possibility of the disease becoming epidemic in the area is severely reduced, and thus the chances of causing losses.

The vast majority of senesced soybean petals from stage R3 to R4, as well as senesced bean flowers or dead parts of crucifers, serve as a source of energy to ensure the primary infection of healthy tissues by ascospores and subsequent colonization of the host. According to the bibliography, an exogenous source of nutrients is necessary for infection to occur, or infecting from wounds. This fact helps to explain soybean crops with the presence of sclerotia in the soil, but in low altitude, dry and hot regions they do not present levels of economic damage from the disease, often not justifying chemical control. With this, it is obviously concluded that white mold is related to the flowering stage of the hosts. However, ascospores can infect plants that are mechanically injured or have necrotic lesions caused by other pathogens or pests. In highly favorable locations, the development of the disease is observed before flowering, especially in soybean crops, starting with the streets where machines (pulverizers) travel. It is worth mentioning that this fungus has a high genetic variability and mutation rate. It is a polyphagous fungus, capable of attacking more than 692 species of plants, especially broad leaves. Thus, even first-year soybean areas are not free from suffering damage from the disease. In Brazil, white mold affects sunflower, beans, cotton, canola, turnip, potato and soybean crops. As far as is currently known, the disease has not developed in grasses such as corn, wheat, sorghum and brachiaria.

After the pathogen enters the plant borders, it will spread easily and quickly (with high aggressiveness) to leaves, petioles, internodes and also nearby plants, after all, it has already obtained its primary source of food, and from that its main function is to spread and produce mycelium and sclerotia in abundance over the largest possible area, as long as the environmental conditions favor it for this, such as soybean density, smaller row spacing, high altitude regions, where temperatures are generally colder. night, increasing dew and the time the plants are wet. The cortical tissues are invaded quickly, until the death of the aerial part of the plant occurs, this being one of the causes that made white mold a priority disease, often gaining more economic importance than rust itself in some regions, where the conditions of climate and land use favored its rapid increase. An example of this is the sequence of soybeans, beans and sunflower, where a massive production of sclerotia can occur in all three crops, in relation to the fungus productivity in the area.

But, in general, colonization is non-selective (not specific to tissues such as xylem). The development of symptoms is associated with oxalic acid and pectolytic enzymes produced by the fungus. Initially, oxalic acid penetrates the tissue, reducing the pH from 6,8 to 4,0, thus providing an optimal pH for the action of the pectolytic enzyme. Remembering that one of the main components of the plant cell wall, which provides rigidity and serves as a barrier to the entry of pathogens into plants, is the pectin gel, which aggregates cellulose and hemicellulose. The action of the pathogen results in regions of the plant with waterlogging symptoms around the lesions, which can be variable, depending on the host's tolerance or resistance to the acid. The lesions frequently appear from the first to the fifth node and on secondary branches of the soybean. The lesions expand from the inflorescence to the main stem, colonizing the tissues and enlarging the lesions. The disease increases until the maturation of soybeans, reaching an incidence rate of 90%.

The major problem with white mold in soybean cultivation in Brazil is the difficulty of control and management. The bibliography has shown for some years now, and not just in Brazil, that there is no total control of the disease, just a reduction in incidence. The way to mitigate the damage caused by the fungus is to integrate measures to reduce the spread of the fungus in the area. The most effective and profitable control measure for this disease in soybean fields is to act on the inoculum and not the disease. The control actions developed for the disease (interaction of the fungus with the plant) minimize the profitability of the process. However, actions aimed at reducing the production of sclerotia and their viability and maintenance in the area take longer to achieve, but are more profitable. As the objective is to make money with soy, it is necessary to find a way to use both in order to obtain the best economic return on investment in technologies aimed at controlling this disease, such as the use of fungicides.

The vast majority of Brazilian soybeans (more than half of the 31,5 million hectares of soybeans in 2015) are soybeans on soybean residue, without rotation. It is no secret that oilseeds are the great economic enabler of an agricultural property or company today, but it is necessary to consider that if, due to economic factors, it is not possible to carry out an adequate crop rotation, a technology rotation must be carried out, using different cultivars, which allow changes in density and spacing between rows without compromising productivity, plant architecture that makes wetting time difficult and, of course, fungicides.

The most critical regions or areas, invariably, must provide for the cultivation of grasses (Poacea) successively, with the aim of promoting a drastic reduction in inoculum in the area. The integrated use of antagonists, in problem areas, should also be considered as a medium - long term aid, such as Trichoderma spp. After all, if currently the profitability of the crop is compromised due to white mold, in the future it may be possible to produce soybeans without white mold in the area, and antagonistic soil fungi help a lot.

Embrapa Soja has been leading a network trial for a few harvests with partners from several research institutions on the chemical management of white mold. The objective is to generate research results to assist in the registration and recommendation of fungicides. The trials have been carried out since the 2008 harvest in the states of Goiás, Bahia, Mato Grosso do Sul, Mato Grosso, Minas Gerais, São Paulo, Paraná and Santa Catarina. It is suggested that soybean producers likely to face this devastating disease update themselves today by obtaining the technical circular of network trials for white mold in Brazil.

The use of fungicides is very effective before the disease becomes established. Once it manages to infect the dead petals, the pesticides will have a low control effect. Knowing this, the most critical phenological stage for chemical management is the beginning of flowering. Thinking about using the fungicide much more on the disease inoculum, the apothecia and ascospores, than on the plant and even less on the disease.

Lucas Navarini, IFRS - Campus Ibirubá/RS; Carlos Machry, Dionatan Nicola, Francine Simmi, IFRS - Campus Ibirubá/RS, CNPq Fellows

Article published in issue 205 of Cultivar Grandes Culturas.