Management of white mold in soybeans

One of the problems that concerns soybean producers is the spread of fungal diseases, which until recently were not serious problems, such as Sclerotinia sclerotiorum. Responsible for considerable productivity losses, white mold has increased its importance as a soybean pathology in regions of Mato Grosso, Mato Grosso do Sul and Rio Grande do Sul. Adjusting the spacing between rows and sowing density are practices that can help in management. 

In years with periods of above-average rainfall, white mold can cause severe losses in productivity. On average, productivity losses are 10% to 20%. However, in conditions favorable to the occurrence of the disease, productivity losses can be greater, reaching, in some cases, up to 70%. Chemical control of this disease is difficult and costly, requiring the use of combined control measures to obtain any practical results. Some management practices may favor the control of white mold. Among them are the use of certified seeds, crop rotation, the succession of soybeans with non-host species such as corn, increasing row spacing and reducing the plant population to the recommended minimum. The spacing between rows and the plant population, associated with a vegetative canopy that allows an arrangement of plants capable of fully exploiting the potential of each cultivar and its particularities, constitute a key factor in increasing productivity.

Symptoms     

The first symptoms of white mold on plants are light green spots with a “soggy” or “buttery” appearance, which evolve into a light brown color, and soon develop abundant formation of dense, white mycelium. Within a few days, the mycelium turns into a rigid, black mass. This is the sclerotium, which is the resistance form of the fungus. Sclerotia vary in size from a few millimeters to a few centimeters and are formed both on the surface and inside the stem and infected vegetables. Sclerotia lying on the soil, under conditions of high humidity and temperatures ranging from 10ºC-21°C, germinate and develop, on the soil surface, sexual reproduction structures, called apothecia. These produce ascospores that are released into the air and are responsible for plant infection. 

Plant arrangement

Adjusting the spacing between rows and sowing density are practices that can interfere with the occurrence of the disease. This practice improves aeration and the infiltration of solar radiation into the crop canopy, which reduces the development and proliferation of the fungus. Both aeration and solar radiation reduce the germination of sclerotia and the development of apothecia. Based on this knowledge, a study was carried out in São Desidério, Bahia, with the objective of evaluating the effect of plant arrangement (spacing and plant population) on the occurrence of white mold (S. sclerotiorum) in soybean cultivation.

The study was carried out in an experimental area at Sementes Eliane, in São Desidério, Bahia. The cultivar used was FTS 4188. This cultivar has maturation group 8.8, with a cycle that can vary from 130 days to 135 days. The recommended plant population ranges from 180 thousand plants/ha to 200 thousand plants/ha. Each treatment consisted of strips measuring 7,6m wide by 200m long. The treatments tested were combinations between plant population per hectare and spacing between sowing lines, as illustrated in Table 1.

The number of sowing lines per plot varied according to the spacing used in each treatment. The evaluation was carried out when the culture was at the R5.5 phenological stage. To assess the incidence of S. sclerotiorum, in each plot sampling was carried out at four points. Each evaluation point consisted of two meters of crop line, where each plant was evaluated individually to obtain the percentage of plants infested by S. sclerotiorum.

Results

The results obtained indicate that, for this situation, the percentage of infested plants showed an inverse relationship with the spacing between rows and a direct relationship with the plant population (Figure 2). In other words, the greater the spacing between lines, the lower the incidence of S. sclerotiorum. Otherwise, when the plant population increased, the percentage of plants attacked by white mold also increased. Therefore, in treatments in which greater spacing between rows was associated with lower population, the percentage of plants attacked was lower. These data agree with the recommendations of researchers and institutions, who state that by increasing the spacing between rows and reducing the population to the recommended minimum, greater ventilation and greater penetration of solar rays into the crop canopy are promoted, which reduces the incidence of white mold. The producer often experiences productivity losses when changing these parameters (increased spacing and reduced density). However, soy is a plastic crop, with the ability to maintain productivity in greater spacing and with lower population density.

The results obtained demonstrate the lowest percentage of plants attacked by S. sclerotiorum in the treatments in which the largest spacing was used, that is, 0,76m, for all tested populations (Figure 2). In this way, it is clear that the arrangement of plants can be a resource used within the management system of S. sclerotiorum in soybean crops, especially in crops with a high incidence of this pathogen.

HOSTS

In addition to soybeans, the fungus infects a wide range of cultivated plants and weeds, with the exception of grasses. Therefore, S. sclerotiorum is a polyphagous fungus, with plants from more than 75 families, 278 genera and 408 species as hosts. It is recommended to rotate crops with non-host species such as corn, oats, brachiaria and other grasses to reduce the incidence of S. sclerotiorum in crops. 

PREVENTION AND MANAGEMENT

The use of quality seeds is a way to prevent the pathogen from entering crops. Transmission by seed can occur both through dormant (internal) mycelium and through sclerotia mixed with them.

Spraying fungicides based on thiophanate methyl and carbendazim can reduce the incidence of the disease. Preventive applications, starting from the soybean pre-flowering period, prevent the colonization of the fungus in plant tissues.

The appropriate plant arrangement for each cultivar can also benefit the crop due to better infiltration of fungicides into the canopy.

The biological control of S. sclerotiorum from spraying fungus spore suspensionTrichoderma spp. provides a reduction in the germination of sclerotia.

Likewise, planting crops that keep the soil surface covered, such as brachiaria spp., imposes a physical barrier to the germination of sclerotia, reducing the incidence of the disease.

Other preventive measures, such as avoiding the movement of machines between areas infested and not infested by the fungus, cleaning machines and equipment, avoiding disturbing the soil so that sclerotia that were buried are exposed and germinate, among others, prevent and reduce the crop infestation.

Jerson V. C. Guedes (UFSM); GiliardiDalazen (UFRGS); Rodrigo B. Rodrigues (DuPont); Jonas A. Arnemann (UFSM); Ivair Valmorbida (UFSM)

Article published in issue 190 of Cultivar Grandes Culturas.