Aggressive pathogen: losses due to white mold

White mold, caused by the fungus Sclerotinia sclerotiorum, is one of the main and most destructive diseases of beans, characterized by being endemic in some regions of Brazil. According to the National Agricultural Society (SNA), the disease is present in 9% of agricultural production areas in Brazil, including beans, soybeans, cotton, among others. Several control strategies must be adopted, from the origin of the seed to be used to the transit of agricultural machinery, as once installed in the area, eradicating the pathogen is almost impossible.

Characteristics of the pathogen

The causal agent Sclerotinia sclerotiorum It is considered a polyphagous fungus, as it attacks several types of plants, with more than 400 host species described (BOLAND, 1994). Among the climatic conditions favorable to the development of the disease are mild temperatures (18ºC – 23ºC) and high relative humidity, with areas irrigated with bean cultivation in winter being the places where the disease is most severe (OLIVEIRA, 2005). beginning from the resistance structures of the fungus present in the soil, called ''sclerotia'', which, when germinating, originate the apothecia, which contain the agents responsible for the infection, the ascospores, which when released can be deposited on the plants and initiate infection (TORMEN, 2014). The disease assumes greater importance from the flowering period to the pod formation period, requiring careful monitoring during these stages. Typical symptoms in plants are waterlogging in the tissues and the presence of mycelia with a cottony appearance. At more advanced stages, agglomerated mycelia mature and form sclerotia, which can be deposited on the external and internal parts of the plant (LINK & JOHNSON, 2007).

Seed quality

The seed is the main means of spreading the disease, and can carry the pathogen in the form of dormant mycelium inside or sclerotia, mixed with the seeds. The use of uncertified seeds or seeds of dubious origin is among the main causes of the increase in the incidence of white mold in areas of Brazil. Given this, the use of certified and healthy seeds becomes a potential means of preventing the disease. However, only a small proportion of bean producers usually adhere to this practice, facilitating the spread and increased incidence of the disease. In addition to the use of certified seeds, their chemical treatment with fungicides (Methyl Thiophanate, Fluazinam and Thiram, among others) is an important alternative for controlling the disease.

Cultural control

According to Görgen et al. (2009), apothecia need light to complete their development and, consequently, straw production becomes an important tool for reducing the incidence of the disease. The function of straw is to form a physical barrier against the development of apothecia and the release of ascospores that can reach the plants and initiate infections (LOBO JUNIOR & SANTOS, 2013). This straw must come from grass species with high potential for mass production, such as those of the genus brachiaria, that the pathogen does not parasitize. The coverage will also allow an increase in the activity of microorganisms that help in the degradation of sclerotia present in the soil. In addition to straw, increasing the spacing between rows of plants is also a management alternative, as it will allow for less density and greater aeration in the crop, preventing the formation of a microclimate favorable to the development of the disease. In places where agricultural operations were carried out and which contained the disease, it is necessary to clean the machines, as their movement between areas is also a form of spread. The burial of sclerotia in soil preparation operations at depths of 20cm to 30cm makes the fungus' resistance structures unviable.

chemical control

The application of fungicides must be carried out preventively, however, this practice may be less effective than expected when used alone to manage the disease. The critical period for the occurrence of white mold occurs between flowering and pod formation, which determines the need for good coverage of the flowers through the application of fungicide (VIEIRA et al., 2001). Application can be via land or irrigation, with a protection period described as around 12 days to 14 days, requiring an average of two to three applications, which may vary according to the incidence of the disease and environmental conditions for its development. Fungicides with the active ingredients Fluazinam, Procymidone and Fluopyram are highly effective in controlling the disease. Under experimental conditions, the active ingredient fungicide Dimoxystrobin + Boscalid has emerged as another effective alternative.      

Biological control

Research results have demonstrated effectiveness in controlling white mold through the use of biological agents, more specifically fungi of the genus Trichoderma. These agents are naturally present in the soil and produce substances that inhibit the growth of various fungi that cause diseases in plants, such as white mold, through the degradation of sclerotia and parasitism. However, as it is a living organism, it requires care regarding environmental and application conditions so that it can colonize and express satisfactory control. Fernandes et al. (2007) found that isolates from Trichoderma showed greater growth at temperatures between 20ºC and 30ºC, with temperatures below 20ºC significantly reducing the parasitic activity of the fungus on the sclerotia. Straw can also be another influential factor that enhances the action of the biological agent. Görgen et al. (2009), when evaluating the effect of straw Brachiaria ruziziensis in conjunction with application of Trichoderma harzianum 1306 for the control of white mold in soybeans, found an increase in parasitism on sclerotia, reducing the incidence of the disease in oilseed cultivation and increasing productivity. Therefore, there will be a need to provide adequate conditions for the development of the biological agent, so that it can act significantly on the pathogen.

 In bean cultivation

According to a survey carried out by the National Supply Company (Conab), the area planted with beans, in the 2013/2014 harvest, accounting for the three harvests, covered around 4,37 million hectares, with an estimated production of 3,30 million tons . These production values ​​reflect the scenario in which the crop is inserted in Brazil, where the predominance of low technology results in productivity far below the genetic potential of the cultivars, despite there being cultivation systems based on high technology, where production levels can surpass 3000kg/ha. In addition to the strong climatic influence that can lead to significant losses, the crop's susceptibility to diseases brings challenges related to management, so that the productive potential of the cultivars is reached.

Final considerations

The aggressiveness of the pathogen and the potential of the disease to interfere with agricultural production remains clear. However, the association of different control measures can significantly reduce the incidence of white mold in crops, allowing the maintenance of production potential and making production systems more sustainable.

 The complete article is present in issue 193 of Cultivar Grandes Culturas.