White mold is caused by the fungus Sclerotinia sclerotiorum, being able to infect more than 400 plant species, with great importance in Brazil in several crops that produce grains such as soybeans, beans, sunflower and canola.
It is a difficult disease to manage, considering the absence of resistant cultivars, the wide range of alternative hosts and the ability to produce abundant amounts of sclerotia, their survival structures in the soil, which allow the maintenance of primary inoculum from one crop to the next. .
The sclerotia can germinate emitting mycelium, or carpogenic germination, depending on environmental conditions. Carpogenic germination is the main reproductive form, which consists of the formation of apothecia by the sclerotia, which are cup-shaped structures. They produce ascospores, sexual spores of the pathogen, the main form of plant contamination. Apothecia (Figure 1) produce ascospores for a period of five to 10 days, and it is estimated that more than two million ascospores are generated in this period. Upon reaching the aerial part of soybean plants, ascospores need a mild temperature (15 °C to 25 °C) and a leaf wetness period of two to four hours, preferentially colonizing senescent flowers.
The symptoms of the disease become more visible when the white, cottony mycelium appears, usually with sclerotia, which is the most evident sign of plants infected with S. sclerotiorum (Figure 2). As the disease progresses, producing mycelium in abundance, new sclerotia are formed that will replenish the primary source of inoculum for crops in subsequent years.
Success in chemical management is often difficult, as the efficiency of fungicides must coincide with favorable conditions for infection to occur, that is, soybean flowering, ascospore release and favorable climatic conditions. However, in soybean cultivars with an indeterminate growth habit, favorable environmental conditions for long periods in the crop cycle and the presence of the pathogen make fungicide applications inefficient.
In addition, fungicides have limited movement (systematicity) in the plant, and none moves down the plant, that is, via the phloem. The lack of ability to move probably contributes to the low effectiveness in controlling white mold.
Considering these difficulties with chemical fungicides, the use of biological agents has shown success in the management of the disease. Among the biological control agents with potential to control S. sclerotiorum studied, there are bacteria and fungi. In Brazil, the use of Trichodermas and Bacillus prevail as biocontrol agents for S. sclerotiorum, being used in the integrated management of white mold.
In the biological management of white mold, it is important to understand that the origin of the disease, in most cases, begins with the germination of sclerotia. In this sense, the management of the disease should be directed towards the reduction of sclerotia and techniques to make it difficult for them to germinate. In addition, in management, it is also important to make efforts to minimize the dispersion of ascospores, as well as to seek to make them unfeasible.
Some strategies are important in the sclerotia reduction process, such as crop rotation and cover crops, considering the use of non-host cultures of S. sclerotiorum, such as corn, wheat, oats, barley. In fields with a growing history of white mold, it is essential to break the pathogenic cycle by introducing non-host species by subsequent crops. Using no-till is also a strategy, although more sclerotia are found near the soil surface in this system, they degrade faster in no-till soils compared to plowed soils. Another important factor is the use of cover crops, being necessary to carry out an inspection of the quality of the seeds, to avoid seeds contaminated with sclerotia, as can be seen in Figure 3, contaminated forage radish seeds.
As for the strategies to hinder the germination of sclerotia and the dispersion of ascospores, the plant population and the adequate spacing are factors that can contribute to avoid the favorable microclimate for the germination of sclerotia. Another factor is to avoid excessive irrigation, especially during critical periods of infection, from early flowering (R1) to early pod development (R3). To avoid ascospores dispersion, the use of vegetation cover in the previous crop, with an abundance of straw in the soybean crop, constitutes a physical layer, making it difficult for ascospores to disperse (Figure 4).
Monitoring the crop is essential, as it allows understanding of the disease's progress, and, therefore, preventive management actions that can be taken. Biological control, on the other hand, is a strategy that, when well used, presents excellent results. For the control of this pathogen by means of antagonists, the preferred form of application is spraying in the initial phase of culture development, considering the sclerotia as a biological target, which are colonized and degraded (Figure 5).
However, several farmers also apply the antagonists after the crop is harvested, generating a positive effect in reducing the initial inoculum for the next crop. The use of biological agents in the system is also a great alternative, regardless of the host, as they reduce the inoculum source over time, and, consequently, reduce the incidence of the disease in host cultures.
However, several farmers also apply the antagonists after the crop is harvested, generating a positive effect in reducing the initial inoculum for the next crop. The use of biological agents in the system is also a great alternative, regardless of the host, as they reduce the inoculum source over time, and, consequently, reduce the incidence of the disease in host cultures.
For the efficiency of biological control, application in humid environmental conditions is necessary for the establishment of biological agents. In this sense, the presence of straw has favored the establishment of biologicals, and, with that, improving their efficiency. In addition, the association of cover crops, mix of species, in a crop prior to soybean, has also favored soil biota and improved biocontrol.
In short, the management of white mold requires the joint adoption of cultural measures, the use of fungicides and biological control agents, with the objective of preventing and controlling plant diseases.
Cultural measures should focus on reducing the amount of inoculum, hindering the germination of sclerotia and the dispersion of ascospores, essential factors for reducing the incidence of white mold in the field.
Biological control should advocate biological agents positioned so that they reach the biological target, in this case the sclerotia, as well as in environmental conditions that allow their stabilization.
Chemical control, despite its limited efficiency, must be maintained under conditions of high disease pressure, and can only be completely replaced after the continued use of biologicals, reduction of initial inoculum and low disease incidence.
More information on the management of white mold can be obtained in chapter 18 of the book "Bioinsumos na Cultura da Soja".
