White mold caused by fungus Sclerotinia sclerotiorum It is currently one of the main diseases of soybean crops, due to the damage caused and the difficulty of control (Furlan, 2009).
This disease affects the productive potential of soybean crops by causing losses that can vary up to 60%, such as those observed in the southwest of Goiás (Silva et al, 2009), however, with the potential to reach 100% in some areas, as described by Jaccoud Filho et al (2010)
The fungus is polyphagous, having as hosts plants from 75 families, 278 genera and 408 species (Boland and Hall, 1994), which include crops with economic potential such as soybeans, beans, cotton, sunflowers, tomatoes, potatoes and tobacco, and weeds. such as picão, carrapicho, caruru and mentrasto. In addition to the wide range of hosts that multiply the fungus, white mold forms resistance structures called sclerotia, which can remain in the soil for several years, keeping their germination power intact, which makes control difficult.
The adoption of isolated measures has not efficiently controlled white mold. According to Balardin (1999), disease control must involve a set of practices, such as the use of resistant cultivars, seed treatment, balanced mineral fertilization, use of fungicides in the aerial part and crop rotation/succession. These strategies, when used in an integrated manner, are highly effective and provide economic returns.
Among these recommendations for managing white mold, practices such as cultural control with the formation of straw for the direct planting system (SPD) can be highlighted. Cultural control by covering the soil with straw has the potential to inhibit the formation of apothecia, as demonstrated in experimental conditions (Ferraz et al, 1999). In an area highly infested by S. sclerotiorum, Görgen et al (2009) found the efficiency of brachiaria straw as a physical barrier to the production of apothecia and considered it as a premise for the biological control of this pathogen in SPD. Straw production is efficient in reducing the population of sclerotia in the soil, which represent the main source of inoculum and a fundamental part of the pathogen's life cycle (Clarkson et al.
Control measures must consider that the severity of white mold on different hosts is, in general, proportional to the pathogen inoculum density in the soil. Therefore, reducing the sclerotia population is essential for effective control of this disease. Furthermore, practices that reduce the formation of apothecia, the release of ascospores and the production of new sclerotia help control the disease. Thus, different ways of preparing the soil can influence the dynamics of white mold, enabling different responses in the incidence of this disease in soybean plants.
The experiment
An experiment was carried out on the Aurora Serios farm, in the municipality of Correntina/Bahia, under field conditions in the 2011/2012 harvest, in an area with a history of incidence of the disease. A randomized block experimental design was adopted, with four treatments and nine replications. The treatments were: T1 - Direct sowing without straw; T2 - Sowing in scarified soil, followed by a harrow; T3 - Sowing in the molded soil, followed by a harrow and T4 - Direct sowing in the brachiaria straw (Brachiaria ruziziensis). The plots consisted of five lines spaced 0,56m and 15 meters long, with the two central lines considered a useful area for data collection. Sowing was carried out with a seeder on 26/10/2011 using 7,2 seeds/m of cultivar M9144RR that were treated with fipronil + pyraclostrobin + methyl thiophanate (200ml/100kg of seeds). Before sowing, the seeds were inoculated with commercial liquid inoculant at a dose of (200ml/100kg of seed). The other cultural treatments adopted for the experimental plots were the same as those applied to soybean cultivation in commercial planting areas, with chemical management of weeds and pests, according to the field survey carried out for these biotic elements and in accordance with the technical recommendations for culture.
The incidence of white mold was evaluated on all plants in the two central rows of the plot, totaling approximately 200 plants 85 days after sowing (phenological stage R3).
The data obtained were subjected to analysis of variance and, when significant, the means were compared using the Tukey test at 5% significance, using the Assistet program.
Table 1 presents the results of the number of plants with white mold symptoms. The number of plants with disease symptoms varied from 0,13 (T4 treatment) to 11,34 (T1 treatment), with all treatments being different from each other.
Table 1 - Average number of plants with white mold symptoms recorded in the different treatments
Treatments | Plants with white mold symptoms | |
T1 | 11,34 | A |
T2 | 1,88 | C |
T3 | 3,33 | B |
T4 | 0,13 | D |
CV% | 17,80 | |
Treatments
Plants with white mold symptoms
T1
11,34
A
T2
1,88
C
T3
3,33
B
T4
0,13
D
CV%
17,80
*Values followed by the same letter in the column do not differ statistically from each other, using the Tukey test at 5% significance.
The lowest number of plants with the disease was recorded when direct sowing was carried out in brachiaria straw (0,13 plants, T4 treatment). Brachiaria straw can inhibit the formation of apothecia (Ferraz et al, 1999), an increase in sclerotia parasitism by pathogens present in the soil and a reduction in the initial inoculum of S. sclerotiorum, as proven by Görgen et al (2009). The straw acts as a physical barrier that reduces light on the soil, which can cause a delay of several weeks in the production of apothecia (Sun and Yang, 2000). This delay could result in a lower number of inoculum at a time when soybeans would be most vulnerable to infection, which according to Danielson et al (2004) is from stage R2 to R5. All of these factors may have contributed to the results obtained in the T4 treatment.
When the soil was scarified, part of the sclerotia may have been buried, which resulted in a reduction in the source of inoculum and, consequently, less infection of soybean plants by the disease (average of 1,88 soybean plants with white mold /plot), as can be seen in treatment T2 (Table 1 and Figure 1).
Figure 1 - Average number of plants with symptoms of S. sclerotiorum in each repetition, in the different treatments
When the moldboard plow was used to prepare the soil (T3 treatment), an average of 3,33 plants with symptoms of white mold were obtained, showing that with the use of this implement, even with the upheaval and inversion of the soil layers, the Plant infection by the disease occurs at values higher than sowing in scarified soil (T2 treatment) and direct sowing in brachiaria straw (T4 treatment).
The treatment where the highest values of plants with symptoms of the disease were found was T1 (direct sowing in the soil without straw), because, without scarification, the use of the moldboard plow and without the “protection” of the straw, there is no physical barrier or impediment to light penetration, among others, and the sclerotia, under ideal conditions of climate and plant phenology, have all the conditions to germinate, produce apothecia and spores that will infect the plant and cause disease in the crop.
Of all the treatments studied, it is concluded that direct sowing into straw contributes to a lower incidence of plants with white mold, and this practice can be part of integrated management to control this disease.
Click here to read the article in Revista Cultivar Grandes Culturas, issue 177.
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Pruning strawberry roots, in the pre-planting stage, is an interesting technique to minimize problems with poorly planted seedlings. However, its adoption requires criteria and must be accompanied by other measures.
