Control of white mold in soybeans

White mold, caused by Sclerotinia sclerotiorum, is a serious disease in several crops, capable of causing losses of up to 70% in soybean crops. Spread often occurs through the infected seed itself, so detecting the presence of mycelium in these materials, possible through the help of a specialized laboratory, is essential to prevent damage.

The disease known as white mold, caused by the fungus Sclerotinia sclerotiorum, is responsible for significant losses in numerous agricultural crops of economic importance in the world. Damage varies according to crop susceptibility levels, climatic conditions and the management adopted. The first report of the disease in Brazil was in 1921, in potato cultivation. Currently, the pathogen affects beans, soybeans, sunflower, cotton, canola, potatoes, tomatoes and lettuce in several producing regions of the country. White mold is a disease caused by the fungus Sclerotinia sclerotiorum and, according to the results of cooperative chemical control trials over the last six harvests, it causes average reductions of 21% in soybean productivity, reaching up to 70% in some isolated crops. According to surveys carried out by the National Supply Company (Conab) in the 2015/16 harvest, and considering the regions with the highest incidence of the disease in the country, it is estimated that the soybean production area infested by S. sclerotiorum is approximately 7,7 million hectares. As white mold is a disease that affects several other crops, it is of fundamental importance to observe the sequence of crops in the succession of crops in areas infested with the fungus, interspersing non-host crops as a strategy to reduce the inoculum of the pathogen. Other white mold management measures must also be adopted in an integrated manner, such as the use of good quality seeds treated with appropriate fungicides; formation of straw for uniform soil coverage, preferably from grasses; the choice of cultivars with plant architecture that favors good aeration between plants (few branched and with small leaves) and with a shorter flowering period; plant population and row spacing suitable for cultivars; use of chemical control, through foliar sprays, mainly during the period of greatest susceptibility of the soybean plant (beginning of flowering until the beginning of pod formation); use of biological control, through the propagation of antagonistic agents in the soil; and cleaning machines and equipment after use in an infested area to prevent the spread of sclerotia (JULIATTI et al.2010).

The spread of white mold in crops often occurs through the infected seed itself. Although sclerotia are easily detected in seed lots, the mycelium is unlikely to be detected without the help of a specialized laboratory.

The criteria used to detect fungi in seeds, in general, consist of stimulating microorganisms to produce structures, or metabolites, that allow their identification. For the fungus S. sclerotiorum, visual observation of the presence of sclerotia is made using the filter paper test (blotter test). The Neon-S method has also been used to detect this pathogen in soybean and bean seeds since the 2010 harvest. This method consists of adding a pH indicator (bromophenol blue) to the culture medium. and is based on the formation of a yellow halo around the seed due to the presence of oxalic acid produced by the fungus, which acidifies the environment, changing its color. However, this method can lead to false positive readings due to the appearance of other fungi that can also alter the pH of the medium. In this sense, several works have been developed and/or methodologies adjusted in order to minimize some problems in tests already established as standard. Alternative methods such as the modified paper roll and the modified Neon-S method have shown promise for safer and faster diagnosis of the pathogen.

637 samples/lots of soybean and bean seeds, from the Triângulo Mineiro and Alto Paranaíba (Figure 1), covering the period from 2008 to 2012. The objective was to verify whether increasing the incubation period could contribute to optimizing the reliability of the Neon-S test.

Three detection methods were compared: paper roll, Neon-S and Neon-S2 (modified Neon-S test, changing the reading time from seven to 15 days). Seeds from the Pérola bean cultivar and the soybean cultivars BRS Valiosa RR and Nidera 7255 RR were used. The seed lots were naturally infected, with the exception of the cultivar Nidera 7255 RR, which had its seeds exposed to infection by the pathogen in Petri dishes for 72 hours.

The paper roll technique did not allow detection of the presence of white mold in soybean seeds collected in 2008 and, for other crops, it presented a lower capacity for detecting the pathogen compared to Neon-S2 (21,88 and 31,25 % of positive samples, respectively). Despite the detection sensitivity of S. sclerotiorum in seed lots was higher for the Neon-S2 method in relation to the roller method, the difference was not significant when compared with the Neon S method, although numerically the incidence percentage of the modified method was higher (Figure 2). Still, detection of S. sclerotiorum using the Neon-S2 method is more reliable due to incubation for 15 days, since the formation of sclerotia, during this period, close to the infected seeds, confirms the presence of the pathogen and avoids false-positive readings.

The importance of this detection method implies better property planning, such as choosing seed lots free from the pathogen and directing a more appropriate seed treatment including specific fungicides for the fungus. Sclerotinia sclerotiorum. This way, the farmer will be able to choose the best treatment and the best batch of seeds for each situation, avoiding future crop failure or contamination of an area free from the pathogen and disease. If this strategy had been used in Brazil in recent years, there would certainly have been a smaller area contaminated with the fungus, which covers more than 7,7 million hectares. The network of laboratories that provide this type of service is small, but it is worth evaluating seed lots before purchasing if the service is available in the region. Highlighting the importance of checking seed quality and that samples must be representative, a standard sample must have an aliquot of one kilo, packed in paper bags. LAMIP UFU is able to carry out detection tests for Sclerotinia sclerotiorum and issuing reports proving the sanitary quality of the main crops grown in Brazil. To do so, it is necessary to send identified lots to Avenida Amazonas S/N, Bloco 2E, room 106, Jardim Umuarama, Uberlândia, Minas Gerais, CEP 38400 - 902. The cost per sample is around R$100,00.  

Fernando Cezar Juliatti, Roberto Resende dos Santos and Tâmara Prado de Morais, Lamip - Univ. Uberlândia Fed

Article published in issue 213 of Cultivar Grandes Culturas.