Reaction of soybean cultivars to sudden death syndrome and disease scoring methods for screening resistance

Abstract

Soybean sudden death syndrome (SDS), caused by Fusarium solani f. sp. glycines, is found in more than 2.0 million hectares of soybean fields in Brazil. Identified for the first time during the 1981/82 crop season, in São Gotardo, Minas Gerais; it had already spread to 99 counties in Central and South Brazil by the 1999/2000 crop season, causing an estimated yield loss of US$ 53 million. Since no efficient control measure is yet available , it has been carried out through resistant cultivars. One of the difficulties in selecting resistant cultivars lies on the lack of a reliable source of resistance and of a screening method to distinguish the reactions among cultivars. The objective of this study was to define a criterium for differentiating soybean cultivars reaction to SDS and to find sources of resistance to the disease, based on leaf symptoms. The study included eight soybean cultivars in four replication and were carried out in a greenhouse in two experiments. Plants were inoculated by the colonized toothpick method and assessed 21 days after inoculation , using five different assessment criteria. Results showed that during the determination of the AP % (affected plants percentile ), which revealed the different reactions to SDS among soybean cultivars, the levels of chlorosis were not significantly different from those which took into consideration the incidence of leaf chlorosis or necrosis, regardless of the severity of leaf symptoms. The %AP and the scoring method using a scale of 1 to 5 were the best procedures to assess the reaction of soybean cultivars to SDS through leaf symptoms. Cultivar FT Estrela was used as a highly susceptible standard SDS (%AP = 96.32%). Genotypes PI 567734, PI 520733 and MG/BR 46 (Conquista) were the most resistant to SDS with AP % of 30.79%, 31.30% and 35.34%, respectively. They could be used as a source of SDS resistance in crosses in breeding programs.

Screening Method for Distinguishing Soybean Resistance to Fusarium virguliforme in Resistant × Resistant Crosses

“…Ranking of the eight genotypes also correlated well with the field data, showing that the method could be used to predict SDS foliar reaction in the field. Similar correlations between greenhouse and field phenotypic results have also been observed by others using a variety of assays (de Farias Neto et al, 2008; Gray and Achenbach, 1996; Hashmi et al, 2005; Klingelfuss et al, 2002; Li et al, 1999; Mueller et al, 2002; Njiti et al, 2001; Stephens et al, 1993b). While the genotype ranking does not match up identically with field ratings, the observed rank changes are not of concern because the genotypes known to be resistant still performed the best.…”

“…Greenhouse and growth chamber assays are also commonly used to evaluate soybean germplasm for resistance to SDS (de Farias Neto et al, 2008; Hashmi et al, 2005). An array of different inoculation methods have been developed, such as soil amendments with oat ( Avena sativa L.) seeds (Stephens et al, 1993b; Melgar and Roy, 1994), sorghum seeds (Hartman et al, 1997; Mueller et al, 2002), cornmeal ( Zea mays L.) (Gray and Achenbach, 1996; Njiti et al, 2001), and toothpicks (Klingelfuss et al, 2002; Melgar and Roy, 1994) infested with F. virguliforme . In some methods, inoculum is homogeneously mixed with the plant growing medium and then applied in a layer below the seeds at planting (Hashmi et al, 2005; Njiti et al, 2001).…”

Inoculation methods under greenhouse conditions for evaluating soybean resistance to sudden death syndrome

“…Researchers have evaluated SDS resistance under greenhouse conditions by inoculating plants with FSG infested oat (Stephens et al, 1993) or sorghum seeds (Hartman et al, 1997;Mueller, 2001), cornmeal (Njiti et al, 2001) or toothpicks (Klingelfuss et al, 2002). Stephens et al (1993) evaluated the SDS reactions of 12 soybean cultivars, infected with FSG infested oat seeds, in pots, in a greenhouse and in the fi eld, under natural and under inoculation conditions.…”

Métodos de inoculação para quantificação de resistência em soja a Fusarium solani f. sp. glycines, em casa-de-vegetação

“…Em todas as leituras, foi observada diferença significativa na severidade de doença entre as cultivares Conquista e FT Estrela (Figura 1), confirmando assim, a diferença no nível de resistência à SMS entre esses dois genótipos (Klingelfuss et al, 2002). Foi observada maior diferença entre as cultivares e entre os tratamentos nas avaliações realizadas aos 18 e 21 dae, no primeiro e segundo experimentos, respectivamente (Figuras 1 e 2).…”

“…Cada parcela foi constituída de um vaso plástico, contendo aproximadamente 2 kg de substrato, composto por solo previamente desinfestado, areia e matéria orgânica na proporção 2:1:1. Em cada vaso, foram colocadas duas sementes da cultivar Conquista (moderadamente resistente) e duas da cultivar FT Estrela, considerada como padrão de susceptibilidade (Klingelfuss et al, 2002). Após a germinação, fez-se o desbaste, deixando uma planta de cada cultivar.…”