Sclerotia morphology traits and mycelial growth rate are not informative variables for population studies of Sclerotinia sclerotiorum

Abstract

The usefulness of sclerotia morphology traits and mycelial growth rate to infer phenotypic variability in populations of Sclerotinia sclerotiorum associated with common beans in Minas Gerais State (MG) was assessed in two experiments. Isolates (131) were collected in the Northwestern, Alto Paranaíba, Zona da Mata and Southern regions of MG and the mycelial growth rate, the number, weight and dimensions (length, width and thickness) of sclerotia and mycelial pigmentation were assessed. There was a high variability among isolates, even among colonies of the same isolate. The variance of the first experiment for the mycelial growth rate, number, weight, length, width and thickness of sclerotia, were 0. 01, 190.15, 0.01, 0.62, 0.12 and 0.05, whereas in the second, the variances were 0.01, 28.48, 0.002, 0.18, 0.06 and 0.03, respectively. Most correlations between traits were of low magnitude except that between length and width (r = 0.84, 'P < 0.001). Regarding colony pigmentation, non-pigmented, pigmented and highly pigmented isolates were identified, but for most isolates this characteristic varied between experiments. No groups based on phenotypic traits were detected. The morphological traits evaluated in the present study are not suitable for characterization of populations of S. sclerotiorum. Key words: Phaseolus vulgaris, sclerotium, phenotypic variability, white mold.Several biological features make the ascomycete Sclerotinia sclerotiorum (Lib.) de Bary a highly threatening fungal pathogen to important crops in Brazil. Sclerotinia sclerotiorum is a homothallic fungus that does not produce conidia, but abundant sclerotia (Bolton et al., 2006). Sclerotia are asexual resting structures that allow pathogen survival when no host is available (Adams & Ayers, 1979). The pathogen has a host range that encompasses over 400 plant species (Boland & Hall 1994). Furthermore, pathogen propagules (sclerotia) can be easily spread over long-distance by contaminated seeds. Sclerotinia sclerotiorum causes white mold in crops such as cotton, soybean, potato, sunflower etc., and is the main soilborne disease affecting common bean (Phaseolus vulgaris L.) in Minas Gerais State (MG), Brazil.In Brazil common bean is produced in the springsummer, summer-fall, and fall-winter growing seasons (Vieira et al., 2010). Considering the three growing seasons, MG is the second largest producer, but ranks first during the fall-winter, with an average yield of 2600 kg ha -1 (CONAB, 2013). In the fall-winter growing season, moderate temperatures and high humidity provided by irrigation create conducive environmental conditions for white mold epidemics (Paula Júnior et al., 2006). Planting white mold resistant cultivar would be attractive alternative for disease management. Nevertheless, to date there are no resistant varieties available to growers. Sound breeding programs require proper understanding of the pathogen variability. The variability of populations of S. sclerotiorum has been investigated for different crops around the world an...

Meta‐analytic modelling of the incidence–yield and incidence–sclerotial production relationships in soybean white mould epidemics

“…Studies on the relationships between white mould incidence and sclerotial production under field conditions are scarce. Studies conducted in vitro for evaluating isolate variability in sclerotial production (Li et al ., ; Lehner et al ., ) may be of limited value to extrapolate to field conditions. Hoffman et al .…”

“…Studies on the relationships between white mould incidence and sclerotial production under field conditions are scarce. Studies conducted in vitro for evaluating isolate variability in sclerotial production (Li et al, 2008;Lehner et al, 2014) may be of limited value to extrapolate to field conditions. Hoffman et al (1998) estimated the number of sclerotia per 300 g of seed across five soybean cultivars, but the disease incidence-sclerotial production relationship was not investigated.…”

European apple canker: morphophysiological variability and pathogenicity in isolates of Neonectria ditissima in southern Brazil

“…Colony diameter was assessed in two different ways every 24h by digital pachymeter by orthogonal dimension and by two perpendicular dimensions. Results were converted to AMG (mm h −1 ) according to LEHNER et al (2014) and MGI according to the adapted formula described by MAGUIRE (1962). Isolates with AMG values ≥60.0mm were assigned as the highest-myceliumgrowth group at the end of the evaluation time.…”

Morphological and Pathogenic Characterization of Sclerotium rolfsii, the Causal Agent of Southern Blight Disease on Common Bean in Uganda

“…Differences in sclerotia number and size among isolates of S. rolfsii have been reported (Le et al 2012;Narayan et al 2017;Xie et al 2014). In a related study, Lehner et al (2014) observed variation in the number of sclerotia produced by isolates in two different experiments using a related fungal species (S. sclerotiorum). Although Lehner et al (2014) and Li et al (2008) found no correlation between mycelial growth rate and the number of sclerotia produced by isolates of S. sclerotiorum, we found that these factors were negatively correlated for isolates of S. rolfsii.…”

“…Morphological and phenotypic diversity of pathogens may be related to variation in virulence that is important in the selection of pathogen isolates used for the evaluation of disease resistance in crop breeding programs. Variability among S. rolfsii isolates and related species such as S. sclerotiorum that infect different crops has been investigated in studies relating to sclerotia morphology traits and mycelial growth rates (Lehner et al 2014;Narayan et al 2017;Vleugels et al 2013), pathogenicity in susceptible host plants, and genetics using molecular markers (Jebaraj et al 2017;Narayan et al 2017).…”