Population structure and phenotypic variation of Sclerotinia sclerotiorum from dry bean (Phaseolus vulgaris) in the United States

Abstract

The ascomycete pathogen Sclerotinia sclerotiorum is a necrotrophic pathogen on over 400 known host plants, and is the causal agent of white mold on dry bean. Currently, there are no known cultivars of dry bean with complete resistance to white mold. For more than 20 years, bean breeders have been using white mold screening nurseries with natural populations of S. sclerotiorum to screen new cultivars for resistance. It is thus important to know if the genetic diversity in populations of S. sclerotiorum within these nurseries a) reflect the genetic diversity of the populations in the surrounding region and b) are stable over time. Furthermore, previous studies have investigated the correlation between mycelial compatibility groups (MCG) and multilocus haplotypes (MLH), but none have formally tested these patterns. We genotyped 366 isolates of S. sclerotiorum from producer fields and white mold screening nurseries surveyed over 10 years in 2003-2012 representing 11 states in the United States of America, Australia, France, and Mexico at 11 microsatellite loci resulting in 165 MLHs. Populations were loosely structured over space and time based on analysis of molecular variance and discriminant analysis of principal components, but not by cultivar, aggressiveness, or field source. Of all the regions tested, only Mexico (n=18) shared no MLHs with any other region. Using a bipartite network-based approach, we found no evidence that the MCGs accurately represent MLHs. Our study suggests that breeders should continue to test dry bean lines in several white mold screening nurseries across the US to account for both the phenotypic and genotypic variation that exists across regions.

Selective sweeps in populations of the broad host range plant pathogenic fungusSclerotinia sclerotiorum

“…Several studies have 80 demonstrated genotypic differentiation of S. sclerotiorum populations from different 81 geographic regions. For example, Kamvar et al showed that populations of S. 82 sclerotiorum from Mexico shared no multilocus haplotypes with populations from the 83 United States, Australia and France (26). Similarly, Lehner et al showed that 29 % of 84 variance within a sample of isolates from southeastern Brazil and the United States was 85 explained by differences between populations from the two regions (27).…”

Something in the agar does not compute: On the discriminatory power of mycelial compatibility in Sclerotinia sclerotiorum

“…For instance, a recent study found a total of 15 MCG, but only seven of them were congruent, which may suggest four loci controlling mycelial compatibility segregating in the population (Attanayake et al 2012). However, considering that identical MCG from disparate regions and haplotypes have been found, this explanation does not scale well (Kohli et al 1992;Ford et al 1995;Kamvar et al 2017).…”

“…The conflict between MCG and VCG documented by Ford et al (Ford et al 1995), while perplexing, may serve to explain complex relationships that continue to be observed between MCG and haplotypes (Schafer and Kohn 2006;Attanayake et al 2012;Kamvar et al 2017).…”

“…With a sufficiently large number of loci, the probability for two unrelated strains to converge on the same MCG or haplotype is extremely low (Kohli et al 1992). However, when MCG and haplotypes have been combined to detect clonal lineages in the past, the results revealed a disconnect between MCG and haplotype (Cubeta et al 1997;Atallah et al 2004;Attanayake et al 2012;Kamvar et al 2017).…”

“…The inconsistencies between MCG and haplotypes have been acknowledged throughout the literature (Cubeta et al 1997;Atallah et al 2004;Malvárez et al 2007;Kamvar et al 2017).…”