Ergosterol concentration and variability in genotype-by-pathogen interaction for grain mold resistance in sorghum

Abstract

A lack of understanding of host-by-pathogen relations can hinder the success of breeding for resistance to a major disease. Fungal strain pathogenicity has to be understood from the virulence it can cause on susceptible genotypes and host resistance indicates which genotypes have resistance genes. Where the two worlds meet lies the place where researchers match the prevalent pathogen in the area of production with resistant varieties. This paper uses ergosterol concentration analysis as a measure of fungal biomass accumulation to assess levels of resistance in host genotypes. 11 sorghum genotypes were inoculated with 5 strains of fungi that are known to be associated with grain mold disease of sorghum. The resulting interaction was analyzed using GGE Biplot analysis and Cluster analysis which showed that none of the genotypes were resistant to Phoma sorghina and Curvularia lunata. Three genotypes were resistant to Fusarium thapsinum. One fungal strain (Alternaria alternata) does not contribute any significant damage in the grain mold disease. Fusarium graminearum causes very little grain mold disease. There was no correlation between the fungal strains. Visual scoring did not correlate with ergosterol accumulation. Resistance to grain mold in sorghum is shown to be due to vertical or specific resistance genes. Sorghum breeders should, therefore, identify predominant fungal strains in their localities and then locate and tag these resistance genes in their germplasm and pyramid them in commercial varieties.

Fig. 1

Mean ergosterol concentration across 11 genotypes inoculated with Fusarium graminearum, Fusarium thapsinum, Curvularia lunata, Phoma sorghina, Alternaria alternata and a control in the greenhouse at Bloemfontein, South Africa in 2006

Fig. 2

Separation of means for the fungal strains (Fusarium graminearum, Fusarium thapsinum, Curvularia lunata, Phoma sorghina and Alternaria alternata) plus a control over the 11 genotypes

Fig. 3

Pathogen strain-focused (genotype-centered) GGE biplot based on mean ergosterol concentration (μg/g of grain) of 11 sorghum varieties inoculated with spores of 5 fungal isolates (Fusarium graminearum, Fusarium thapsinum, Curvularia lunata, Phoma sorghina and Alternaria alternata) plus a control. The fungal isolates were used as entries (blue) and genotypes as testers (red numeric)

Fig. 4

Dendrogram showing clusters of fungal isolate groups (Fusarium graminearum, Fusarium thapsinum, Curvularia lunata, Phoma sorghina and Alternaria alternata) plus a control based on their virulence to the 11 sorghum genotypes

Fig. 5

Relationship among the 5 fungal strains (Fusarium graminearum, Fusarium thapsinum, Curvularia lunata, Phoma sorghina and Alternaria alternata) plus a control according to their virulence to the 11 genotypes

Fig. 6

Ranking of the 5 fungal strains (Fusarium graminearum, Fusarium thapsinum, Curvularia lunata, Phoma sorghina and Alternaria alternata) against the control according to their virulence to the 11 genotypes

Fig. 7

Separation of means for ergosterol concentration for the 11 genotypes for all pathogens

Fig. 8

The genotype coordination view to rank genotypes relative to a typical stable genotype (center of the concentric circles)

Fig. 9

Genotype-focused GGE biplot based on mean ergosterol concentration (μg/g of grain) of 11 sorghum varieties inoculated with spores of 5 fungal isolates (Fusarium graminearum, Fusarium thapsinum, Curvularia lunata, Phoma sorghina and Alternaria alternata) plus a control. The genotypes were used as entries (blue numeric) and fungal strains as testers (red)

Fig. 10

Dendrogram showing clusters of genotypes based on their differential resistance/susceptibility to the five fungal isolates