Identification and Characterization of Cultivated Sunflower Lines with Basal Stalk Rot Resistance Against Diverse Sclerotinia sclerotiorum Isolates

Basal stalk rot (BSR) of cultivated sunflower ( Helianthus annuus) is caused by the necrotrophic fungal pathogen Sclerotinia sclerotiorum. This disease is economically significant and limits sunflower production in the northern Great Plains region of the United States. Resistance to BSR is quantitative, controlled by many genes exerting small effects on the level of resistance. This genetic complexity hinders efforts to develop sunflower hybrids with sufficient resistance. Field-based disease trials have successfully identified cultivated sunflower genotypes with partial BSR resistance but failed to determine the degree of resistance. Consequently, the objectives of this study were to (i) distinguish between highly and moderately resistant genotypes by reevaluating 60 cultivated sunflower genotypes exhibiting partial resistance in field trials using a newly developed greenhouse inoculation method with improved resolution; (ii) determine if selected genotypes identified in objective (i) are broadly resistant to diverse S. sclerotiorum isolates; (iii) assess potential host genotype–pathogen isolate interactions between sunflower genotypes and S. sclerotiorum isolates; and (iv) determine if resistant genotypes carry alleles of molecular markers previously associated with BSR resistance loci and assess the frequency of these alleles in resistant germplasm. The results of this study identified sunflower inbred lines HA 124 and HIR 34 exhibiting high levels of BSR resistance against all tested S. sclerotiorum isolates. Additionally, significant host genotype–pathogen isolate interactions were detected between sunflower lines and S. sclerotiorum isolates. This information will guide breeding efforts to improve BSR resistance and facilitate prioritizing highly resistant lines for genetic mapping and further characterization.