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Evaluating inoculum source, application and timing in screening for resistance to Sclerotinia sclerotiorum on sunflower cultivars

Marlese Bester: University of the Free State

<div>Sunflower head rot, caused by <em>Sclerotinia sclerotiorum,</em> is responsible for economic losses of up to 75%. Resistance screening requires consistent inoculation techniques (IT) that replicate natural field reactions of the host to the pathogen. The aim was to optimize laboratory inoculum production and to evaluate the viability and efficacy of inoculum sources, application methods and timing on disease incidence in field and greenhouse trials. Techniques used were sclerotia planted adjacent to seedlings, spray and milled grain mycelium, ascospore suspension and head punch using <em>S. sclerotiorum</em> colonized grains. Treatments were applied at bud, flower and head ripening growth stages, respectively. Inoculation technique and cultivar main effects were observed but no IT x cultivar interactions were recorded in the field. IT x growth stage interactions were observed in both greenhouse and field trials. An ascospore suspension sprayed at flowering resulted in the highest disease level in the greenhouse. In contrast, spray mycelium at bud stage yielded the highest disease level in the field. Cultivar responses to IT were quantified as percentage head rot incidence (HRI) and regression analysis was used to determine response type and relationship between observed HRI within a cultivar and disease potential. Results highlight the need for multiple considerations when screening for <em>S. sclerotiorum</em> resistance.</div>