Parasitic Stress Dynamics in a Novel Host: Responses of Arctium lappa L. to Multiple Races of Orobanche cumana Wallr.

Sunflower broomrape (Orobanche cumana Wallr. var. helianthi) is traditionally regarded as strictly host-specific, parasitizing cultivated sunflower (Helianthus annuus L.). Arctium lappa L. (burdock) was recently identified as a novel susceptible host, but whether this susceptibility is uniform across different sunflower O. cumana races remained unknown. Here, three naturally occurring A. lappa populations from north-eastern Bulgaria and a susceptible sunflower inbred line (AD-66) were challenged under controlled glasshouse conditions with three O. cumana races of increasing virulence (F, G, and G+/H), and tubercle number was recorded 45 days after inoculation (n = 50 plants per host × race combination). Race F was largely unable to establish infection on A. lappa (0–4% infected plants) while remaining highly infective on sunflower (96%), whereas races G and G+/H retained substantial infectivity on A. lappa (68–96%), though with up to two-to-three-fold lower mean tubercle numbers than on sunflower. A negative binomial generalized linear model showed that race accounted for the largest share of explained variation in tubercle number, with weaker and less robust host and host × race effects. Additive Main Effects and Multiplicative Interaction (AMMI) and genotype plus genotype-by-environment (GGE) biplot analyses further resolved this interaction into a single dominant axis, along which A. lappa was disproportionately susceptible to race G+/H and disproportionately resistant to race F. These results demonstrate that, under controlled glasshouse conditions, A. lappa is a race-dependent, alternative host for O. cumana. Whether this translates into a field-relevant reservoir capacity beyond the sunflower cropping cycle remains to be established.