Distribution and virulence of Orobanche cumana Wallr. in sunflower weed communities of northeastern Croatia

Orobanche cumana Wallr. is a parasitic angiosperm distributed in many sunflower-producing countries worldwide, where it can cause a severe yield loss. Field surveys in sunflower fields during July-August in 2017, 2019, and 2021 were conducted in the northeastern part of Croatia to investigate broomrape attack rate, weed community where broomrape was present, and weed management strategies used by the farmers. Although, it was observed the difference between the morphometric characteristics of broomrape in the investigated area (Baranja, Syrmia, and Slavonia), the most concerning fact is a trend of increasing attack rate from 2017 to 2021, recorded in the whole region. Weed communities that co-exist with broomrape consist of a total of 31 species, where late spring broadleaf weeds predominate (58% of the total weed infestation), followed by the late spring grass weeds (16%). The most abundant was Ambrosia artemisiifolia followed by Chenopodium album, Xanthium strumarium, Setaria viridis, Cynodon dactylon, Sorghum halepense, Convolvulus arvensis and Orobanche cumana. No phytosociological affinities were observed between broomrape and weed communities in sunflowers. The full CCA model explained 82.81%, while the first two axes explained 55.92% of the total variation in community composition. The most important predictors were herbicide and sunflower hybrid choice, while preceding crops, sunflower sowing rate and broomrape attack rate accounted for significantly less variance.