<p><i>Loranthus europaeus</i> is a hemiparasitic plant that grows on oaks, receiving water and nutrients from the sap of the host trees. Over the past decade, this mistletoe, once found in isolated pockets within the Zagros forests, has increasingly threatened the region's oak tree populations. This research compares the radial growth and xylem vascular structure of branches from mistletoe-infested and healthy oak trees to elucidate the impact of parasitism on tree growth from 2002 to 2021. For infested trees, the period from 2002 to 2010 was characterized as a transitional period between normal and infested growth, and was used to identify xylem traits that predispose oak trees to mistletoe attack. This analysis revealed that vessel density (VD) is a more reliable indicator of oak susceptibility than ring width (RW) measurements, as later infested trees had higher VD than healthy trees. Since 2011, mistletoe-infested trees have exhibited reduced RW and vessel lumen area but an increased percentage of vessel lumen area in the xylem and VD compared to healthy trees. During the last 11&#xa0;years, the RW and VD of infested trees were reduced by 29.6% and increased by 6.7%, respectively, while the corresponding values in healthy trees showed a 6.6% increase in RW and a 4% decrease in VD. It should be noted that the last decade was characterized by a series of drought periods, which exacerbated the negative effects of the mistletoe. This study highlights the fact that <i>L. europaeus</i> is a significant threat to oak stands in the Zagros forests. Nevertheless, this stress is not given as much attention as other biotic stressors in forest management, mainly due to the gradual reduction of the trees' vitality. Thus, further studies are needed to develop a successful forest management plan to control this important ecologically impactful hemiparasitic plant.</p>