Mistletoe (phoradendron spp.) infest many taxa of plants, often causing the branches of the host species to become swollen and distorted and making the trees more susceptible to insect infestations. this is an example of which symbiotic relationship?

Parasitism

Explanation:

Parasitism is a symbiotic relationship in which the two organisms belonging to different species lives in close association. In this association, one organism receives the benefit and the other is harmed. The organism which receives the benefit is the parasite and the organism which is harmed is the host.

Mistletoe is a parasite for the host plants. This causes the infection to the  branches of the swollen plant species and makes the trees more susceptible to other insects infestation. This is an example of parasitism.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: