Leyland cypress and arborvitae are high-risk choices for single-species privacy screens.  In fact, a diversity of species is always the safer choice for privacy screens, but Leyland cypress and arborvitae are more disease- and insect-prone than most.  Recently I submitted Leyland cypress foliage samples from a New Bern property to the Plant Disease and Insect Clinic on campus at NC State University.  I was suspicious of Passalora needle blight, but suspicion and plant pathology are two different things.  The samples were placed in an incubation chamber and after two weeks it was clear that the foliar symptoms were caused by the canker fungus Neofusicoccum (or more familiarly, Botryosphaeria).  According to the Clinic report:  "This is a common disease on Leyland cypress.  Botryosphaeria needle blight/canker most often occurs on plants that are under considerable stress, and an effective control strategy should include keeping the plants growing as vigorously as possible; for example providing adequate water during extended dry periods and avoiding heavy fertilization.  If possible, the dead twigs should be pruned off and removed from the area.  They should be pruned at least two inches below the dead area.  Effective chemical control for this disease is not available."

According to a 2023 article by Keith O'Herrin of the Union County Extension Center, "The best advice in the case of Leyland cypress is to only use them as single, large

specimen trees.  Using this tree as a vegetative screen will only end in heartbreak."  Lest we be accused of unwarranted hostility against Leyland cypress, please note the second image below, of Botryosphaeria damage to an extensive screen of 'Green Giant' arborvitae.