We've received a large number of arborvitae samples at the Plant Clinic in the last few weeks. Most have been affected with Phyllosticta thujae, a fungal needle pathogen. There are numerous Phyllosticta species which cause leaf spots on a wide range of host plants, including many deciduous trees. On arborvitae, this pathogen causes brown needles at the tips of branches.
Phyllosticta refers to the asexual stage of the pathogen. The sexual stage (and the pathogen's correct name) is Guignardia. These pathogens are generally considered weak pathogens, attacking stressed plants and causing cosmetic damage.
Guignardia species produce three types of spores: ascospores (sexual spores), conidia (asexual spores), and spermatia (male sex cells). We most often see the conidia in the lab. On this sample, we found both the conidia (clear, globose spores, identified with a blue arrow in the first picture) and spermatia (small, thin spores which remind me of femurs, identified with the yellow arrow in the second picture).
Because diseases caused by Guignardia and Phyllosticta usually pose little threat to the overall well-being of healthy plants, chemical management is not always needed. If the cosmetic damage is problematic, a general foliar fungicide may be used. Always follow the label of any product you choose to apply. Removing diseased tissue during dry weather can help reduce the amount of inoculum in the environment. Reducing stress on the affected plants by light mulching, regular watering, and fertilizing when appropriate is often beneficial to increase plant vigor.