The arborvitae is a cold-loving evergreen that thrives mostly in the cooler northern states of the U.S. This member of the cypress family produces soft, rich-green needle foliage that grows into a dense canopy. It is a vigorous grower that grows well in moist, well-drained soil environments that provide partially shaded, afternoon sunlight. Though hardy, the arborvitae is susceptible to several diseases that are relatively harmless when treated early.
Cypress canker is a disease that is most prevalent during the cool, rainy periods. This fungal disease travels by spores that are swept onto the tree, mostly by water splashes and wind. The spores infect the bark and woody areas of the arborvitae, killing the tree’s tissue as it germinates. Infected arborvitaes will display a fading or lightening of color with yellowing of the foliage. Cankers will develop on or near the areas where this fungus entered the tree’s system. The cankers are accompanied by a flow of resin. Chemical treatments are mostly ineffective as a cure but are successful when used as a preventative measure. Infected areas must be pruned from the arborvitae with sharp, sterile pruning shears that are sterilized in between each cut. This eliminates the potential of spreading the disease during the pruning process.
The arborvitae’s young foliage and developing shoots are the target of phomopsis blight or shoot blight. This fungal disease, transported by spores, is most active during the wet, spring months. The infected foliage and shoots become blighted with yellowing and browning needles that take on a singed appearance. The fungal spores on these discolored areas produce small, black pockets of fungal spores. If not pruned away, these spores will lie dormant on the foliage, only to infect the following year’s new foliage. The older needles of this evergreen are not susceptible to phomopsis blight. Infected areas should be pruned from the arborvitae to prevent continued outbreaks. The interior of the arborvitae’s dense canopy should be thinned, at least, annually to promote increased sunlight and air circulation throughout, which will reduce the potential for disease and promote healthy cell development.
Like many fungal diseases, botrytis blight is most concerning during spring’s cool, rainy season. These fungal spores lie dormant in the arborvitae’s dead tissue and are transported to its healthy areas during the rainy season. This blight causes the arborvitae’s needles to brown and wilt. The infected needles develop a grayish-colored mold on its surfaces that moves like dust when disturbed. Fungicidal applications are effective when applied thoroughly and combined with proper pruning. Dead, infected and stunted areas should be trimmed away from the tree when the tree is dry. You may have to wait several days after a long rainy period before you can complete the pruning.
Pestalotiopsis blight is a fungal disease that only infects relatively weak and ailing foliage. In most cases, the arborvitae will have pre-existing conditions and diseases before it is infected with pestalotiopsis blight. Similar to other blight diseases, pestalotiopsis causes the foliage to discolor, from yellow to dark browns and blacks. Pestalotiopsis can pass through the infected foliage and infect the supporting stems, as well. Pestalotiopsis blight infections can be prevented by routine care. Infected areas should be removed and discarded in order to remove cross-infections.