A homeowner in my neighborhood here in Napa contacted me with a concern about an evergreen tree in his front yard. He prizes it for the screening it provides and for its year-round greenery. The tree grew rapidly and now, about 15 years after planting, it has achieved considerable size. His concern was about a few branches fading to brown. I recognized the tree as a Leyland cypress, and it was showing symptoms of a notorious and predictable, lethal fungal infection.

I had been wondering for a long time when someone would call me about this problem. I could see it coming after the new homes were built near us and the landscapes were planted with Leyland cypress trees in a number of the front yards. I knew it was just a matter of a few years before they would start to die off. The tree is inherently susceptible to coryneum canker, a fungal disease pretty much guaranteed to kill Leyland cypress trees here in the Napa climate.

According to “Diseases of Trees and Shrubs” (Sinclair, Lyon, Johnson), the disease is also called cypress canker, technically Seiridium cardinale, “…one of the famous examples of a disease innocuous to a tree species in the natural forest but devastating to the same species planted beyond its natural range.”

Leyland cypress is not a naturally occurring forest tree but both parents are native to moist coastal climates. It is a hybrid of trees from two different genera: Nootka cypress (Chamaecyparis nootkaensis), and Monterey cypress (Hesperocyparis macrocarpa) giving it the botanical name: X Cuprocyparis or X Cupressocyparis leylandii, depending on what reference books you check. The main points are: 1. The tree needs plenty of moisture. 2. The fungal disease develops more aggressively where there are prolonged dry periods with high daytime temperatures in summer.

With a quick search I found Leyland cypress originated as an unexpected hybrid. Seedlings were collected at an arboretum in Wales in the 1880s. Their beauty, rapid growth and ease of propagation led to them being planted widely in England, Europe, and in the US from Florida up to the Massachusetts coast.

They were quite popular in the nursery trade here on the West Coast back when I was in retail nursery work in Napa in the late 70s, but over the years, experience showed they are not suited to warmer, dry summer locations. They grow quickly for a few years, then about the time they begin to provide substantial screening and shade, they start to die of the disease.

Surprisingly, cypress canker may also infect and kill Italian cypress. Personally, the only case I recall seeing was in a row of young Italian cypress trees that mistakenly had the irrigation interrupted in summer.

One pest control applicator told me he had good control of the disease with systemic fungicide treatments. That could provide some hope for a property owner who is committed to saving sick trees.

This disease and a different disease with similar symptoms, commonly found locally in Giant Sequoias and occasionally in coast redwoods, is sometimes misdiagnosed as a borer infestation. One of the symptoms is a flow of resinous pitch on the trunk and affected branches. That can be the tree’s response to any wound. Borers may attack infected and stressed trees, but the underlying problem is the fungal infection fostered by stress. In that case, a topical insecticide spray against borer infestation is probably worthless. The money would be better spent on irrigation.

There are some alternatives for evergreen conifers for screening in our region, but the dark green ones are few. One local favorite is Thuja Emerald Green. Another is Thuja Green Giant. Both need permanent irrigation through dry periods but at least they are more resistant to cypress canker.

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Bill Pramuk is a registered consulting arborist. Visit his website billpramuk.com, email questions to info@billpramuk.com or call him at 707-363-0114.