The Leyland cypress, a hybrid of the Monterey cypress (Cupressus macrocarpa) and the Nootka false cypress (Chamaecyparis nootkatensis), is a popular landscape plant because it grows to a height of 70 feet or more, in very short order. Though few diseases seem to bother Leyland cypresses in their native England, Austin Hagen, a plant pathologist at Auburn University, describes two serious fungal infestations that have lately begun striking the trees in the United States.
The fungus Seridium cardinale causes Seridium canker, which emerged in the mid-1920s in California and promptly killed off many Monterey cypresses in that state's Central Valley, where it's hot and arid. By the mid-1980s, the disease was spreading to Leyland cypresses in California. It has since reached the southeastern United States.
Seridium cardinale invades the tree by entering wounds on the trunk or branches. Cankers then form on the tree's twigs, on the supporting branches and then on the main trunk. At that point, the canker turns the foliage on the top or lateral branches of the trees yellow or brown, and then the foliage dies. The cankers exude quite a lot of sticky resin. The disease can kill substantial parts of the tree.
The fungus spreads by sending out spores that reach unaffected trees via splashed water, contaminated garden tools, diseased cuttings and possibly through insect activity. The fungus spreads best during warm, wet periods, but it grows best during hot, dry weather.
Hagan writes that preventative measures such as proper soil preparation, planting, mulching and watering will help keep Leyland cypress trees healthy. Plant your trees in soil amended with generous amounts of compost, so it will drain well. If you're planting Leyland cypresses to make a screen, set them 15 to 20 feet apart, measured from the center of one tree to the next. If the weather gets hot and dry, water the trees every five to seven days, making sure you soak the soil completely each time.
If cankers develop on your trees, strip off and dispose of affected twigs and branches. Clean gardening tools you use on the cypresses thoroughly with anti-bacterial soaps or rubbing alcohol.
Botryosphaeria (Bot) Canker
The fungus Botryosphaeria dothidea causes Botryosphaeria, or Bot canker, which is appearing on Leyland cypress trees in the Deep South. Infected trees display similar symptoms that appear similar to those caused by Seridium canker---the foliage turns yellow or brown and cankers appear at the bases of branches, but Bot cankers don't ooze resin.
The fungus spreads by splashing water from one tree to the next, and it needs water to germinate. It generally attacks trees already suffering from drought, exceptional cold or heat or a combination of the three factors.
Hagan recommends planting Leyland cypress trees in the fall, in holes that are three to four times the size of their root balls. Mix in sawdust or rotted pine bark to amend the soil and set the trees in their holes so that their root balls are either at or just below the level of the adjacent soil. If the soil is too heavy or the area you want to plant the trees in is flood-prone, consider building raised beds for them. Apply plenty of mulch around the trees' roots, to hold moisture in, and be careful not to damage the tree trunks with the lawnmower or other tools, since wounds serve as entry points for the fungus.
Again, if the weather is hot, water the trees every five to seven days until they are soaked. Cut any discolored foliage off the trees immediately and discard it.
At present, arborists don't recommend using fungicides on Leyland cypresses to treat either of these diseases.