How to Prevent Your Arizona Cypress Trees from Rotting

Arizona cypress trees (Cupressus arizonica) are evergreen conifers that are native to the southwestern United States. With their attractive blue-green foliage, these trees make a beautiful addition to landscapes in USDA zones 7-9 However, they do have one drawback – they are prone to developing a fungal disease called cypress canker This disease leads to rotting of the wood, causing dieback and even tree death. In this article, you’ll learn what causes cypress canker and how to prevent your Arizona cypress trees from rotting.

What Causes Cypress Canker?

Cypress canker is caused by several species of fungus in the genus Seiridium. The most common is Seiridium cardinale. The fungus enters the tree through wounds or natural openings and grows in the sapwood. As it spreads it kills sections of tissue, interrupting the flow of water and nutrients. This leads to canopy dieback rotting wood, and eventual death of the tree if the disease is left uncontrolled.

The fungus thrives in wet conditions. Anything that increases moisture around the trunk or causes water to accumulate on the branches sets up prime conditions for infection. Some potential causes include:

• Overhead irrigation spraying the foliage
• Excessive mulch piled against the trunk
• Poor drainage
• High rainfall or high humidity
• Crowded planting areas with poor air circulation

Trees that are stressed or injured are also more vulnerable to the disease. Stress may be caused by drought, high winds, or root damage from construction. Pruning wounds, sunscald injuries, and other mechanical damage also provide entry points for the fungus.

Preventive Care for Arizona Cypress Trees

The best defense is preventing cypress canker in the first place by maintaining good cultural practices:

• Avoid overhead irrigation. This is by far the most important measure. Drip irrigation or a bubbler system is ideal. If using sprinklers, avoid spraying the tree canopy.

• Allow space for air flow. Give trees adequate spacing at planting and prune back encroaching branches from neighboring trees or shrubs.

• Remove thick mulch layers from around the trunk. 2-3 inches is sufficient; less is better. Mulch should be gradually tapered as it approaches the trunk.

• Improve drainage. Make sure trees are not planted in low, wet spots. Consider amending soil with organic matter or constructing a berm to improve drainage.

• Water judiciously. Overwatering encourages fungal growth. Water deeply but infrequently, allowing the soil to partially dry between irrigation.

• Promote tree health. Keep trees vigorous by fertilizing in early spring and pruning out dead branches. Address any signs of stress immediately.

• Disinfect tools. Sanitize pruning tools after each cut by dipping in isopropyl alcohol or a 10% bleach solution.

• Scout for problems. Monitor trees for early symptoms of cypress canker like resin droplets, discolored bark, or thinning foliage. Remove affected branches promptly.

Early Treatment Options

If you detect cypress canker early, treatments may save the tree or at least slow the disease’s progress:

• Improve air flow with selective branch removal. Remove nearby branches that rub or overcrowd the infected tree.

• Prune out infected branches. Remove 6-12” below visible symptoms. Sterilize tools between each cut.

• Apply fungicides. Treatments containing thiophanate methyl or copper compounds may protect uninfected trees or slow canker expansion.

• Water carefully. Avoid further wetting the foliage. Rely on drip irrigation or watering at the trunk to avoid spraying.

• Monitor for renewed symptoms. Look for resin bleeding, new branch dieback, or additional canopy thinning to determine if the disease continues to spread.

When Prevention Fails

If a large portion of the canopy is affected, the outlook for the tree is poor. At this stage, fungicide treatments are unlikely to work. The rotting wood also becomes prone to attack from secondary pests like carpenter ants or wood boring beetles. Unfortunately, removal may become the best option for trees extensively damaged by cypress canker.

If you must remove a diseased Arizona cypress, consider replanting with a more canker-resistant Cupressus species such as ‘Monterrey cypress’ or ‘Italian cypress’. Other good evergreen alternatives include pine, juniper, arborvitae, or Leyland cypress. Be sure to remove as much of the old root system as possible and do not replant immediately in the same location. Proper site preparation and drainage improvements are key to preventing canker in new plantings.

With their statuesque forms and soft, feathery texture, Arizona cypress trees make a striking landscape specimen. But their susceptibility to fungal disease means these trees require careful siting and attentive care. By understanding what conditions lead to cypress canker and adopting sound horticultural practices, you can hopefully avoid the rot and enjoy the beauty of a healthy Arizona cypress on your property for years to come.