Redwood Tree Dieback
There are several reasons why redwood trees suffer dieback. These forest trees are adapted to grow in a cool environment with lots of water and dead leaves to cover their roots. When they’re cultivated in hot inland valleys, they may not grow as well. Disease, animal pests and other cultural practices also cause problems. Identifying symptoms is the first step to finding a solution to tree dieback. Does this Spark an idea?
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Cultural Practices
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Gardeners like to plant redwoods with shrubs and ground cover plantings, and sometimes they prune the lower branches on their redwood trees to make room for other plants. Pruned trees are exposed to more sun and higher temperatures, which they don’t like. In addition, nearby plants compete for moisture. For vigorous growth, provide redwoods with a thick layer of mulch and don’t prune their lower branches. Avoid planting trees on south facing slopes because those slopes are hotter. Instead, plant them on northern slopes. Avoid walking on the ground around redwoods because most of their roots grow in the top 12 inches of soil, leaving them susceptible to soil compaction.
Canker
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Dothiorella canker is a fungal disease that causes the twigs, branches and crown of redwood trees to die. Cankers are flattened or sunken areas of dead bark that are sometimes surrounded by callus tissue, and they appear on the branches or trunk of trees. The cankers grow and may eventually encircle an entire branch. Wood under the cankers is discolored. Dothiorella spores are spread by splashing rain or wind, and they infect a tree through wounds or through pores in the stem. Other cankers that infect redwoods include cytospora and seridium. Control cankers by pruning infected branches and disposing of them. Thin groves to avoid crowding. Provide water during dry weather to maintain tree vigor. Avoid fertilizing since it may make the problem worse.
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Phytophthora
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Phytophthora cinnamomi is a root rot disease that is sometimes confused with dothiorella. However, instead of causing some branches to dieback, phytophthora causes the entire tree to turn brown. Infected trees can die in just three to eight weeks, although many trees survive for several years. Symptoms of phytophthora include the appearance of water-soaked lesions and large brown spots, which may be surrounded by a yellow halo. Leaves wilt and turn brown. Cankers may appear on twigs and branches, which then die back. Shoots wilt and die and stem lesions appear. A dark red, amber or black liquid oozes from wet spots on the stem, and these spots attract beetles. Lesions also appear on the root collar and they decay. As of 2010, the only treatment for phytophthora was to quarantine infected trees. A buffer zone of 300 feet is maintained around infected trees.
Armillaria
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Armillaria is a fungal disease that also causes root rot in redwood trees, along with dieback of branches. It’s easily identified by the mats of creamy white fungal threads that grow under the bark. Armillaria usually infects trees that are weakened by other diseases, by insects or by drought. Symptoms include yellowing or browning of foliage and premature leaf drop, along with dieback of twigs and branches. A fermenting watery liquid or resin may also ooze out of the lower trunk of severely infected trees. Small clusters of honey-colored mushrooms appear around the base of the trunk in the autumn. Trees with mild infections may be temporarily saved by exposing the root collar and buttress roots to dry air between mid-spring and late autumn. Remove infected bark and wood on large roots and the trunk and burn them.
Other
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Redwood trees also suffer dieback when they’re exposed to salt found in coastal winds, or when they’re suffering from a lack of moisture due to drought. Several animals can also cause problems. Bears shred tree bark, which can cause dead tops or even kill entire trees, if the damage is severe enough. Other problems are caused by squirrels, wood rats and porcupines that chew the bark, interrupting the flow of nutrients to the branches.
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References
- University of California Cooperative Extension; What's Up with the Redwoods?; James Downer
- Urban Forests EcoSystems Institute; Redwood Canker; Bruce W. Hagen
- Navarro Watershed Working Group; Redwood Top Die-back; Thembi Borras; July 2005
- Urban Forestry South; Diagnosis of Phytophthora ramorum in Trees; Kim D. Coder; April 2004
- Illinois University Integrated Pest Management; Armillaria Root Rot of Trees and Shrubs; March 2000
- California Oak Mortality Task Force: October 2010 Report
Resources
- Photo Credit Comstock/Comstock/Getty Images
