Sudden Oak Death - Phytophthora ramorum

Sudden Oak Death (Phytophthora ramorum) - SOD (also known as Phytophthora canker disease), was originally identified in Germany and The Netherlands in the early 1990's on Rhododendrons .  Since being discovered in the United States, it has been confirmed in forests from California, Oregon, Washington and British Columbia.  The origin geographically of Phytophthora ramorum is unknown and before the early 1990's there were no reports in Europe or the United States.  The areas that do exist in Europe and the United States are believe to have been originally transported from other areas or even the original site of origin.  Phytophthora ramorum's very limited distribution related to the host's distribution suggests a more recent introduction versus a point of origin.  

Image Citation: Joseph O'Brien, USDA Forest Service, Bugwood.org

Two types of disease are caused by Phytophthora ramorum, the first being bark cankers and the second being foliar blights.  Bark cankers may eventually kill the host while foliar blights serve as a reservoir for the pathogen to remain within and be tranferred from the foliar host.  The list of hosts (and foliar hosts) seems to grow with each new report and now includes Coast and Canyon Live Oak, Tanoak, California Black Oak, Coast Redwood, Douglas Fir, Rhododendron, Bay Laurel, California Buckeye, Madrone, Bigleaf Maple, Oregon Myrtle, Toyon, Honeysuckle, Arrowwood, Camellia, Californis Hazelnut, Mountain Laurel, Valley Oak, Poison Oak and Grand Fir.  In lab testing it has been found that both Red and Pin Oaks are susceptible this opens up the potential for spread into the Eastern portions of the US as the Red Oak family is found in most of North America. In the field the White Oak family including the Valley, White and Blue Oaks have not been confirmed as hosts or even shown any symptoms- hopefully this means they are immune to Phytophthora ramorum or at least have a higher tolerance level.

Image Citation: Bruce Moltzan, USDA Forest Service, Bugwood.org

As with many diseases of woody plants the spread of Phytophthora ramorum most likely occurs from contact with foliar hosts, infected material, soil transfer and spreading by rainwater.  Windy, cool and moist conditions are also thought to aide in the spread of the pathogen by further dispersing the spores from their foliar hosts.  Transporting (for nursery sale, wholesale or production) of foliar hosts may also aide in the spread of this disease making it harder to control.  

The symptoms of Sudden Oak Death are easily identified by large cankers on the trunk or main stem, browning of the leaves or even death of the entire plant/tree.  Some infected trees also become host to Bark or Ambrosia Beetles, or Sapwood rotting fungus-these outside organisms may speed up or even contribute to the death of the host.  Foliar host infection os harder to identify and may not be noticed until it is to late.  With a foliar host you may notice deep gray or brown lesions on the leaf blades, vascular tissues, petiole, or stems of the host.

Learn more about Oak trees and their diseases/pests on our Website www.ArundelTreeService.com  or our blog  www.MeetaTree.com 

What is Oak Wilt?

Oak wilt effects all Oak species, but has different results and signs with each group. Ceratocystis fagacearum is the fungal pathogen that causes Oak Wilt.  It is a vascular disease, meaning the fungus is only found in the outermost xylem of the tree. This fungus is thought to be native of the Eastern US but the problems identifying and isolating it delayed the recognition of the true effects of the fungus until the 1980's.  Oak wilt is a very aggressive disease, currently it is one of the most serious tree related diseases in the Eastern US-killing thousands of Oaks each year in both forest and landscape settings.

Oaks in the Red Oak family- such as Black, Northern Red, Northern Pin and others with pointed edged leaves are particularly susceptible and when infected usually die over the course of a single season-some declining to the point of complete defoliation in a matter of weeks.  Infected Red Oaks will begin wilting from the top of the tree down, the leaves will gradually become bronze in color and fall off of the tree.

Oaks in the White Oak family- such as White, Swamp, Bur, or others with rounded edged leaves are less susceptible to Oak wilt and when infected can live for several years, losing only a few branches each season also from the top down.  Symptoms in White Oaks are very similar to that of the Red Oaks.








The Live Oak however is not so predictable, it's infection level and timeline is effected by many other variables in the environment.  Usually however, Live Oaks infected will die within a six month period from the first sign of decline.  Lives Oaks foliar symptoms differ from those of the Red and White Oak.  An infected Live Oaks leaves will develop chlorotic veins that eventually turn necrotic prior to falling from the tree.

Oak Wilt fungus spreads in two basic ways.  Spores can be transferred from an infected tree to a healthy tree by insect movement or The fungus can move from the roots of an infected tree to those of a healthy tree through root grafts.

Oak Wilt is very similar to Dutch Elm disease but considered to be more controllable.   On good factor to consider is the nitidulid Beetles that carry the Oak Wilt Fungus do not have chewing mouth parts, so in turn would need another creature or a damaged portion of the tree to make the transfer.  This is different from the Beetles that carried Dutch Elm disease, they do have chewing mouth parts and are able to enter any tree without help.

When an Oak has died from Oak Wilt, trees should be chipped and then burned or covered with plastic sheeting to speed composting.  The heat from the chips composting should destroy or severely enervate the fungus.  Logs from infected trees should never be moved to unaffected areas, even for use as firewood.

More at www.ArundelTreeService.com or www.MeetATree.com


Downloadable Oak Wilt fact sheets: 

http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_043443.pdf

http://na.fs.fed.us/pubs/howtos/ht_oakwilt/identify_prevent_and_control_oak_wilt_print.pdf


Photos sources:
Joseph O'Brien, USDA Forest Service, bugwood.org (Photos# 1,3,4,6 & 7)
Forestthreats.org with Google Earth overlay (Photo #2)
Wikipedia (Photo #5)