Oak Wilt (Ceratocystis fagacearum)

Origin

The exact origin of Oak Wilt fungus (Ceratocystis fagacearum) is not known, however there aretwo hypotheses1. One hypothesis suggests that the pathogen may be native in origin and had only been prevalent at a low level of incidence, until increased disturbances such as development and natural resource extraction created conditions favourable for disease epidemics. Another hypothesis suggests the pathogen may have been introduced with possible origins in Central or South America, however, further research is required to correctly determine the true origin ofCeratocystis fagacearum2. Oak Wild has not yet been detected in the Credit River Watershed.  

Description

Oak Wilt is a vascular disease of oak trees, caused by the fungus Ceratocystis fagacearum,which develops in the outer sapwood of oak trees1.  Infected trees react by developing tylosesand gums which act to dam or block the infection, consequently blocking the flow of nutrients and water, resulting in tree wilting and subsequent death3, 4.

All native species of oak have been found to be susceptible to Oak Wilt, although species in the red oak group (red, black, pin) are more seriously affected3, 5. Red oak group species are usually killed within the first year of becoming infected, whereas white oak group species (white, bur)are more resistantFor trees in the white oak group, fungus distribution within the vascular system is more restricted, giving them a better chance for recovery3, 5, 6.

Once a red oak tree has died, sporulating fungal mats, sometimes referred to as “pressure pads” form under the bark of the dead tree1, 4, 7. The mats attract bark beetles (Nitidulidae) that feed on these mats and pick up spores on their bodies1, 7Bark beetles can then carry and transmit the spores to healthier trees not yet infected by the Oak Wilt fungus (Ceratocystis fagacearum)1, 7.

Fungal mats are only produced on dead red oaks7. The fungal mats are usually produced in late fall and early spring and are viable for 2-3 weeks. 

Oak Wilt fungus (Ceratocystis fagacearum) can also travel below ground. Infected trees can transmit the disease to healthy trees through interconnected roots1. Wood material harbouring fungal mats can spread the pathogen over great distances through human transport, such as moving firewood containing the fungus5.

Signs and Symptoms 

  • Leaves turn dull green, brown or yellow. 
  • Discolouration of leaves progresses from the edge towards the middle. 
  • Premature leaf drop. 
  • White, grey, or black fungal mats, sometimes referred to as “pressure pads”, just under the bark. They sometimes emit a sweet “Juicy Fruit” smell. 
  • Vertical cracks in the bark on trunk and large branches are a result of fungal mats, as they exert outward pressure on the bark1, 45.

Ecological Threat

The introduction of Oak Wilt to Canada could significantly impact the amount of oak seen in the natural landscape. The loss of oaks in urban areas may decrease available shade and may impact property values. Hardwood forest products could be affected, especially within the forest industry and local forest based economies. The loss of native oak species could lead to decline in biodiversity, and reduce habitat and food for wildlife across both urban and forest landscapes4, 1.


Oak wilt aerial photo (Robert L. Anderson, USDA Forest Service, Bugwood.org)

Preventative Strategies

Preventing Oak Wilt from entering and establishing itself in Canada is the best way to protect oak species in Canada1. Currently there is no available cure for Oak Wilt infected trees4, 5. The best approach for infested areas is to reduce infection by means of identifying and removing infected trees, preventing and severing root grafts, and minimizing wounds, especially during the flight period of potential vector insects4, 5It is recommended not to not prune oak trees from April to July, and not to move firewood5Educating others about the signs, symptoms and threat of Oak Wilt to Canada, can help prevent the spread of the fungus1.  

 If you see signs and symptoms of Oak Wilt (including summer leaf drop and sudden die off of oaks), report sightings to: 

  • Invading Species Hotline1-(800)-563-7711 

References:

  1. French, D.W. and W.C.Stienstra, 1980.Oak Wilt. Extension Folder 310 – Revised 1980.Agricultural Extension Service, University of Minnesota. 6 pp.
  1. Canadian Food Inspection Agency.2018.Ceratocystis fagacearum (Oak Wilt) – Fact sheet.”Web. 26 July 2018. http://www.inspection.gc.ca/plants/plant-pests-invasive-species/diseases/oak-wilt/fact-sheet/eng/1325629194844/1325632464641 
  1. ForestInvasives. 2015. “Oak Wilt” Web.26 July 2018.  https://forestinvasives.ca/Meet-the-Species/Pathogens/Oak-Wilt 
  1. Gibbs, J.N and D.W. French, 1980.The transmissionof Oak Wilt. Web. 27 July 2018.https://www.nrs.fs.fed.us/pubs/rp/rp_nc185.pdf 
  1. Juzwiket al. 2008. “The origin of Ceratocystis fagacearum, the Oak Wilt Fungus” Web. 27 July 2018. http://www.tsusinvasives.org/dotAsset/8a9e865d-5e4f-43a2-b0da-b41f95c22509.pdf 
  1. Invasive Species Centre.2018.”Oak Wilt”. Web. 26 July 2018.  https://forestinvasives.ca/Portals/0/oak_wilt_factsheet_2018.pdf?ver=2018-03-02-203911-070 
  2. Davies, C.S, 1992.Environmental Management of Oak Wilt Disease” Web. 30 July 2018. https://link.springer.com/article/10.1007/BF02400071 
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