page_title: Oak wilt
section_title: Root Graft or Overland Infection?
hierarchical_section_title: Oak wilt / Management / Root Graft or Overland Infection?
caption_attribution_description: English: Site has been cleaned up of oak wilt infections (present and future) and buckthorn
context_page_description: Oak wilt is a disease affecting oak trees caused by the fungus Bretziella fagacearum. Symptoms vary by tree species but generally consist of leaf discoloration, wilt, defoliation, and death. The fungus spreads from diseased to healthy trees overland by insect or other airborne vectors and underground by naturally grafted tree roots. Management of the disease historically meant preventing infection by not wounding oaks during certain time periods, removing diseased oaks that will produce spores, and breaking root connections with vibratory plows, trenchers, rock saws or hoes. Present methods focus on reducing or eliminating monocultures and restoring ecosystems correctly using soils baseline information. Fungicides are used for preventive treatments in urban areas. Oak wilt poses a danger for oak wood products and shade trees in urban areas.
context_section_description: The first oak in any infection center is always infected from overland spread; additional oaks killed from the disease in that infection center are killed from underground root graft spread.  Before control plans are made, the landowner or land manager (forester, arborist) needs to find out how the infection got there and what type (underground, overland or both) it is.  Root graft infections move slowly, so additional root graft infections may not be seen for 1-3 years.  It is impossible for one oak infected via overland to infect & wilt seventeen additional oaks via root graft transmission, in less than 4 months (see Kerrick, Minnesota oak wilt photo, below).  If one sees more than half dozen oaks wilting during one summer in a new infection with healthy oaks in between means infections started from overland route/storm event, not underground.  In that case, each wilting oak is a separate infection center, not part of one infection center.  One should also look at the site to confirm whether or not it's an overland infection.  Overland infections occur on or near hilltops or other exposed areas, where taller oaks get wounded, then infected, and shorter oaks look healthy.  A spread pattern with no obvious direction usually means multiple airborne/overland infections.  Root graft infections can "fan" out in 360 degree pattern, but tend to head faster in one or two directions because of soil and topography changes and tree species arrangement/diversity. 
    A birds-eye view of an infection center, during the middle of summer, going from oldest to newest based on colors would show: stone dead gray, brown, yellow, orange, green.  So, if one doesn't see these color sequences or, a hub and spoke pattern radiating out, they are probably looking at multiple airborne infections.  Looks can be deceiving because 2-6 large wilting oaks may often have only 1-2 trees left nearby to infect.  Compared-to one oak deep in the woods that is just starting to wilt that can lead-to 2-5 dozen more nearby oaks dying in the next 5 years.  So, proper ground ID & diagnosis is crucial for stopping this disease.  Stopping spread with multiple infections means plowlines will be further out, so natural barriers should be located first.  In the Lake States, a vibratory plow line is installed 54 to 72 inches deep in the ground and 60 - 150 feet away from the diseased oak.  Plowing done correctly ruptures healthy roots that could, in the future, transmit oak wilt to neighboring healthy trees.  Installing plow lines far enough away from diseased oaks is key to preventing root graft disease transmission.  In southern states like Texas, where heavy clay or Caliche soils are prevalent, rocksaws are commonly used in lieu of vibratory plows.  Infected oaks should not be cut down before vibratory plowing because, doing so will speed up oak wilt root graft spread to nearby healthy oaks.  Base of trunk or root flare injections of azole or copper based fungicides can help contain foliar - or top down - infections but are costly and have less success containing root graft - bottom up - transmission of oak wilt.
caption: Oak wilt and buckthorn cleanup on oak savanna in Chisago County, Minnesota autumn/after leaf drop 2007
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