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MNLA E-Training, Project Plant

By Carolyn Diamond, Inspector, Region 3
Pesticide and Plant Pest Management Division
Michigan Department of Agriculture and Rural Development

Host and Distribution
A newly identified pathogen has been found on bur oak (Quercus macrocarpa).  Bur oak is also called burr oak, blue oak, mossy-cup oak, mossy-overcup oak, overcup oak, Savannah oak, prairie oak, and scrub oak.  Bur oak is a species in the white oak section.  Its commercially valuable wood is marketed as white oak.  Bur oak has the largest acorns of all the oaks.  These acorns are an important part of the diets of wildlife.  Bur oak, native to North America, is drought tolerant and widely distributed throughout the United States and Canada.

The fungal pathogen is Tubakia iowensis, commonly called bur oak blight (BOB).  There are five known Tubakia spp. that infect bur oak, but only this newly identified one causes severe leaf symptoms, including defoliation that can and does lead to the death of the trees.  The disease occurs mostly on naturally established, mature stands of bur oak trees.  It was found infecting bur oak in the 1990s in the midwestern United States.  Thus far, BOB is known to occur from northeastern Kansas and eastern Nebraska to central Minnesota and southwestern Wisconsin and across most of Iowa.  In 2008, Iowa reported its first occurrence of BOB, and in 2013 reported 87 counties infested with BOB.  BOB is most severe in eastern Nebraska, Iowa and Minnesota, which coincides closely with the distribution of Quercus macrocarpa.  A few trees in Illinois and Missouri have also been identified as infected.

In late July and August, foliar symptoms begin to appear as purple-brown lesions along the midveins and major lateral veins on the underside of the leaves.  Dark veins become noticeable on the upper-side of the leaves and the lesions progress.  On the leaf blade, large, wedge-shaped areas of chlorosis and necrosis develop, and major leaf mortality may occur.  Individual lesions may coalesce, causing large areas of the leaf to die, giving the tree an overall scorched, wilted appearance that resembles symptoms caused by oak wilt.  During the summer, black fruiting bodies of the fungus form along the dark leaf veins and produce spores that become rain-splashed.  These fruiting bodies can be seen by using a 10X hand lens.  In late summer, black fruiting bodies of the fungus (pustules) develop on the petioles of infected leaves, and mature spores may be seen in these pustules the next spring.  Some of the infected BOB leaves remain on the trees during winter and become the source of inoculum for the next growing season.  If the leaves fall or are blown off of the trees, the petioles may remain.  A hand lens can be used to see the black pustules at the base of the clinging petioles to confirm BOB infection.  Symptoms of BOB usually start at the base and progress up the tree.

Economic Impacts
In Iowa, the economic impact to wood products, businesses and forest landowners is more than $19 million.  A $964 million loss of tree-derived benefits such as energy savings, storm water retention, property value, carbon sequestration, tree removal, and replacement costs are other economic losses, as well as reductions in nut production and wildlife habitat.  It is estimated that 54% of winter deer food is bur oak acorns.  Without the acorns, deer turn to corn.

In the landscape, as always, tree species diversity is the best option.  In the woodland, proper management by an arborist or forester is the best defense against most diseases, including BOB.

Iowa State University reported, "some success with experimental root injections of fungicides in late May or early June, after the leaves have fully expanded, but before BOB symptoms develop.  However, fungicides at even modest rates may cause some leaf browning and twig death in bur oak.  A single propiconazole injection following the recipe for oak wilt treatment may reduce BOB symptoms for two or more seasons.  Trees should not be injected again until the spring following the reappearance of moderate to severe symptoms.”¹

Bur Oak, Paul S. Johnson

United States Department of Agriculture Forest Service Northeastern Area State and Private Forestry NA-PR-02-11 May 2011, Jill D. Pokorny and Dr. Thomas C. Harrington

Bur Oak Blight, USDA Forest Service

¹Horticulture & Home Pest News, Tom Harrington, Iowa State University, Department of Plant Pathology and Microbiology


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