By Katie Duttweiler
Iowa State University Extension
Since childhood I have had a slight aversion towards oaks. The reason? I could never frolic barefoot in my front yard due to the risk of stepping on hard, pointy acorns. But I would never wish oak wilt on any tree, even those in my childhood front yard.
Caused by the fungus Ceratocystis fagacearum, oak wilt is the most important disease of oaks in Iowa. The hot zone of oak wilt is centered on the Midwest and extends south to Texas and west to the Rocky Mountains. Thanks to this disease, thousands of oaks are killed each year across the country.
Oak wilt debilitates oaks by blocking water movement throughout the tree. The presence of C. fagacearum and tyloses plug the water conductive tissue (the sapwood) of oaks. Tyloses are gooey outgrowths that certain plant cells produce in response to fungal invasion. Parts of the plant above the blocked sapwood cannot receive water and subsequently wilt and/or die.
While no oaks are immune to oak wilt, different types have different levels of susceptibility. Two basic groups of oaks are the white oaks (including white, bur, chinkapin and swamp oaks) and the red oaks (red, black, scarlet, shingle and pin oaks). Species in the white oak group range from moderately susceptible to tolerant of oak wilt. Infected trees in this group often lose individual branches each year and take one to seven years to succumb (if at all). Trees in the red oak group, on the other hand, typically die within a few months to a year after infection.
For both white and red oaks, wilt symptoms begin at the top of the tree and progress downwards. Leaves on wilting branches turn brown from the edges inward, and heavy defoliation of symptomatic and green leaves also occurs. When wilted branches are cut, it is common to observe brown streaks in the sapwood, just underneath the bark. These symptoms appear in late spring or early summer for trees in the white oak group and mid- to late summer for trees in the red oak group.
Oak wilt is spread from infected to healthy trees in two ways: underground, by natural root grafts, or above ground, by insects. Root grafts are the union of roots originating from different trees (generally of the same oak species). The fungus can travel through these unions and therefore easily spreads between trees connected by root grafts.
Insects spread oak wilt by transferring C. fagacearum spores from infected to healthy trees. Insects are important for the spread of oak wilt between trees that are too far apart or incompatible for root grafting. Mats of the oak wilt fungus growing through the bark attract sap and bark feeding beetles (often the picnic beetle in Iowa). Sticky spores from the fungal mat are carried and deposited to a new host, since the beetles are also attracted to the sweet sap that leaks from wounds on healthy trees.
Management recommendations for oak wilt start with avoiding wounding during the period of greatest risk for insect transmission of the fungus. The greatest risk of transmission occurs during April 1 to July 1, due to abundant sap production by the trees and high activity levels of sap feeding beetles. After July 1, the risk of pruning falls considerably. But since a small risk of insect transmission remains until the first hard frost, it is best to prune oaks only between November 1 and April 1. If the tree is wounded during the infection risk period, a dressing such as paint can be applied to the wound to make it less attractive to picnic beetles.
Once a tree is infected with oak wilt, management becomes more challenging. Potential root grafts between infected trees and adjacent healthy trees can be destroyed, either by digging a 5-foot-deep trench or by releasing highly toxic soil fumigants. It is advisable to bring in a professional arborist for intensive treatments such as trenching or fumigating.
Another option is injection of fungicides into threatened oaks. Again, hiring a professional makes more sense than doing it yourself. Trenching, fumigating and fungicide injection are viable options only for especially high-value oaks in the urban landscape.
For more detailed information about coping with oak wilt, see ISU Extension publication Oak Wilt - Identification and Management, (SUL 15). It can be ordered through any ISU Extension county office or the ISU Extension Distribution online store at www.extension.iastate.edu/store/ or by calling (515) 294-5247.