Oak wilt threatening trees across Iowa

QUASQUETON — Unlike the tree affliction caused by the emerald ash borer, oak wilt does not threaten to wipe out every oak in the state.

But that's little consolation to Clayton Ohrt, whose picturesque, predominantly red oak timber overlooking the Wapsipinicon River has already lost scores of mature trees to the disease State Forester Paul Tauke considers 'Iowa's most persistent long-term oak ailment.'

More will likely succumb before the disease is controlled, said Department of Natural Resources forester Jeff Seago, who is helping Ohrt redefine his timber management plan.

'I hate to see it happen. That timber is one of my prized possessions,' said Ohrt, who has been planting trees, shrubs and native grasses on his farm for several decades.

Ohrt said he noticed last year that many of his trees had withered leaves and thought it might have been an after effect of the 2012 drought.

When they failed to leaf out this spring, Ohrt said he knew he had a serious problem and called Seago.

After inspection tours this summer, Seago said he found scores of dead and dying mature red oaks, as well as ample signs of classic oak wilt symptoms — withered, discolored leaves that lead rapidly to defoliation and darkened patches of fungus, called mycelium mats, that crack the bark and emit a sweet smell that attracts insects that further spread the disease.

Although Seago said he is confident that oak wilt is killing Ohrt's trees, he said the disease has not yet been confirmed by laboratory tests. Landowners who suspect oak wilt should not harvest trees or undertake other management steps until the disease has been confirmed, he said.

When it is confirmed, Seago said he will likely recommend a 'sanitation cut.'

'Basically, that entails removal of all visibly infected trees as well as some healthy-appearing trees beyond the affected area,' he said.

If the trees are large enough — typically, with trunk diameters of at least 20 inches — and not decayed, they may be suitable for lumber, according to Seago. Smaller trees and those already decaying can be used as firewood, he said.

Seago said oak wilt is a serious problem because the disease, caused by a fungus, spreads quickly from tree to tree and 'does not respect property lines.'

'We are concerned about that,' said Dan Cohen, director of the Buchanan County Conservation Department, which manages three nearby public tracts of timber.

'Knowing it's nearby, we'll step up our monitoring efforts,' he said.

The 186-acre Guy Grover Timber and Tree Plantings across the river from Ohrt's timber has 'fantastic stands of red oaks, and the trees are also prominent in the 15-acre Putty Root Preserve on the same side of the river, Cohen said.

'That river corridor is all timber, both public and private. It would be a shame if the oak wilt spreads,' he said.

Oak wilt can spread from infected trees to healthy trees through root grafts connecting nearby oaks and by insects that carry the fungus from one tree to another.

Although Ohrt's timber still has many healthy red oaks, Seago said they are all vulnerable.

Red oaks have an open vascular system that allows the fungus to spread quickly within the tree, Seago said.

White oaks are also susceptible to the disease, but they can sometimes survive it, he said.