Bur oak hammered by Hypoxylon
True, classic oak dieback and decline affecting mature bur oaks have been occurring across southern Minnesota over the last few seasons. This dieback is likely what's called, Hypoxylon dieback of oak. Affected trees have wilting and yellowing leaves and branches and, ultimately, dieback. Old stromata, fruiting bodies produced by the Hypoxylon fungi, appear on the bark of trees with dieback. In all cases, the trees are mature and under stress from a variety of cultural and/ or environmental disturbances. The likely Hypoxylon species associated with main stem cankers are known to be opportunistic fungi. Unable to colonize trees of normal vitality, they quickly colonize weakened trees.
But again this is not to be confused with oak wilt disease which can look very similar. Oak dieback and decline can be distinguished from oak wilt primarily by the distinctive, large cankers low on the main stem, and secondarily by branch mortality patterns. Oak wilt does not involve either canker formation or fruiting body formation on bur oaks. With oak dieback and decline, whole branches discolor and die at the same time and the leaves hang on to the twigs. With oak wilt, only part of a branch may die and the leaves wilt and fall off during the summer and early fall.
|Forest tent caterpillar
The pest survey cycle is complete for FTC in the Alexandria Area. Aerial detection, homeowner calls and ground checks conducted in June and July confirmed the extent of FTC caused defoliation to stands around lakes in the Alexandria Area. Pockets of defoliation were also confirmed in adjacent areas administered by the Willmar Area. Egg mass counts from basswood twigs cut in mid-September confirmed the potential for moderate defoliation in 1999. Sampling confirmed the presence of pupal and egg parasites; but it will be the severity of the winter, that determines the survival of the egg masses and the true potential for outbreaks in 1999.
Cocoons of forest tent caterpillars were found in a few of locations in Region 2 this summer, notably, in one location just east of Ely and in a couple of spots south of International Falls. Is this an omen that we will have a leaf-less summer next year? Probably not. Widespread defoliation is unlikely since it takes a while for large outbreaks to develop and we did not have any areas of significant defoliation this year in northern Minnesota. However, it appears likely that some areas of defoliation will occur next year.