August 14, 2015
WINTER INJURY TO OUR MICHIGAN PLANTS
As with 2014, 2015 was a very difficult winter for woody plants. Many trees and shrubs exhibited winter injury symptoms this past spring and are continuing to do so into the summer. On evergreen plants such as conifers, overwintering needles sometimes turned reddish brown and dropped by spring, leaving some branches or in some cases entire trees without needles. Believe it or not many of these conifers have produced new shoots with new needles, essentially ensuring their survival. Regrettably, some plant owners removed their affected trees too soon, believing they were dead. Deciduous trees and shrubs exhibited symptoms such as slow foliar emergence, wilting foliage after emergence, and no emergence of foliage. Obviously, the latter two symptoms are troublesome and may indicate plant death. However, it is recommended that trees and shrubs be allowed some time for possible recovery. Whether conifer or deciduous, the cambium of stem tissue should be checked for viability. The presence of a green succulent cambium indicates life, but it may take some time for new foliage to develop. Buds should also be checked for viability. We stress patience to all because issues with winter injury may even be observed into late summer.
This serious disease is once again reaching epidemic proportions in some locales in Michigan this year. The fungal causal organism only attacks Pyrus (pear) species as its deciduous host. The fungus is apparently capable of virtually killing large established pears within two years of repeated infections. Control of the disease is proving to be far more challenging than originally believed. Will this make Bradford and other pear species unusable for landscapes? Only time will tell. Although ornamental pear has been overplanted, especially after the Emerald Ash Borer debacle, we would be at a loss to lose Pyrus as a landscape tree to yet another invasive organism.
It is generally believed that Oak Wilt (OW), caused by the deadly fungus, Ceratocystis fagacearum, is increasing in incidence throughout Michigan. While exotics such as Dutch Elm Disease and Emerald Ash Borer are “broadly devastating,” a disease such as Oak Wilt is lethal but tends to be “locally devastating.” In some manners, Oak Wilt is more difficult and costly to contain and manage than some of the broad ranging pest and disease problems. During the past spring and summer, OW has made its presence known around Michigan. While the vast majority of oak trees killed by the OW fungus occur via root graft transmission, most new infection sites develop as a result of injury to oaks at the inappropriate time. Most of the property owners who encounter Oak Wilt have never heard of it before.
NORWAY MAPLE BRANCH BY BRANCH DECLINE:
The first time we’ve seen a very widespread Norway Maple branch by branch decline occurred during the spring and summer of 2014, after the very severe winter. In some urban locations, more than 90% of trees seemed to be affected. According to reports around Michigan, the branch by branch decline is apparently recurring again this season. In fact, some city foresters are claiming that some trees are so severely declined that their removal is imminent. As a brief symptom description, one branch wilts and the foliage eventually turns tan; while the first branch is declining another branch begins to wilt and so forth. Investigations of the 2014 outbreak found that winter injury followed by an opportunistic Valsa canker-causing fungus as a likely scenario.
IS IT REALLY SPRUCE NEEDLECAST?:
In the past several months, the severity of Rhizosphaera needlecast and the difficulty in its management has increased. The problem is not spruce needlecast. Spruce needlecast has been confused with a variety of issues. One of the causes of needle loss from trees is winter burn/desiccation, which has been quite common this past winter and spring. Canker diseases such as Cytospora and Phomopsis can also mimic needlecast diseases. Differentiating needlecast from other causes is fairly straightforward. Rhizosphaera needlecast rarely kills branches…unless there has been several years of severe defoliation. Neither does winter desiccation…”usually.” Phomosis and Cytospora cankers will kill branches and also, obviously, lead to needle loss. To check for viability in branches, always look for a live, green cambium directly beneath the bark.
SOURCE: MSU, David L. Roberts, PhD, Senior Academic Specialist, College of Agriculture and Natural Resources July 2015
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