Apple growers in Indiana and much of the Midwest report older trees are failing or have failed due to winter injuries sustained in the sub-zero temperatures this winter. Cultivars such as Mutsu, Jonagold, and Rome seem to be affected at a much higher rate than Golden Delicious, although there have been reports of Golden Delicious failing in southern Indiana. In Michigan, Empire and McIntosh have also sustained winter injury.
Determining cause of plant death from a dead specimen is challenging, because tree death is caused by many reasons, including environmental conditions, pathogens, or insects. Oftentimes, these are not simple “one organism” caused “one problem” situations.
What Is Decline?
Pathologists use the term “decline” to describe diseases of complex causes. Within a disease complex is:
- A predisposing situation that slightly challenges and weakens the plant but doesn’t kill the tree outright;
- An inciting event that may have occurred because the tree is stressed, or an event so severe by itself that it could profoundly affect the tree;
- Minor contributing factors that normally aren’t problems by themselves, but serve as straws that break the camel’s back when piled onto a stressed tree (see chart).
Tree death this summer is due in part to the harsh and long winter of 2013-2014. However, this stress is not isolated to just this year, but to several years of stress these trees have endured. Heavy cropping, as many of us observed in 2013, was a direct result of crop loss in 2012. Both caused stress to the trees. Older literature discusses the relationship to heavy crop load and lack of trunk maturity, and failure to undergo proper dormancy.
One possible contributing factor would be late fertilizer application that forced trees to continue to grow instead of undergoing dormancy. Another contributing factor that may have resulted in tree death was the dramatic change in temperature that prevented dormancy from becoming fully established. In the absence of full dormancy, plants are susceptible to freeze injury. Finally, growers should use a dome when spraying trees to minimize herbicide drift, thereby avoiding situation three.
Trees are actually quite slow to respond. The dieback process growers observed in the spring was occurring all fall and winter long, but the symptoms weren’t always obvious. Injury to the stem and roots results in water not getting to the leaves. This means that the leaves can’t produce food to feed the roots. The roots, lacking food (energy) can’t send water up to the leaves. As you can see, this becomes a negative feedback loop of decreasing water and decreasing food. It takes a lot of water and energy to turn this around.
Rapid Tree Death
Winter injury can result in rapid tree death. Damaged inner bark begins to die, and turns brown, failing to conduct water to leaves, or sugars from photosynthesis to roots. Healthy bark will appear yellow-green. A few small slices throughout the branches allow you to readily distinguish between dead and healthy branches. Severe damage can result in bark splitting or sloughing off.
In its most severe form, winter-damaged trees never break bud. In other instances, trees may bloom and begin to leaf out in the spring, only to wilt, fail and die as temperatures begin to warm, as the dying cambium cannot keep up with demands made by buds for water. Depending on how severe the winter injury was, and overall tree health prior to this freeze event, trees may begin a process of dying. It is important to note that less damaged trees may recover or they can continue to decline for the next few years before dying.
How To Keep Your Trees Healthy
Even if your trees survived the winter, it is important to keep in mind that even moderate cold injury could make trees susceptible to infestation by borers and infection by opportunistic pathogens like Botryosphaeria canker, Nectria canker, Phytophthoras, black rot, and even dead man’s fingers. For stressed trees, making sure crop load is well managed to the light side, and keeping plants irrigated through any drought event, will go a long way in preventing any more nasty surprises next spring.
Keeping trees healthy is hard. You want growth, but not too much growth, which is often weak, and very susceptible to fire blight. Having the soil tested every few years by a reputable lab to know your nutrient status is really important. This University of Massachusetts Extension bulletin, provides great information on nutrient management.