Lightning is on its way, along with spring and summer thunderstorms. Throughout Missouri, any location averages about 50 thunderstorms each year. Although thunderstorms occur during any month of the year, 60% of them rumble across the state between May and August. During these storms, people and trees are at risk from lightning strikes. A lightning strike can produce very high temperatures and an electrical charge of 100 million volts. An average of 25 million lightning strikes are detected annually in the United States.
Because trees are often the tallest objects in a landscape, they are especially vulnerable to lightning strikes. When lightning strikes only one side of a landscape tree, it can often recover with only a minor vertical scar along the affected area. However, when a strike passes through the whole tree, splintered bark and exploded wood around the entire trunk usually results in tree death. Electrical current or lightning can also pass from the tree trunk and into the roots, causing tree decline and death over time. After a lightning strike, remove any broken branches that may create an immediate hazard. For partially damage trees, wait a few months to determine if trees will recover. Also, during this time, irrigate trees to minimize tree stress during periods of drought.
For apple trees, damage resulting from a lightning strike is easily misdiagnosed after a healthy tree or its limbs suddenly die. Often the tallest tree limbs are struck by lightning with localized dieback. After a severe strike, the bark separates from the wood in the tree top, the central leader collapses, and its leaves turn brownish-black. The damaged leaves remain attached to the limbs and have sharply bent petioles, unlike the normal arc of healthy leaves and their petioles.
In orchards, lighting can damage individual free-standing or trellised apple trees. The number of trees damaged or killed by lightning will depend on the strength of the initial strike, the size and moisture content of the trees attached to the wire, and the conductivity of the posts within the trellis system. Usually, damage along the trellis will decrease from the center of the strike because metal posts and/or trees attached to the trellis will dissipate the charge. Pressure-treated wood line posts can also direct the current to the soil surface when posts are wet from precipitation, resulting in a line of damaged trees occurring between two line posts.
Damage from lightning is often confused with fire blight, which is caused by bacteria. Shoots damaged by lightning have a distinct margin between heathy and necrotic tissue and bacterial ooze is absent. Several weeks after a lightning strike, the pith in the center of the damaged shoot appears segmented (Figure 1). Presumably, the normally hydrated pith rapidly desiccates and shrinks into segments from heat associated with the electrical charge of lightning. At the base of a partially damaged shoot, leaves may appear normal, but there is a ring of darkly-colored tissue just under the bark in young xylem (wood) when the limb is cross-sectioned (Figure 2). In spite of this necrotic ring, the cambium survives and produces healthy new xylem that grows over the dead tissue. Several weeks after lightning damage, fungal bracts can occur on tree trunks of dead trees.
Most often diagnosis of lightning damages is based on the appearance of dieback in the highest part of the trees, lack of other disease or insect pests, and weather records indicating lightening activity in the area.
REVISED: September 29, 2015