No. 16 Article 6/July 11, 2008

Soybean "Mystery"

A spot of unusual-looking plants appeared in a soybean field at the University of Illinois South Farms on Monday morning, July 7. This field was in corn managed uniformly in 2007, and the soybeans had not been sprayed with any postemergence herbicides. The soybeans were planted in 15-inch rows in late May, and nothing has been done to the field since.

The affected spot is about 20 feet wide and 30 feet long. Affected plants appear stunted, with most of the effect appearing at the top of the plants, where leaves seem to have wilted. In the center of the affected area, a few plants are dead.

Damaged area in soybean field.

Plants in affected area of the field.

Dead plants in the center of the affected area.

Given the rapid appearance of this problem, the facts that no chemicals have been applied since planting and the field was treated uniformly in 2007, and the complete lack of any symptoms like this in the rest of the field or in nearby fields, we have to conclude that the problem is the result of a lightning strike. There was heavy lightning early in the morning on July 7, and this field appears to have been struck.

We had a lightning strike in a soybean field at Monmouth a number of years ago; my recollection is that plants were killed completely, but in an area only a few feet across. In the current case, more electrical current seems to have traveled through tissues with the highest water content, affecting upper stems and leaves more than older tissue. I saw a lightning strike in a cornfield a number of years ago in which foot-tall corn plants simply toppled over down the row, in the direction away from the strike. Lightning probably does not strike the same way in all cases, but it appears that tissue with a lot of water is most vulnerable, probably because it conducts electricity, and also because it tends to be younger and to have less woodiness.

We can take away from this that lightning can cause symptoms that might in some cases look like a disease, such as phytophthora with its dead tops of plants. It's also instructive to look at the pattern of damage and consider that it's rare for damage to occur out of the blue like this and to be confined to only a small part of a single field, without regard to topography and with relatively uniform symptoms within the affected area. We'll watch to see if these plants recover. With death of the upper stem likely, most recovery might be through growth of branches, providing lower buds are still alive.--Emerson Nafziger

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