Dan Egel, Extension Plant Pathologist, SWPAC, Purdue University
In the 1970’s and 1980’s in southern England, patterns began to appear in wheat fields. Paranormal explanations began to appear before a couple of pranksters owned up to the deeds. The accompanying photos might be thought of as crop circles. These circles, like the ones in southern England, have an explanation that doesn’t have anything to do with aliens or ancient rituals.
The slightly raised circular patterns in the watermelon in the photos are as a result of an infection by the virus Papaya Ring Spot (plant viruses are named for the first plant in which they were found). The virus was spread to the watermelon plants on which these fruit grew by aphids, insects with sucking mouthparts. The surface patterns shown here do not affect the interior of the watermelon and do not pose any harm to humans. However, growers may not be able to sell fruit with patterns such as shown here.
Regular readers of the Picture of the Week may remember that about a year ago, this author submitted photos of pumpkins with symptoms of Papaya Ring Spot Virus. The symptoms shown in October 2012 were much different than the photos shown here. Although it is not clear why plant viruses cause different symptoms at different times, the plant host and the timing of infection may have alter how the symptoms appear.
Click image to enlarge
Circular pattern on watermelon caused by Papaya Ring Spot Virus
Close up of pattern on watermelon