The light circles in this field of mature rice are areas in the field affected by sheath blight caused by Rhizoctonia solani. This fungus also attacks soybeans. Sheath blight has been the most economically significant disease in Louisiana since the early 1970s. Sheath blight begins at the water line and progresses up the plant. When it reaches the top of the canopy, a circular area locally referred to as a "bird's nest" appears. Plants in this area will produce little grain. These spots often coalesce, forming large areas of the field that appear grayish. Sheath blight is characterized by large oval spots on the leaf sheaths and irregular spots on leaf blades. Fungus mycelium grows up the leaf sheath. The fungus can spread in the field by growing from tiller to tiller on an infected plant or across the water surface to adjacent plants.