Based on the information provided and the context of the comparison between the North and South, it appears that the industry's decline that would hurt the southern economy the most is agriculture.
This conclusion can be drawn because the southern economy was historically more dependent on agriculture, particularly cotton and other cash crops, compared to the North, which had a more diversified industrial economy. Given that the graph indicates a higher percentage (likely representing dependency or contribution to the economy) related to agriculture for the South, a decline in this industry would have a more significant negative impact on the Southern economy than declines in industries such as iron production, manufacturing, or railroad construction.