While both describe some incredibly poor working conditions, neither suggest that illegal immigrants are large factor. The UC Davis piece says that it existed but the numbers were exaggerated, the Guardian piece says there might be 100,000 in a labor force of 23 million.
As I understand it Africans are the main labour source for many farms in Spain.
Some articles to read:
https://migration.ucdavis.edu/cf/more.php?id=40
https://www.theguardian.com/business/2011/feb/07/spain-salad...