Spinach leaf miner?
Hi - The tan blotches on spinach leaves may be a sign of insect damage caused by spinach leaf miner. The immature maggot stage burrows within leaves of spinach, beets and chard, causing blotchy dead patches. The remainder of the leaf is edible.
This Penn State link provides some useful information about controlling the leaf minter. http://extension.psu.edu/plants/gardening/news/2014/control-spinach-leafminer-at-the-time-of-plantin...