I was reading one of those spam inbox emails this morning which listed the dirty dozen and clean 15. Why are potatoes listed as one of the dirty dozen but sweet potatoes make the clean 15? WHAT?!?!
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"The sweet potato is only distantly related to the potato (Solanum tuberosum) and does not belong to the nightshade family." http://en.wikipedia.org/wiki/Sweet_potato
second, "Over 50 percent of potato sales are to processors for french fries, chips, dehydrated potatoes, and other potato products; the remainder goes to the fresh market. Although potatoes are grown year round, the fall crop comprises roughly 90 percent of potato production.
Potatoes are the fourth-most-consumed food crop in the world, after rice, wheat, and corn." http://www.ers.usda.gov/topics/crops/vegetables-pulses/potatoes.aspx
Why do these topics matter? Well, I think it matters because 1) They are only distantly related and I assume (may be making an ass of myself) that potatoes and sweet potatoes therefore have different fungicides, pesticides, etc. applied to them.
Secondly, potatoes are a much more highly desired and eaten crop than sweet potatoes. So there's a lot bigger business and therefore a lot more money to be made in shipping, driving, and flying potatoes than sweet potatoes. I think that influences a lot of the potato pesticide practices, because I don't think they necessarily segregate the potatoes that are going to frozen foods or fast food or the grocery store and treat them differently. My guess is that JR Simplot or whoever treats all the potatoes after harvest and then segregates them, preserving their investment and future source of income.
I have never worked in the food industry though, but were I a potato company, I would be hell bent on preserving my crop and getting the best price possible for it!
I garden, and it is well known that potatoes get diseases. The various blights that kill tomatoes make potatoes produce tiny crops. And when you have square mile after square mile of potato fields in some parts of the country that is bound to happen a lot, specially since some of the blights overwinter in the soil or vegetable refuse.
Not so for sweet potatoes, who are quite adapted to high moisture, high disease tropical climates. Their problem is that they will not do much here in Michigan, without cover (most are grown in the South), and also they are absolute deer candy and there is no chance of growing them outside a deer fence.
They're completely different plants, and often grown in different parts of the country. They probably benefit from different soil treatments and pesticides. But, are you sure you're reading the original Environmental Working Group list and not an alternate or made-up version? The original list that I'm looking at shows potatoes near the top (most likely to have pesticides) and sweet potatoes near the bottom (least likely).
Scroll to the bottom of this page to see the Environmental Working Group's methodology. Note that this is basically a probability + pesticide count measure. The score involves the probability that a given sample would have pesticide residues (after washing and standard preparation) and multiple pesticide residues. It doesn't mean that every potato is "dirty" or that every sweet potato is "clean." It also treats all pesticides the same and doesn't distinguish more harmful from less harmful ones.