James, you are correct: the potato flavor comes from microorganisms that infect the coffee fruit and seed, due to skin damage. This skin damage can happen in a variety of ways, but it is commonly thought that the distinct "potato defect" (which is common also in neighboring Burundi and also occurs in Tanzania, Zambia, and Kenya) is due to damage from an insect called Antestia.
It's very difficult to sort out the "potato" defect, as it is largely invisible. Flotation of cherry before pulping helps, as does densimetric sorting of finished coffee. However, I personally feel that if you taste enough cups of any Rwandan coffee, no matter how good, you will hit the potato defect at some point. It has cropped up even in some of the best Rwandan coffees. Of course, it's far more common in poorly cultivated, sorted, and prepared coffee. When cupping in Rwanda, especially lower quality coffees, it feels like you're in a raw potato soup factory sometimes. Apparently the specific chemical which causes this effect is 2-methoxy-3-isopropyl-pyrazine. So, hold the 2-methoxy-3-isopropyl-pyrazine.
But seriously, this compound is secreted by a number of bacteria which infect fruits. I presume these bacteria are more prominent in the microflora of East Africa, and that's why this problem is more present there, although potato cups can happen from coffees all over the world.
Addressing this problem is a high priority in Rwanda and all over East Africa, and the primary strategies are:
1. Insect Control
2. Careful Picking (no split skins)
3. Pre-pulping floatation tanks
4. Densimetric sorting (Oliver tables)
All have been successful to some degree, and this year I tasted way less potato than I have in years past.
Specialty Coffee Association of America