I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
Regarding your potatoes, if the rusty spots are on the foliage this is a disease that will eventually kill the topgrowth, but most likely not until they form a decent crop of tubers. If you can catch such a disease early in the future sprays can prevent it from taking out the plants foliage and thus provide more carbohydrates for forming tubers.
If the rusty looking spots are on the tubers themselves you may be seeing potato scab disease. This disease is caused by bacteria that create brown, corky lesions on the outer surface of the tubers. The potatoes are still useable if you cut away the damaged areas when peeling them. This problem is more common in neutral to high pH soils (acidic soils are not favorable for this disease) and when an application of manure or compost has been made prior to planting the potatoes. Make any such additions after harvest to allow them plenty of time to decompose fully before the next potato season. Some varieties have shown resistance to the disease but these varieties are not commonly available at local garden centers and may not be the best producers in this region of the country. Select certified disease free seed potatoes to avoid introducing the problem into your garden. Crop rotations so that potatoes are not replanted in that location for 5 or 6 years can also help reduce problems with scab. There is no practical method of chemical control for this disease in home gardens.
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