http://en.allexperts.com/q/Plant-Diseases-715/2009/2/pecans-growing-turn-black.htm Try this web site - it might be helpful.

The web sit will probably have more info, but around here, black pecans mean zinc deficiency. We do three things to combat this:
First, apply compost, compost tea, and/or humic acid regularly over the next year, to release the zinc already in the soil.
If that doesn't work, apply zinc sulphate in the following amount: 10 lbs., plus 1 lb. per inch of tunk diameter. That should be applied in foot deep holes, about 2 feet apart, around the drip line of the tree.
If that doesn't work, spray the emerging foliage 2-3 times with a 1 tsp. per gal. solution of zinc sulphate, possibly mixed with Yucca extract, every spring. Considering the size that pecan trees grow to, that is a last resort, since it involves renting major power equipment to reach the top of the tree.

Blimey, I didn't realise they needed so much attention.

Only in alkaline soil, Cammomile. Most parts of the southern 2/3 of the U.S., they grow like weeds.