A lot is talked about terra preta, and that biochar was used in South America by natives (and is still used today). But it was also used in many other region, Japan, South-East Asia, Europe, just to name a few...
One of the uses of Biochar was the reduction of plant diseases, or the increase of resistance of plants. There are a few ways to explain it (see links below), but one I like is that biochar increases the diversity of organisms in the soil trough its structure, and that this biodiversity leads to healthier plants. Beneficial (symbiotic) organisms can help the plant to cope with the diseases. Some research is done in this direction now, but as it reduces the need for pesticides guess if it is likely to be financed...
One historical european source (which I can't find anymore) stated that biochar was used against rust in wheat.
I did some trials with
Botrytis, and the plants grown in the biochar were significantly less affected (no statistics would have been needed to see it). So I can tell that it works sometimes.
http://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-100-9-0913
http://hortsci.ashspublications.org/content/47/12/1736.full
http://jxb.oxfordjournals.org/content/early/2013/02/04/jxb.ert026.full
