"The recent and ongoing potato discussion brings several questions to mind:"
1. Could the Irish potato farmers controlled the blight if they had rotated their crops with barley or oats or some other Gramineae?" One of the reasons that potato cultivation was so widespread in Ireland was that it could be grown virtually anywhere under the "lazybed" technique, versions of which were employed by the Incas and are widely in use today. Just place the tubers on peat, manure or whatever, dig a drainage trench on either side while covering the tubers, and wait until harvest. Grains could not have been cultivated in most of the places where potatos thrived, and the peasants who grew most of the potatos for subsistence did not have access to the more arable land. For a vivid account of this, see Henry Hobhouse, _Seeds of Change: Five Plants that Transformed Mankind_ (NY: Harper & Row, 1986). I'm sure there are better accounts.
On the question of genetic diversity (raised in another posting), the genetic stock for Irish potatoes apparently originated in the tropical Andes, and was introduced to Europe ca. 1580. Early potatos produced poorly in Europe because they were adapted to a more tropical photoperiod. Before they were suited for cultivation in Ireland they underwent some 250 years of human and natural selection as garden curiosities whereby they were allowed to reproduce sexually (a practice uncommon in commercial or subsistence production). The potato blight was introduced later (ca. 1830?), possibly from Chile, where races of resistant subspecies were cultivated (the higher elevation races of the tropical subspecies have also shown resistance). All the European potatoes proved vulnerable to the blight. Resistant strains were developed during the latter half of the century. The relatively narrow gene pool of the European potatos, the practice of propagation by tuber, the cool moist climate of Ireland, made for particularly vulnerable conditions. This information is drawn from Jonathan Sauer, _Historical Geography of Crop Plants: A Select Roster_ (Boca Raton: CRC Press, 1993).
"2. If organic agriculture grew out of a reaction to the increased use of agricultural chemicals in production systems, why can the description of "organic" be applied to any system that relies on non-synthetic inputs?"
No.
"3. Wasn't the Irish potato famine really an example of unsustainable organic agriculture? "
I don't think such generalizations are in order here. There are other important social aspects to the story that haven't even been mentioned. The relation of peasants to "lords" and the colonial relation of Ireland and England. It does seem, however, that we have the knowledge and the genetic diversity (without the herbicides) to avoid such crop failures today.
"4. Wouldn't Bill Duesing make a great neighbor? I think so."
But he lives in Connecticut!