Since it is the case that nasties can be carried around a plot by wind, by your footwear & tools & by water running or splashing, it really isn't worthwhile practicing crop rotation on small areas; you have to forego growing certain crops for three out of four years.
The best way to ensure against disease on your plot, is the quality & assurance of what you plant in it:
DON'T:
Buy in plants or sets if you can avoid it.
Seeds have very few diseases, & you won't get onion white rot from seeds.
Garlic: I grow my bought seed garlic in tubs first year, so I can be sure there's no visible fungal diseases, & only if it's safe do I plant it in my plot.
I grow my onions in the same two 12x6 beds beds every year, without trouble, because I keep trouble away.
Potatoes, similarly, I grow certified seed potatoes & my own saved seed potatoes only.
Brassicas, again, clubroot affects ONLY the roots of brassicas, so seeds WILL be free of that.
I grow all of them in the parts of the plot best suited for them (light, drainage etc.) year after year
The only proviso with this is that if you get carrot or onion root flies, or other pests like eelworm, you have to abstain from growing the affected crops for a couple of years, however, if you have a small plot, you'd have to do that to eradicate them anyway, as they will very quickly sniff out new sources of food.
You also have to be very careful about maintaining soil nutrients, especially in the case of potatoes, which are heavy feeders.
Meadowlark is an excellent food gardener, but his plot is like a small farm, almost, & rules which he will benefit from do not always have the same value for smaller plots.
I fully intend to grow two successive crops of first-early potatoes in the same ground this year, just re-layering with seaweed & comfrey in between.