Fred1949
Here is the website address for the Ohio State University ,college of Food, Agriculturte and Enviromental Sciences....
http://ohioline.osu.edu/
Under the "Yard & Garden" section you will see a number of interesting bulletins for all types of common garden vegetables.....
One you may find of particular interset to resolve your problem is the bulletin on "Fusarium & Verticillium Wilts of Tomatoes, potatoes.
Also check out the Farm section as they have great bulletins on soil prep, fertilization and weed control methods.