Dying Genovese Basil

Hello. Regarding my Genovese basil plants – 24 in all – previously, I had written you and spoke by phone to your greenhouse about how these plants were dying and what I could do. Nothing seems to help; they are still dying one by one.
Seems they rebound for a few days and then the dying process begins anew; leaves wilt and drop off from the top down and the stem turning black from the top down as well. I would say, that i have just three healthy plants left and do not have much hope for their survival considering the trend! Is there anyway to verify what is causing this destruction?? Most of the plants are in a raised bed and last year these same species did extremely well there. When the dying began I removed three plants 75' away replanting them in an isolated area of the vegetable garden next to the other Richter plants. One died a day or so after transplant; the other two look like they are on their way out. Have continually inspected for bugs and used Safer Soap as recommended. There has been no visible infestation; except for some minor leaf eaters which i was able to control. Would appreciate receiving any suggestions regarding this dilemma.
P.S. All of the other species plants from Richters: tarragon, lemon balm, oregano, sage and thyme are doing just the opposite – they are flourishing; bumper crops!!!

That the other herbs are flourishing while the basils are not leads me to suspect the problem is fusarium wilt. This is a soil-borne fungus that attacks the stems of plants cutting off the water supply to the leaves. It can ravage plants just the way you describe. For more information about the disease see the Richters InfoCentre section of the Richters website and then in the Richters InfoSheets.
There is no cure. The disease persists in the soil for several years at least, maybe longer, and could be present from infected plants previously grown in the same area, even if they did not show as much damage as the ones you have now.
The only solution is to plant fusarium resistant Nufar basil in future years. You could otherwise try planting in a different area which has never had basil before, but chances are that the soil there is infected also.