Stokes seeds say's that if you keep the pH between 6 and 7 that you can avoid clubroot

The pH of the soil determines whether or not your plants will be susceptible.

If the pathogen is in the soil, an acid pH means trouble.

So, the real question is, what is the native pH of your soil?

I was part of a long discussion on club root last week with another New Zealand gardener and the results of that discussion is that a soil well endowed with organic matter and a pH above 7.0, brassicas really prefer a soil pH that is more alkaline, the club root pathogen will not be active and will not affect the plants.
Crop rotation in small gardens is grossely over stated. Very few backyard gardeners have enough room to rotate their crops so other steps, making the soil into a good, healthy soil to grow strong and healthy plants.

In the past, I have had trouble with club root and fungal/viral problems with nightshades. I now rotate these crops and do not compost them if the plants show signs of disease. This has worked for me and I have had no club root for many years. "Some crops prefer to feast on the leftovers of nutrients applied the previous year. This list includes members of the cabbage family, solanceous plants such as tomatoes and potatoes, most root crops, and leafy crops like spinach and lettuce." THE ORGANIC GARDNER'S HANDBOOK OF NATURAL INSECT AND dISEASE CONTROL Hope this helps

Thanks guys. We have put in a rotation- brassicas, solanum/nightshade family and relatives (ie potatoes, tomatoes, eggplant and capsicum) legumes, and root crops and miscellaneous. We will maintain a brassica and solanum roation and relax a bit on the others. There is plenty of compost in the soil but we need to check the pH. It is good to hear that strict rotation isn't necessary. I feel an organic garden should involve interplanting with companion crops and generally a diverse mixture rather than the more monoculture style beds full of one type/family of crop. It's also so much more fun!