Control of weeds in the crucifer family is critical to prevent disease transmission to cole crops. Try to choose sites without cruciferous weeds. All crucifer seeds should be treated to reduce or eliminate these pathogens. Seed companies will usually treat seed on request. Hot water treatment reduces seed viability, but is still one of the best means available to prevent disease spread.
All diseases have the potential of being transported to a farm on infected transplants.
The best alternative is to grow your own transplants from clean seed in soilless mixes or in sterilized or fumigated seed beds. Any practice which promotes the rapid drying of foliage and soils will help minimize disease incidence. Start by choosing a planting site with good air and soil drainage. When planting, orient rows in the direction of prevailing winds for better circulation of air through the foliage. Avoid over-planting or crowding plants as this increases the drying time.