The thing with sclerotinia is that lots of crops can get it - potatoes, carrots, peas, parsnips lettuce, brassicas to name some.
Tends to occur with the sclerotia (the fruiting bodies) release the spores and then landing on plants to start the infection. As wide a rotation as possible. Good health regime - bin any infected material - I wouldn't compost.
Good air flow and if growing in a tunnel try not to have the conditions too wet or humid.