Clubroot is a common disease of crucifers. Actually your question is not clear. Because of this disease the root biomass will be higher over normal pants. I don't now why you are asking about the grain straw ratio. 

Ramesh has rightly mentioned. Can you please elaborate what exactly you are looking for. 

You did not unterstand my question... Many factors (temperature, soil pH or soil structure) have an impact on the occurence of clubroot. I will explain: You have high quantives of straw in the soil after grain havest and the time between harvest and seeding of winter oilseed rape is not enough for reduction of straw. In Germany it is possible you have only a tight time frame between harvest and seeding. Is there an effect of this straw influencing the living conditions of Plasmodiophora brassicae?

Thank you for your answers! I hope my explanation will help you.. 

Dear Jan, I could understand your problem. There is no report, but you can try crucifer plant residue incorporation, it may control clubrot. I am giving below a link also for further study. Crucifer plant residues incorporation in the soil was shown to reduce the population of soilborne plant pathogens and the effect was mainly attributed to the release of toxic volatiles such as mercaptan, methyl sulphide and isothiocyanate (Gamliel & Stapleton 1993). The improvement in populations of actinomycetes antagonistic to Foc was recorded in amended soil, which might have also contributed in reducing the pathogenic population. Efficiency of cruciferous residues in controlling wilt indicated that amending soil with mustard residues or oil- cake during hot summer days with one irrigation or at the time of rainy season led to significant reduction in wilt incidence on cumin (Mawar & Lodha 2002). The reduction in wilt incidence was significantly improved when amendments were applied during summer compared to their application in rainy season, which may be a cumulative effect of bio- toxic volatiles released during decomposition of residues at high soil temperatures (38-42°C) and subsequent microbial antagonism. Allyl- glucosinolate is one of the predominant glucosinolate in Brassica spp. and is generally converted to allylisothiocyanate (ATIC). The concentration of the ATIC was found to be directly related to heating of the soil up to 45°C (Gamliel & Stapleton 1993). However, isothiocyantes were not detected at low temperatures (Lewis & Papavizas 1971).

http://www.indianspicesociety.in/josac/index.php/josac/article/viewFile/368/305