Effects of different cultivation systems (soil management, nitrogen fertilization) on the epidemics of fungal diseases in oilseed rape (Brassica napus L. var. napus)

The effects of different Cultivation systems on disease incidence and severity in oilseed rape were investigated on two sites in Schleswig-Holstein in the period 1997-1999. The cultivars 'Joker' (Futterkamp site) and 'Falcon' (Hohenschulen site) were grown either by conventional methods (plough/drill), by methods with reduced tillage such as drilling following timed cultivation (cultivator/drill combination) or sowing underneath the soil by a rotary-cultivator (rotavator/drill combination). The cultivation systems differed further in the amount of mineral N fertilizer application (0, 120, 240 kg/ha). The disease incidence (DI) and severity (DS) of oilseed rape pathogens (Phoma lingam, Verticillium dahliae, Sclerotinia sclerotiorum, Pseudocercosporella capsellae, Botrytis cinerea, Alternaria brassicae, Peronospora parasitica) were assessed in untreated control plots. Despite varietal differences, pathogen reaction to different cultivation systems on both sites was largely similar. During the 3-year investigation period, P. lingam, the causal organism of root collar and stem rot (black leg), dominated. As a rule, the disease was present from the earliest plant growth stages through harvest whereby first symptoms of black leg could be determined from flowering growth stage onwards. The typical S. sclerotiorum infection of the rape stem remained very low in all trial years with a maximum of 8% DI and a DS of 0.3. Notably late Sclerotinia infections of the upper lateral branches and pods Occurred at Futterkamp in 1998 and 1999 and in 1998 at Hohenschulen with a DI of up to 50%. The causal organism V dahliae was determined on all sites and in all trial years up to harvest. The most important leaf disease pathogen was P. parasitica. More severe infections of P. capsellae appeared on leaves in 1998 at the Hohenschulen site only. B. cinerea and A. brassicae were present throughout the year at low infection levels. The soil management systems plough/drill and cultivator/drill or a rotavator/drill combination did not affect the main foliage pathogens (P. lingam, P. parasitica). The DI of vascular pathogen V. dahliae was reduced with plough/drill compared to cultivator or rotavator/drill systems. Black leg infection by P. lingam was not affected by the cultivation system. Increased N fertilizer application resulted in increasing infection by P. parasitica. There also was a trend towards increased DS of V dahliae, although results were not always significant. On the other hand, foliage and root collar infection by P. lingam was not affected by N fertilizer application rates. Compared to the results of increased N applications, decreased applications resulted in increased infection by S. sclerotiorum on upper side branches and pods at later growth stages (GS 92). Harvest yields rose with increased N fertilization, and there was a trend towards increased yield through inversion cultivation (plough/drill). Very high oil contents were achieved in all three trial years with these yields correlating negatively with N fertilizer applications. Protein contents rose significantly with increasing N fertilizer applications. Glucosinolate content was not significantly affected by N fertilizer.