Epidemiological studies on winter oilseed rape (Brassica napus L. var. napus) infected by Phoma lingam (teleomorph Leptosphaeria maculans) and the effects of different fungicide applications with Folicur® (tebuconazole)

Leptosphaeria maculans ascospore release, the infection of foliage and root collar by Phoma lingam and the effect on oilseed rape yield as affected by weather conditions were studied at different sites in Schleswig-Holstein from 1997 to 2003. Folicur((R)) (tebuconazole) and Derosal((R)) (carbendazim) treatments in autumn, spring, at flowering and combinations thereof, were also investigated regarding the control of infection and crop loss as well as effects on plant morphogenesis (stem length, lodging tendency, and pod stability). The results emphasized that precipitation, especially in September, was of particular importance for L. maculans ascospore release. This effect was reduced in October and November whilst the influence of temperature increased. A very close correlation could be determined between L. maculans ascospore release and leaf infection in autumn (September to November, r = 0.82(**)). The correlation between the autumn infection of the leaves and root collar infection was highly significant (October to November, r = 0.83(**)). Leaf infection by P. lingam was reduced by a fungicide application in autumn as well as in spring. Effective control of P. lingam on the root collar was only observed after the autumn treatment. Both autumn and spring treatments resulted in growth- regulating effects (stem axis shortening and reduced lodging tendency). Particularly the reduced lodging tendency achieved after spring application proved to have a greater yield effect than that of the reduced root collar infection achieved through autumn spraying. Across years and sites, the fungicide treatments led to significantly increased yields when compared with the untreated control. Yield increases of 3.1 dt/ha resulted from the combined autumn/flowering application, 4.6 dt/ha from the spring/flowering treatment, 5.6 dt/ha from the combined autumn/spring/flowering treatment and 3.0 dt/ha from spraying at flowering. The yield increase resulting from the combined autumn/spring/flowering treatment can be attributed to the control of P. lingam on foliage and root collar and to the growth- regulating effects of the autumn/spring fungicide applications. Additionally, yield increases were. attributed to pod stability achieved after spray application at flowering as Sclerotinia sclerotiorum infections remained minor over the years and sites. The increased yields achieved under Schleswig-Holstein conditions with the tolerant cv. 'Express' through fungicide application was largely due to the growth-regulating effects (stem axis shortening, reduced lodging tendency, increased pod stability) as well as to control of P. lingam infection.