Effekt av olika kvävegödselmedel på utvecklingen av klumprotsjuka i salladskål (Brassica rapa ssp. pekinensis)

Abstract: The price trend on rapeseed the latest years has led to an increasing interest to grow the crop among many farmers in Sweden. The price trend has resulted in more than a doubling of the oilseed area since the early 2000s. For many farmers it is an advantage to increase cultivation of rapeseed because of the high proportion of cereals in the crop rotation on many farms, but it is not just benefits. The risk of the fields being infected by clubroot increases within frequent rapeseed cultivation and it often leads to subtantial economic losses. Clubroot is caused by Plasmodiophora brassicae that is a soilborne pathogen in genus Protozoa and its resting spores can be found in the soil for up to 18 years. To avoid increase of the amount of resting spores the cultivation of rapeseed must cease until the risk of being affected by the disease is over. Resistant cultivars are available but despite resistance they increase the amount of resting spores. Previous experiments and studies have clearly shown that the pH value plays a role in how the disease of clubroot develops. It is also known that fertilization with nitrogen, such as nitrate and ammonium, affect the pH in various directions. The aim of this study was to investigate if nitrogen fertilization can affect the development of clubroot. An experiment was performed that lasted ten weeks as Chinese cabbage was grown in pots in a growth chamber. Two different levels of spores concentration (1000 spores per gram soil and 35 000 spores per gram soil) and three different nitrogen fertilizers, calcium tetrahydrate, ammoniumnitrate and urea were compared in two different levels corresponding 60 N kg/ha and 120 N kg/ha with an unfertilized control. It is considered that there is no way to fight the pathogen but the results in this experiment indicate that fertilization reduces the percentage of diseased plants in an initially infected soil. Nitrogen fertilization has reduced disease index in five of six tests and urea proved to have a good effect. The amount of nitrogen has been more important than the choice of nitrogen fertilizer. The positive effects of nitrogen fertilization motivates high levels of nitrogen in an early stage when rapeseed plants are susceptible to clubroot infection. The results need to be repeated under field conditions in order to draw clear conclusions.