Svampar associerade med raps : betydelse för uppkomst och grödetablering

Abstract: Oilseed rape is an important crop in Sweden and is grown on about 90000 ha today. The interest in oil seed pro-duction is increasing because of growing demand and high prices for oil seed both in Sweden and Europe. However, an increased intensity in the oil seed production can bring higher risk of yield losses due to fungal diseases. The purpose of this study was to investigate what different fungi are associated with oilseed rape under field conditions and if their incidence is correlated to crop establishment problems. This study was carried out in three parts: 1) investigations on fungal community structure in roots and rhizosphere soils from young spring- and winter rape plants sampled in failed respectively good establishment patches in four fields, 2) investigations on the pathogenicity of fungi isolated from oilseed rape in controlled environment and 3) investigations on the effect of different fungicide seed treatments on germination and early plant development in a field soil naturally infested with damping off- and Verticillium wilt pathogens. The studies on community structure using a combined approach of standard fungal isolation techniques and DNA based methods revealed a high fungal diversity in three out of four studied fields of spring rape and winter rape. For some unknown reason, very few fungal species were identified from the fourth field. Fungi belonging to establishment related pathogenic groups e.g. Rhizoctonia, Fusarium, Phoma and Alternaria were found in both spring- and winter rape. Also Verticillium, a pathogen attributed to wilting in late crop stages was found in both crops. In general, the plants from patches with good establish¬ment seemed to carry a higher number of fungal species than plants from patches with a poor establishment. Possibly, the potentially pathogenic fungi exist in equilibrium with non-pathogenic fungi in the good establish¬ment patches and whereas in poor patches fungal pathogens are favoured by being able to attack plants already weakened by other factors. This hypothesis, however, needs further investigations. The greenhouse study on effect of potentially pathogenic fungi on emergence and early growth showed that all tested fungi affected the emergence/plant growth compared to the uninoculated control. Rhizoctonia solani proved to be the most severe pathogen. Damping off-like symptoms and/or hampered plant growth due to Fusarium and/or Alternaria were also observed but the damage caused by these fungi was not statistically different from the control in this short term study. The greenhouse seed dressing study demonstrated that most of the tested compounds did not protect the seed-lings from damping off. In general, most treatments affected the rape seedlings negatively in some way. The only fungicide showing good effect was metalaxyl which is known to be effective only against oomycetes. This indicates that the field soil used in the experiment probably also carried oomycetous pathogens. Lack of detection of oomycetes in the community study is most likely due to the choice to use only fungal specific primers. The results indicate that there is a risk of losses due to bad crop establishment of oils seed caused by pathogenic fungi. This risk might be accentuated by low seed rates in especially winter rape. The combination of low plant density in the oil seed fields and an increased population of pathogenic fungi can increase economic risks. In a situation with increasing intensity in the oil seed production it is important to have knowledge about the role of fungal pathogens on early oil seed crop establishment.