Compost and its effects on soilborne plant pathogens

Abstract: Certain microbial residents from composts are known to possess the ability to suppress soilborne plant pathogens. Trichoderma spp. and Gliocladium spp. are commonly found in composts and are perhaps the most wellknown hyperparasites of fungal pathogens, which may also be naturally present in compost. Trials have many times proven them to be effective against plant diseases caused by soilborne pathogens such as Pythium spp., Phytophthora spp. and Fusarium spp. In this study, extracts obtained from eight different composts of various origin and maturity were applied to oilseed rape and cucumber seedlings under greenhouse conditions. The test plants (oilseed rape and cucumber) were thereafter challenged with the test pathogens. Oilseed rape was inoculated with Verticillium longisporum (former V. dahliae) and cucumber with Pythium sylvaticum. V. longisporum is a wellknown fungal soilborne plant pathogen that causes wilt disease in oilseed rape and other hosts. Since V. longisporum apart from being soilborne also is vascular, it is more difficult to control compared to opportunistic soilborne fungal pathogens, e.g. Pythium spp. Many experiments have proven compost amended media to be suppressive to several pathogens including Pythium spp. On the other hand, little research seems to have been done on composts regarding control of V. longisporum. In the present study, P. sylvaticum failed to infect cucumber seedlings, probably due to its weak pathogenic nature among other factors. The trials were hence focused on compost mediated suppression of V. longisporum in oilseed rape. The results showed that V. longisporum infected oilseed rape seedlings to various degrees, depending on the compost extract applied. Both fungi and bacteria could be isolated from the compost extracts that were evaluated for their in vitro antagonistic activity against V. longisporum. Predominantly fungal isolates were demonstrated to be strongly antagonistic towards the pathogen. Most bacteria isolated from the compost extracts showed no or only slight antagonistic activity towards V. longisporum, with the exception of two isolates that exhibited strong antagonistic activity. The results obtained suggest that compost has potential to function as biological control of V. longisporum.