Development of disease-suppressive organic growing media

Vigorous seedlings are an important base for vegetable production. Beside the availability of appropriate amounts of nutrients, the health of seedlings is decisive. Soil-borne diseases are a challenging problem in organic seedling production. Here, we present results on the development of disease-suppressive growing media. Three aspects were examined: (i) use of different components of growing media (peat, coconut fiber, wood fiber, compost), (ii) influence of selected organic nitrogen fertilizers and (iii) use of different microorganisms (including commercial biocontrol agents (BCA)). Three plant-pathogen systems were used in this study: cucumber-Pythium ultimum, cress-Pythium ultimum and basil-Rhizoctonia solani. Green waste compost showed a good capability to protect cress against P. ultimum. This effect was improved by using a chitin-containing N-fertilizer. However, an inappropriate storage of the compost diminished its efficacy. In contrast to coconut fibers, wood fibers showed a suppressive activity against P. ultimum when used as partial substitutes of peat. None of five tested commercial BCAs could improve the suppressiveness of the substrates against P. ultimum. However, one of newly tested strains of Trichoderma sp. was very suppressive against P. ultimum. The tested growing media showed only small differences in suppressiveness against R. solani on basil. In contrast, two of the new strains of Trichoderma sp., which were intermediately active against P. ultimum, could efficiently protect basil against R. solani. At the moment, we test combinations of different Trichoderma strains, compost, different types of peat and peat substitutes. The aim is to determine whether it is feasible to manufacture growing media which allow the production of healthy and robust seedlings also in the presence of high levels of pathogens.