Supressive effects of yard waste compost amended growing media on soilborne palnt pathogens in organic horticulture

Summary

Suppressive effects of composts have been demonstrated mainly by American scientists with composted hard-wood bark as a part of container media for ornamentals. In this paper we report about experiments with yard waste- (YWC), biogenic waste- (BWC) and cattle manure compost (CMC) towards the soil borne pathogen Pythium ultimum Trow. There are results included from model composting systems and from yard waste composts originating from two commercial composting plants (C-YWC). It was shown that, in comparison to the other tested composts, YWC from the model composting system was superior in reducing disease incidence caused by P. ultimum on peas. In fifteen cases out of eighteen experiments with addition of 15 % compost the amendment of YWC resulted in significantly higher fresh matter compared to the control treatment. The average reduction of disease incidence of YWC was 50 %. Amendment of sterilised sand with BWC resulted in similarly good results for both amendment rates. However, this was not the case with CMC. In total, the addition of CMC resulted only three times in a significantly higher fresh matter than the control. Reduction of disease incidence was markedly lower than that provided by YWC or BWC.
In bioassays using YWC amended peat growing media we could demonstrate the high suppressive abilities of this compost. In these systems which are closer to conditions of the horticultural industry it was possible to ver-ify the results previously obtained with steril sand as the basic growing medium. Due to the favourable chemical characteristics of YWC it was possible to add between 30 % and 50 % (v/v) to a peat growing medium without adverse effects on the cultivated plants. Growing media amended with 50 % of YWC waste com-post contained between 50 and 100 mg N/l as easily available NO3-N and the measured salt content ranged from 0,9 to 1,1 g KCl/l. In the cucumber bioassay amendment of 50 % YWC to the peat growing media markedly suppressed P. ultimum and reduced disease incidence up to 86 %. Results from bioassays obtained with C-YWC confirmed the former results with YWC from the model system. C-YWC from composting trial 1 reduced disease incidence up to 80 % in treatments with a strong disease severity of approx. 70 to 90 %. The compost produced in composting plant 2 during the second composting trial showed consistent effects in suppression of P. ultimum even on an higher infection level. So far, we could observe a slight trend that suppressive effects are dependent on the composting time. Material obtained in the age of 3 to 6 month seemed to be more effective than material from samples of the older compost. Although these results are preliminary, they demonstrate the possibility to produce suppressive composts on commercial composting plants using YW