Effects of soil fumigation and flooding on suppression of Pythium root rotin ornamental bulb culture

The effects of flooding and soil fumigation with cis-dichloropropene or met hylisothiocyanate (MIT) on disease suppression against Pythium spp. were te sted in pot and field experiments in sandy soil. Disease suppression was re duced by both flooding and fumigation treatments, resulting in severe infec tion in Iris and Crocus and reduction of bulb yields. It is demonstrated th at the disease suppression has a biological nature, and that disease severi ty is more related to effects of the soil treatments on the soil microflora than to the initial inoculum density of Pythium. After flooding, disease s uppression was restored within the experimental period of two years, wherea s after fumigation, disease suppression was only partially restored. The ef fect of repeated fumigation in two consecutive years on the disease suppres sion was less severe than the effect of a single fumigation treatment prior to cultivation of a susceptible crop.