SPECIFIC PROBLEMS ON INFECTION AND DISINFECTION OF ARTIFICIAL PLANT SUBSTRATES
Under several conditions it can be useful to disinfect a peat medium after use as bench soil, as a cover layer or as potting soil, because of specific contaminations with nematodes, Rhizoctonia, Fusarium and others.
Peat, originally used as a microbiologically poor element can reach, after use, a high microbiological activity, including plant parasites. Table 1 gives an illustration of the microbiological activity of peat, respectively kept dry or used during six months as an underlayer for benches in ornamentals growing.
Some fungi such as Trichoderma, and especially several species of bacteria escape from the lethal action of chemical disinfectants, especially from the more selective methylbromide.
The surviving microflora, however, may play a prominent part in the further degradation of the pesticidal gases, especially by microbiological processes. The influence of the remaining, non parasitic, microflora on the phytotoxical after-effect is of an utmost importance in intensive horticulture, on the rotation-scheme and on the biological activity of the soil.
Graph 1 illustrates the after-effect of Metam-Na and of Chloropicrin in a peat substratum, of which previously one part was kept dry, the other part was microbiologically enriched. For the phytotoxical after-effect of MIT and particularly for those of chloropicrin the biological activity, also to be measured by Wahrburg apparatus, is strikingly illustrated by the shorter after-effect of only 28 days with regard to 49 days in the biological poor peat. In conditions normally causing a longer after-effect, such as very dry peat by adsorption of the polar MIT, or as a moist peat by adsorption of the polar Na-N-methyldithiocarbamate, or because of the longer persistence of chloropicrine, this difference becomes still larger, because of a difference in the amount of the surviving miro-organisms.
A part from the polarity of the chemicals used, a high organic matter content generally will increase the adsorption.
Graph 2 shows the pesticidal evolution and the phytotoxical after-effect of methylisothiocyanate, of Metam-Na and of chloropicrine in peat and in leaf mulch.
The toxic after-effect is strikingly longer in the microbiologically poor peat than in the biological rich leaf mulch, to be explained by the