SOME SPECIFIC PROBLEMS ARISING FROM METHYL BROMIDE APPLICATION IN GREENHOUSE CUCUMBER GROWING ON STRAW BALES

The timing of methyl bromide application during the preparation of straw bales for cucumber growing induced different effects on the fermentation of the straw and, consequently, on the growth and yield of cucumber plants. Fumigation employed before watering of the bales and adding nitrogen, delayed the beginning of the fermentation by about two weeks. After this time the biothermal process had started, but maximum temperatures generated by the fermentation were several degrees Celsius lower than in non-fumigated straw. Fumigation of the straw (with or without the capping soil) during the fermentation, with temperatures in the centre of the bales reaching 32–38°C, caused sudden interruption of the fermentation and rapid temperature decrease to the level of the ambient temperature. Shortly after setting plants on the bales, quite different fungal communities, attributable to the fumigation timing, were visible on the soil surface. As a consequence of microbiological perturbations caused by fumigation in the course of the fermentation, early growth retardation and marked symptoms of nitrogen deficiency developed on plants grown on straw bales treated before fermentation. Also chlorophyll content in leaves from those plants was substantially lower. In these conditions, the total yield was decreased by 6 % (cv. Taha) and 16 % (compared to controls) in the first and the second year of the experiment, respectively. On the other hand, fumigation performed during the fermentation had no detrimental influence on the plant growth and yield of two cucumber cultivars tested. The extent of uptake of inorganic bromide by cucumber plants was also influenced by the time of methyl bromide application. Fumigation during the fermentation of the straw resulted in a threefold increase in Br^- residues in the fruit over the treatment of straw before fermentation.