System and climate related pythium problems in mobile chrysanthemum growing systems

One Dutch greenhouse company started to grow chrysanthemums in a mobile system. The system’s basic unit is a sub-irrigated V-shaped gully of 8.0 m long and 5 cm wide, filled with a peat-coir mix. The system is hampered by growth differences along the length profile of the gullies and Pythium related yield reductions of up to 10% during the summer period. A series of experiments aimed to mimic the problems, explain causes and to advice on improvements. A Pythium ultimum pathogen from the grower was cultured in the laboratory and disseminated in the irrigation water tanks. In part of the cultivations the plants were subjected to high temperature and low air humidity treatments aimed at creating stress and Pythium susceptibility. Various plugs fit for transporting rooted chrysanthemum cuttings were tested as well. The results show that Pythium ultimum is initially the result of a too high water content in the first 10 days of the propagation phase. This was a direct consequence of precipitation of pure water by the mist system used to keep the relative humidity high. A related factor was a too low EC of