PYTHIUM ROOT ROT IN TULIP FLOWER CULTURE AS VAN BRUGGEN, TLJ DUINEVELD Bulb Research Centre, PO Box 85, 2160 AB Lisse, The Netherlands Background and objectives Pythium ultimum causes root rot of tulips in flower-forcing systems. Root rot results in a reduction of flowerstalk length and loss of quality. Tulip flowers are produced in the greenhouse in plastic boxes filled with substrate, generally potting soil. Research was carried out to determine possible sources of inoculum and to study the effect of cultural practices to prevent contamination of the system by Pythium. Three aspects were studied: introduction of Pythium
  • into the greenhouse by the bulb, infection caused by contaminated boxes and the importance of root residues from a former culture in case substrates are re-used. Material and methods To study the possible transmission of Pythium by the flowerbuib from field into greenhouse an experiment was conducted with tulip bulbs harvested from a field were Pythium was present in the soil. Bulbs were properly peeled or residues of old roots were left on the base of the bulb. Latter can occur in practical bulbculture in case peeling is carried out by machine. Furthermore bulb disinfection with formaldehyde was carried out or not . The bulbs were planted and at flowering rootrot severity was assessed. Another aspect studied was the possible survival of Pythium on the plastic material of the boxes in which the flowers are forced. Plastic flowerpots were collected from a culture suffering from Pythium root rot. These pots were stored for half a year and before the next culture different treatments were carried out to eliminate possible Pythium
  • : thorough removal of substrate and root residues, hot-water treatment for 1 hour at 60oC and disinfection with formaldehyde. Tulips were planted in the pots of the different treatments and at flowering root-rot severity was determined. Re-use of substrate was simulated by adding Pythium-infected tulip root pieces to a fresh substrate (10 g/l). This substrate was immediately re-used or stored half a year before planting the next culture. Furthermore this substrate was steam-sterilized or not. Tulips were planted in the substrates and at flowering disease was assessed. Results and conclusions In case residues of old roots were present at the base of the bulb root rot developed during forcing. Removal of these old roots by proper peeling reduced disease considerably. Disinfection of the bulbs with old roots did not prevent root-rot development completely. Combination of proper peeling and disinfection resulted in a healthy root system. It can be concluded that Pythium is able to survive in the old roots and infection of newly formed roots during flower-forcing can occur. Flower-forcing in plastic pots collected from a culture suffering from root rot resulted in considerable disease in a following culture in the next season. Thorough removal of substrate and root residues did not reduce root-rot severity. Hot-water treatment or disinfection of the pots before the next culture prevented the development of root rot. Apparently also contaminated plastic boxes can cause root rot in practical flower forcing. Stephens [1] reported the development of damping-off in bedding plants caused by Pythium which had survived on used seedling flats. If substrate containing contaminated tulip-root fragments was re-used without steam-sterilization severe root rot developed. Storage of the substrate for half a year could not prevent this. In the steamsterilized substrates no root rot was observed. Steam-sterilization can be applied in case substrates are re-used to avoid the risk of infection of the crop by Pythium from a former culture. It can be concluded that sanitation practices for bulb-material, growing boxes and growing substrate are important to prevent root rot in tulip-flower forcing. Reference 1. Stephens CT, Herr LJ, Schmitthenner AF, Powell, CC, 1983. Plant Disease 67, 272-75.