Research Station for Floriculture and Glasshouse Vegetables, PO Box 8, 2670 AA Naaldwijk, The Netherlands

Background and objectives
In the Netherlands, pot plant cultures (e.g. Saintpaulia, Cyclamen, Spathiphyllum) are grown mostly in ebb and flow watering systems, on benches or on the floor. The nutrient solution in these systems is usually being recirculated. This recirculation of the nutrient solution can cause a huge spread of Phytophthora spp. in the system, if there are infective parts (mycelium, sporangia, zoospores) of this pathogen in the solution [1]. This spread can result in high amounts of infected plants (root rot) and therefore huge losses for the growers. Many Phytophthora species are found in the various pot plant cultures [2]. The disinfection of the nutrient solution in ebb and flow systems is expensive and time consuming. Therefore research on the spread of the different Phytophthora spp. in ebb and flow systems is necessary to make a risk-analysis for growers.

Materials and methods
In an ebb and flow system with benches and recirculation of the nutrient solution the spread of Phytophthora nicotianae, P. dreschleri and P. cryptogea in the system with pot plant cultures of Saintpaulia, Spathiphyllum and Gerbera, respectively, was studied on benches of 2 m2 and tanks with 300 l of nutrient solution. The spread of different amounts of mycelium and zoospores, put in the tanks or in some of the pots on the benches, was studied for 3-4 months (depending on the crop). In half of the treatments irrigation cloth was used on the benches, because of the idea being that mycelium and zoospores could hardly survive the dry conditions on the bench surface. Each experiment consisted of 12 treatments in 4 replicates.

Results and conclusions
In the experiments with P. nicotianae in Saintpaulia and P. dreschleri in Spathiphyllum no infected plants were observed after 3-4 months, as a result of spread of infective parts of these Phytophthora spp. In the experiments with P. cryptogea in Gerbera all plants were infected, as a result of a rapid spread of the infective parts. The amount of infected plants in an ebb and flow system with pot plant cultures on benches depends on the Phytophthora spp. Therefore every different Phytophthora spp. needs to be investigated for possibilities of spreading in these systems. Spread of Phytophthora spp. in nutrient solution is mostly via zoospores. Probably, the spread of a Phytophthora sp. is depending on the strength of the zoospores and the possibility to produce high amounts of zoospores. If a Phytophthora spp. can produce large amounts of strong and vital zoospores, then there is a good possibility of spread in an ebb and flow system, this is considered to be the case for P. cryptogea. For growers of pot plant cultures in the Netherlands, there is a need for more knowledge about the spread of Phytophthora spp. in ebb and flow systems. For example, how many zoospores per liter (thresholds?) are necessary to give a rapid spread of the disease, resulting in large economic losses.

1. Thinggaard K, Andersen H, 1994. Plant Disease 79, 259-263.
2. Rattink H, 1981. Netherlands Journal of Plant Pathology 87, 83-90.