3.8.9
PREVENTION OF PHYTOPHTHORA ROOT ROT IN POT PLANTS THROUGH MANIPULATION OF COPPER IONS IN THE NUTRIENT SOLUTION

K THINGGAARD1 and B TOPPE2

1 Danish Institute of Agricultural Sciences, Department of Omamentals, Kirstinebjergvej 10, DK-5792 Aarslev, Denmark; 2The Norwegian Crop Research Institute, Plant Protection Centre, Department of Plant Pathology, Fellesbygget, 1493 Aas, Norwa

Background and objectives
Problems with Phytophthora root rot in pot plants grown on ebb-and-flow benches with recirculation of the nutrient solution are common. To reduce the use of fungicides, research has been carried out since 1992 to find a new strategy for prevention of Phytophthora. An investigation has shown that watering frequency and electrical conductivity of the nutrient solution influence plant death caused by phytophthora root rot in pot plants of Gerbera jamesonii [1]. Raising the electrical conductivity (EC) from 1.5 to 2.2 mS/cm reduced plant death from 73.6 to 13.4% recorded 33 days after inoculation. This could be due to an increased content of one or more elements in the nutrient solution e.g. it is known that copper ions in low concentration (0.1 ppm) can harm zoospores of Phytophthora and Pythium. In order to test this experiments were carried out with a normal and a high copper ion level in the nutrient solutions.

Materials and methods
Greenhouse experiments were carried out with pot plants of Gerbera jamesonii grown on ebb-and-flow benches with separate systems for recirculating nutrient solutions and inoculation with zoospores of the root pathogenic fungus Phytophthora cryptogea [2]. Nutrient solutions with two levels of copper (0.07 or 0.28 ppm), two electrical conductivity values (1.5 or 2.2 mS/cm), and two iron sources (iron sulphate or iron chelate) were combined in a factorial design. Known concentrations of zoospores were inoculated via the nutrient solutions and the experiments were finished 22 days later.

Results and conclusions
A 72% reduction in plant death was observed by increasing the copper concentration from 0.07 to 0.28 ppm when the iron source was iron sulphate. No effect of increased copper concentration was observed when iron chelate was used as iron source, probably because copper ions were bound to the chelate. Electrical conductivity did not influence the disease severity. It can be concluded that disease incidence was significantly reduced when plants were grown in nutrient solutions with 0.28 ppm and iron sulphate.

Increased copper ion concentrations in the nutrient solution can be a future component of disease management in ebb-and-flow systems and thereby reduce the consumption of fungicides. But further development of the method is needed. Also the method could be less expensive and more effective compared to disinfection of the nutrient solution by heat, ozone or slow sand filtration.

References
1. Thinggaard K, Andersen H, 1995. Plant Disease 79, 259-263.
2. Toppe B, Thinggaard K, 1998. European Journal of Plant Pathology 104, In Press.