Influence of Watering Frequency and Electrical Conductivity of the Nutrient Solution on Phytophthora Root Rot in Pot Plants of Gerbera . K. THINGGAARD, Danish Institute of Plant and Soil Science, Research Centre Aarslev, Department of Ornamentals, Kirstinebjergvej 10, DK-5792 Aarslev, Denmark. H. ANDERSEN, Danish Institute of Plant and Soil Science, Research Centre Aarslev, Department of Ornamentals, Kirstinebjergvej 10, DK-5792 Aarslev, Denmark. Plant Dis. 79:259-263. Accepted for publication 21 November 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0259.

Recirculation of nutrient solutions in nurseries with ebb-and-flow benches will reduce discharges to the environment, but a prerequisite is that the quality and health of the plants are satisfactory. An experiment investigating the possibility of reducing attacks of Phytophthora cryptogea through changes in watering frequency (WF) and values of electrical conductivities (EC) was carried out in pot plants of Gerbera jamesonii grown on ebb-and-flow benches with recirculating nutrient solution. Nutrient concentrations EC 1.5 and 2.2 mS cm-1, respectively, were combined with WF 0.5 and 1.5 times per day, respectively, with or without inoculation with P. cryptogea in a factorial design. Symptoms of root rot were absent from plants on noninoculated benches. Raising EC and lowering WF both inhibited the attack, alone or in combination. Percent plant death caused by P. cryptogea was 73.6% at WF 1.5 times per day/EC 1.5 mS cm-1. By contrast, only 4.7% plant death was observed at WF 0.5 per day/EC 2.2 mS cm-1. Also, the treatments with WF 1.5 per day/EC 2.2 mS cm-1 and WF 0.5 per day/EC 1.5 mS cm-1 reduced the attacks (13.4 and 13.5%, respectively). Percent diseased plants was highest near the inoculation points in all four combinations, and there was no difference in the pattern of spreading, only in the severity of attack. The results show that Phytophthora zoospores in the nutrient solution in an ebb-and-flow system with recirculation could cause an epidemic under certain conditions. The findings also demonstrate that it is possible to reduce attacks of Phytophthora considerably without the use of fungicides by adjusting the WF and EC values appropriately. This is possible without effects on the growth and flowering of the plants, even when the nutrient solutions are recirculated.