The Norwegian Crop Research Institute, Plant Protection Centre, Department of Plant Pathology, 1432 AAs, Norway

Background and objectives
Different varieties of ivy (Hedera helix L.) are among the most commonly produced ornamental foliage plants in the Norwegian greenhouse industry. Root pathogens are known to cause serious problems in this culture, particularly when the growth temperature increases during summer. Since 1988, Phytophthora cinnamomi Rands frequently has been isolated from diseased ivy in Norway and in Denmark [1]. It has been noted that the isolates detected from ivy are characterized by high optimum temperatures. In the present study, we investigated the effect of different temperatures on disease development in H. ;Helix inoculated with P. ;cinnamomi.

Materials and methods
An isolate of P. cinnamomiobtained from H. ;helix was used as inoculum during the whole study. The vegetative growth of the isolate on PDA was studied by using controlled-temperature incubators maintained at 9, 18, 22, 25, 28, 31, 34 and 37C. For further investigations H. ;Helix 'Pittsburgh' was used as host plant. Plants were grown from cuttings in limed and fertilized peat with five individual one-leaf cuttings in each 10-cm pot. To prepare inoculum, agar with fungal growth was added to distilled water and homogenized; 25 ;ml of the homogenized suspension was added to each pot. The effect of temperature on disease development was studied by growing plants in growth chambers at constant air temperatures of 18, 21, 24, 27, 30 and 33C. The lowest temperature was chosen as an optimum growth temperature of ivy, and the highest as a optimum growth temperature of the fungal isolate. A further experiment was designed to investigate the effect of changes in temperature during production. Plants were grown at 20C the first 21 ;days after inoculation. The temperature was then increased to 30C the following 21 ;days. For disease assessment in both experiments individual pots were examined and the number of wilted runners with Phytophthora root rot recorded at 2-day intervals after inoculation. Samples of roots were collected for isolation of the pathogen. Both experiments were repeated once and the analysis of variances conducted by using the of general linear models procedures.

Results and conclusions
Isolates of P. ;cinnamomi obtained from ivy in Norwegian greenhouses, showed high optimum temperature for vegetative growth (31-34C). No mycelial growth was recorded at 9C and just very restricted growth was found at 37C. Experiments in climate chambers revealed that temperatures of 20C and above were necessary for development disease in ivy inoculated with P. cinnamomi. Temperatures between 23 and 33C increased disease severity. No dead plants were found at 18C. Further experiments confirmed the establishment of the pathogen at 20C. The disease developed rapidly when increasing the temperature from 20 to 30C 3 ;weeks after inoculation. Our results are consistent with previous reports which indicate that symptom development and severity caused by P. ; cinnamomi progresses slowly at low temperatures [2]. This emphasizes the importance of adequate temperature control in the greenhouse production of ivy. Where P. ;cinnamomi is the main source of disease, periods with temperatures above 20C should be avoided.

1. Thinggaard K, Toppe B, 1997. Plant Disease 81, 960
2. Grant BR, Byrt, PN 1984. Phytopathology 74, 179-184