Tolerance in Greenhouse Geraniums to Pythium ultimum. M. -C. Chagnon, Graduate Research Assistant, Département de Phytologie, Faculté des Sciences de l’Agriculture et de l’Alimentation, Université Laval, Québec, Québec, Canada G1K 7P4. R. R. Belanger, Research Associate, Département de Phytologie, Faculté des Sciences de l’Agriculture et de l’Alimentation, Université Laval, Québec, Québec, Canada G1K 7P4. Plant Dis. 75:820-823. Accepted for publication 18 February 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0820.

Four cultivars of geranium (Pelargonium × hortorum) were used to study their resistance to two strains of Pythium ultimum. In two separate experiments, the cultivars tested responded similarly to the treatments in terms of plant death, dry weight, plant volume, and symptom evolution. One strain was significantly more virulent than the other, but there was no significant interaction between the cultivars and the strains, indicating that the source of inoculum did not affect the respective response of the cultivars to the treatments. An average of 20% mortality was recorded after the inoculations. The growth of all inoculated surviving plants was severely delayed compared with the controls. However, two of the cultivars, after the initial stress of the inoculations, were able to resume a growth rate comparable to the controls and did not exhibit symptoms (except for growth delay) after 4 wk. The two other cultivars never recovered fully from the initial attack by P. ultimum. This relationship between the remission of symptoms and plant growth resumption of inoculated plants of some cultivars seems to indicate a defense reaction from the plant.