Fusarium yellows, caused by Fusarium oxysporum f. sp. betae, causes significant yield and storage losses in sugar beet. The pathogen can be highly variable and sugar beet cultivars can have differing responses to fungal infection. Environmental factors may be contributing to this variability in disease response and these interactions have not been fully described. We describe how air temperature may influence interactions of F. oxysporum f. sp. betae with sugar beet. Fusarium yellows was assessed at different temperatures in five sugar beet cultivars to determine how increasing temperatures affect disease development. Generally, temperatures of 24 degrees C or higher led to more disease symptoms, while temperatures of 16 degrees C and lower led to fewer symptoms. Additionally, during resistant interactions, Fusarium yellows development did not increase as temperatures increased, indicating that resistance remained effective at higher temperatures. However, some F. oxysporum f. sp. betae isolates had varying abilities to cause disease at the different temperatures tested, with some isolates able to cause more disease at higher temperatures, while for other isolates, mid-range temperatures were optimum for disease development. To gain insight on disease development at higher temperatures, the growth rate of F. oxysporum isolates (both pathogenic and non-pathogenic to sugar beet) was assessed in vitro at 15 degrees C to 35 degrees C. The greatest growth tended to occur at 25 degrees C; however, growth responses varied between isolates and did not always correlate with a disease response. These results suggest that diversity of the fungal population could influence disease severity in the field at different temperatures. ResumeLa jaunisse fusarienne, causee par Fusarium oxysporum f. sp. betae, cause d'importantes pertes chez la betterave a sucre, et ce, tant de rendement que durant l'entreposage. Le pouvoir de l'agent pathogene peut etre fortement variable et les cultivars de betterave a sucre peuvent reagir differemment a l'infection fongique. Les facteurs environnementaux peuvent contribuer a cette variabilite quant a la reaction a la maladie, et ces interactions n'ont pas encore ete entierement decrites. Nous decrivons comment la temperature peut influencer les interactions entre F. oxysporum f. sp. betae et la betterave a sucre. La jaunisse fusarienne a ete evaluee a differentes temperatures chez cinq cultivars afin de determiner a quel point l'augmentation de la temperature peut influencer le developpement de la maladie. De facon generale, les temperatures de 24 degrees C ou plus ont provoque plus de symptomes, tandis que les temperatures de 16 degrees C et moins en ont provoque moins. En outre, au cours des interactions durant lesquelles la resistance entrait en ligne de compte, le developpement de la jaunisse fusarienne ne s'est pas intensifie a mesure que la temperature augmentait, ce qui indique que la resistance demeurait effective aux temperatures plus elevees. Toutefois, certains isolats de F. oxysporum f. sp. betae pouvaient de diverses facons causer la maladie aux differentes temperatures testees, certains isolats engendrant des taux plus eleves de maladie a des temperatures superieures, tandis que, pour d'autres, les temperatures moyennes suffisaient pour provoquer optimalement le developpement de la maladie. Pour mieux comprendre le developpement de la maladie a des temperatures plus elevees, le taux de croissance d'isolats de F. oxysporum (pathogenes et non pathogenes pour la betterave a sucre) a ete evalue in vitro a des temperatures variant de 15 degrees C a 35 degrees C. La plus forte croissance avait tendance a se produire a 25 degrees C; toutefois, les reactions de croissance variaient d'un isolat a l'autre et ne correspondaient pas toujours a une reaction a la maladie. Ces resultats suggerent que la diversite de la population fongique pourrait influencer la gravite de la maladie au champ, et ce, a differentes temperatures.
