The effects of waterlogging, alone and combined with ray blight disease (caused by Stagonosporopsis tanaceti), on pyrethrum (Tanacetum cinerariifolium) plant growth were quantified in glasshouse trials. Six pyrethrum cultivars were initially studied for their response to 6 days of waterlogging and their recovery from waterlogging during 26 days post-waterlogging. Waterlogging caused substantial root death and leaf wilting and accelerated senescence in all cultivars. Root growth was 80% more reduced than shoot growth. Cultivar ‘F’ showed significantly higher root porosity and growth following waterlogging than other cultivars. In contrast, cv. ‘C’ had the greatest growth reduction from waterlogging and poor root-system recovery after waterlogging. Plants of cvv. C and F inoculated with S. tanaceti and then waterlogged were more significantly affected than were those exposed to waterlogging only. For both cultivars, shoot growth under the combined treatment, relative to initial growth, recovered up to 25%, but root growth suffered irreversible damage. The combined treatment decreased the number of stems by 39% compared with waterlogging alone after the post-waterlogging period. In conclusion, pyrethrum cultivars showed differential reactions to waterlogging; but growth in all cultivars was seriously affected by a combination of waterlogging and infection by ray blight.