Estimates of aggregate disease costs can be used for assigning research resources or to evaluate control measures. Most diseases cause production losses, but others affect quality and marketability. Seed-borne diseases also cause problems for the seed production and distribution industry. The aim in this paper is to examine issues relating to the economic impact of a quality-reducing, seed-borne disease, and to highlight differences compared to non-seed-borne diseases affecting yield only. Economic evaluation of quality-reducing, seed-borne diseases needs to incorporate impacts of trading restrictions such as quarantines or embargoes imposed by purchasers. The costs of measures taken to control diseases also represent part of the economic impact of the disease. Full economic costs of a disease include the direct (yield and quality) costs and costs of the control measures. The costs of Kamal bunt of wheat in Mexico were found to include many control costs that have often been overlooked. The optimal amount of resources to invest in controlling a disease depends on the likely annual costs of the disease and of control measures. Before implementing disease control policies, both the costs and the benefits of the policies need to be considered, taking the risks of each option into account, to ensure that the policy itself does not impose greater costs than the uncontrolled disease.