Control of aphid-borne Lily symptomless virus and Lily mottle virus in Lilium in the Netherlands

An overview is presented on the management of viruses of Lilium crops in the Netherlands since the 1960s. This mainly concerns Lily symptomless virus (LSV) and Lily mottle virus (LMoV). Various factors which affect the efficiency of control are considered. The variable symptoms of LMoV in the many vegetatively propagated cultivars grown (c. 340) pose problems in the efficient roguing of diseased plants of some cultivars. Additionally the acceptable incidence of viruses such as LSV which are generally symptomless in field-grown plants may cause problems under the unfavourable light and growing conditions of year-round cut-flower production in greenhouses. The reduction in bulb yield and quality caused by viruses of lilies necessitates a further decrease in the already low virus levels still tolerated in lily bulb stocks. The routine detection of LSV and LMoV by ELISA has been developed extensively over the years. The impact of testing bulbs ('bulb test') during storage has been important in achieving an overall decrease in virus incidence in many stocks of lily cultivars, e.g. in Asiatic hybrids. The 'leaf test' used to assess many other cultivars, e.g. oriental hybrids, in which the bulb test for LMoV is not applicable, will be developed so as to eliminate at an early stage severely infected stocks that were initially intended for further propagation. The spread of the aphid-borne LSV and LMoV generally occurs very rapidly. The low virus incidence in the initially virus-tested stocks obtained by tissue culture procedures is effective in decreasing the access of vectors to virus-infected lilies. The viruses spread mainly in June and July, considerably less in May and least in August/September. Consequently the routine spraying of mixtures of mineral oil and pyrethroid insecticide is generally done weekly in May, June and July and fortnightly in August and September. There are differences in efficacy of the different brands of mineral oil. The rapid propagation of lilies, as done routinely by the scaling of bulbs and by tissue culture procedures, enables the rapid bulking of virus-tested and other stocks of high quality which have the health status required. The impact of the different factors in the management and control of viruses in Lilium crops in the Netherlands is discussed.