Preventative control of non persistent aphid transmitted tobacco virus in France
Non persistent, aphid transmitted virus frequently occur in France and are detrimental to the flue-cured and burley leaf production. In order to set up an integrated management strategy, a study involving every French tobacco producing Cooperatives has been run by ANITTA. In a first step, a survey was performed, based on controlling virus symptoms and sampling for ELISA confirmation, over 58 fields grown with susceptible varieties and located in every part of France (50 in 1998, and 8 in 1999). In light of the results, the second step was devoted to assessing new technical means for virus control through classical field trials. The survey confirmed that in the French situation, PVYN and CMV are the most frequent tobacco virus. PVY occurs in every production area, while CMV occurs mainly in south of France, often on the same field and plants than PVY. Virus pressure has been found to be lower in the most western areas nearby the Atlantic ocean. Sampling greenhouse grown seedlings showed that they were virus free, in every of the 58 studied situations but one. Based on symptom date of appearance and incubation duration estimates, it was found that contamination occurs generally between 15 and 30 days after planting. Crops located on plateau rather than on valley and planted with seedlings treated with imidacloprid in the water, had significantly less viral symptoms. However these factors provided only a partial degree of control. No other relation were shown between frequency of symptoms and the various technical observations made during the survey (type of surrounding crops, preceding crops on the same field, weeds, agro-chemical use…). In 1999, a specific nylon yarn trap for aphids was used and periodically checked. Results of trapping were closely related with symptoms appearance in the next 15 days. Among the different means of control that were assessed in field trials, the following were shown to be efficient for decreasing the symptom frequency, and free from detrimental effects on the tobacco plants : mulching with transparent polyethylene plastic, and weekly spraying of Vazyl→ oil. Combination of both techniques showed a strong decline of the virus incidence.