Symptoms of yellowing of the newest leaves began showing up over the past few weeks in one lavender field (Figure 1). The symptoms showed up on many plants in the field, but the degree of symptoms on each plant was relatively minor. The plants were sent for virus testing and three viruses were confirmed in the plant tissue: tobacco mosaic virus (TMV), alfalfa mosaic virus (AMV) and impatiens necrotic spot virus.
Figure 1. Yellowing of lavender leaves as a result of virus infection.
Both AMV and TMV are very common viruses that affect a wide host range. They are very easily transmitted from plant to plant by insect feeding, transfer on pruning equipment and plant to plant through physical contact. AMV has been reported before on lavender (see article below by Dr. Curtis Swift in Colorado for more details). TMV has been found in plants at the Simcoe Research Station with similar yellow symptoms in the past. Plants can be infected with viruses but have very little damage from them. In our experience with TMV in the past, the plants grew out of the symptoms after a few weeks and they never returned. The combination of the three viruses in the current case may be why symptoms are beginning to show up. It is unknown if the viruses will have any effect on plant growth or yield. Some stunting of the affected plants may occur.
Transmission of the viruses in the field is most likely from insect feeding and pruning equipment. Insect feeding is very difficult to control in lavender, and because these viruses can be easily spread by any insect wandering into the field from another crop, controlling insects in the field will not necessarily prevent spread of the viruses.
Repeated sterilization of pruning equipment is the best way to prevent spread of the viruses. It is also important not to take cuttings from areas with virus symptoms. Plants without symptoms in areas with virus infections may still contain viruses. See the article below for control suggestions and sterilization procedures but bear in mind that many products registered in the US are not registered for use in Canada.