Managing viruses in small fruit crops
/by Matthew Peill
Of the many pathogens that can affect small fruit producers in a growing season, viruses are generally among the hardest to manage and the least well understood. Viruses are microscopic infectious particles that must infect their host to multiply (replicate). Viruses replicate by hijacking their host’s cellular processes. This interferes with normal cell functioning and in turn causes disease.
In small fruit crops such as strawberries and raspberries, virus infections can reduce vigour and yield, with severe cases causing crop failure. Viruses are difficult to diagnose based on visual symptoms alone, as many symptoms of viruses look like nutrient deficiencies, pesticide damage, and other physiological conditions, and require specialized tests for accurate detection.
Unlike other pathogens, viruses can’t currently be managed using pest control products, and it’s not possible to eliminate viruses from plants once they’re infected in the field. Due to their damaging effects, it’s crucial to effectively manage viruses through an integrated approach, which requires an understanding of what viruses are present and how they’re spread.
In the Maritimes, the primary viruses of concern in strawberries are Strawberry mild yellow-edge virus (SMYEV) and Strawberry mottle virus (SMoV). The primary viruses of concern in raspberries are Raspberry leaf mottle virus (RLMV) and Rubus yellow net virus (RYNV). All four of these viruses can be transmitted (vectored) by aphids, with the Strawberry aphid (Chaetosiphon fragaefolii) the primary vector of SMYEV and SMoV, and the Large raspberry aphid (Amphorophora agathonica) the primary vector of RLMV and RYNV.
Most commonly grown strawberry and raspberry varieties are asymptomatic when infected with a single virus, but observable symptoms develop when two or more viruses simultaneously infect the same plant. If viruses are present and vectors are not controlled in a planting, virus infections can spread rapidly. Preventing the introduction and accumulation of viruses on-farm and monitoring vector populations are essential to prevent viral disease development.
One of the best ways to prevent the introduction of viruses is to use certified virus-free plants (available from select nurseries). Certified plants are inspected and produced using specific standards to ensure that they’re free of viruses as well as other diseases and pests. Planting a field with virus-infected plants puts you at an immediate disadvantage in managing viral disease development and can provide a source of virus inoculum that can then spread to other fields. By using certified virus-free plants, one potential source of virus introduction to a planting and farm is eliminated, but viruses can still spread to a planting from other infected fields and environmental sources.
Most producers have fields of varying ages on their farm. Older fields can be reservoirs for viruses and vectors that can spread to new, uninfected plantings. The longer a field is in production, the greater the probability that virus infections will occur and accumulate over time. For strawberries, it’s suggested that fields not be held for more than two harvests, as virus levels and vector populations increase dramatically after that period. In raspberries, which have longer production cycles, it’s recommended that virus-infected plants within a field are removed (if possible) and then replaced if the field is still high-yielding.
Wild strawberries and raspberries can also be sources of virus inoculum and vectors and should be managed in areas bordering cultivated strawberry and raspberry fields. If fields are to be carried over for multiple seasons, monitoring and controlling vector populations is essential.
Preventing the establishment of aphid vector populations in new plantings is highly effective in controlling virus infections. Once in a field, aphid populations can increase exponentially, so monitoring fields is critical for early detection and implementation of controls.
The most critical time to prevent vector establishment and virus spread is during aphid flight periods. Winged aphids can carry viruses several kilometres and can introduce viruses to otherwise uninfected fields. Strawberry aphids and raspberry aphids both generally have two flight periods, the first in early summer and the second in mid-fall. Flight periods vary from year to year, as well as by geographic location, so monitoring and proper identification of aphids in fields is necessary to accurately determine when controls need to be implemented.
Annual virus testing is also an excellent tool to manage viruses. Knowing if fields are infected with viruses and to what extent allows producers to make informed decisions on which fields to carry over for another season and how to best focus vector controls. If you suspect that your strawberry or raspberry planting is infected with viruses or would like to perform annual testing, please contact an appropriate Perennia extension specialist to advise you.
(Matthew Peill is a molecular biologist with Nova Scotia’s Perennia Food and Agriculture Inc.)
