Strawberry virus survey 2023

Erica Pate & Katie Goldenhar

Ontario strawberry growers have been managing viruses for almost a decade now. Strawberry viruses, including strawberry mottle (SMoV), strawberry vein banding (SVBV), strawberry mild yellow edge (SMYEV), strawberry pallidosis-associated virus (SPaV), and strawberry polerovirus 1 (SPV1) cause plant decline, stunted plants, and reduced yields. Symptoms can include leaf cupping, asymmetrical leaves, chlorotic foliage, and leaf mottling. Typically, symptoms develop when two or more viruses are present. Unfortunately, symptoms can be easily confused with herbicide damage, cyclamen mite injury, or nutrient deficiency, and to make diagnosis more challenging symptoms cannot always be associated with a particular virus. The combinations of different viruses and the cultivars affected may result in different symptoms.

Most strawberry viruses are vectored by aphids, so growers have been vigilant with aphid management over the last 10 years. Over the last few years growers are finding their fields are lasting longer and vigour has improved, compared to previous years where viruses were reducing yields and vigour, and fields were only lasting for 1 or 2 harvests. 

In Ontario we have done multiple virus surveys over the years to determine the incidence and frequency of strawberry viruses, with the most recent surveys in 2017 and 2023. The 2017 survey determined the level of strawberry viruses in Ontario after 4 years of virus vector management. Results indicated virus levels had decreased, but not as much as anticipated. Further decrease was anticipated following the 2017 survey if growers obtained virus free transplants and continued to effectively manage virus vectors.  

In 2023 we collected 6 samples from 16 farms across the province, including the same farms that were involved in previous surveys. Results from the 2023 survey showed the trend of decreasing incidence of viruses continued for most of the viruses included in the survey (Fig. 1).

The incidence of three viruses was lower compared to 2015; incidence of strawberry polerovirus was slightly lower than 2017. However, incidence of strawberry pallidosis virus continued a trend of increasing incidence. While these other viruses are vectored by aphids, strawberry pallidosis virus is vectored by whiteflies, suggesting whitefly management may be warranted.

Overall, 12.5% of all samples in 2023 were positive for more than one virus, compared to 27% of samples in the 2015 survey, demonstrating the efforts made to control strawberry aphids are having a positive effect on virus management. However, the incidence of some viruses was higher in 2023, and multiple viruses were identified in some samples. We still find unproductive fields that lack vigour with multiple viruses present.

Vigilant aphid management over the last 10 years is working to control viruses and improve the vigour and longevity of strawberry fields. It is very important growers maintain a regular aphid management program, and whitefly control may also be needed.

Managing viruses:

If possible do not plant new fields next to older fields, and begin with clean plants from an accredited plant grower. Once plants are established begin to monitor for aphids, and continue monitoring through the season. Aphids need to be controlled season-long. Monitor and manage aphids in new and bearing fields, regularly through the season. In a well-managed field aphid numbers should always be very low. For a list of insecticides registered for aphid management visit the Ontario Crop Protection Hub. If whiteflies are present choose products that have efficacy against aphids and whiteflies.

Monitoring strawberry aphids:

To monitor for strawberry aphids this spring, check the underside of new leaves and look for the pale, green bodies (Fig 2). Strawberry aphids can be distinguished from other aphids by knobbed hairs all over their body- a hand lens or microscope is necessary to see the hairs. Strawberry aphids can be winged or wingless- it is the winged form that needs to be controlled. Collect 60 new, partially folded leaves and count the number of aphids per leaf. Currently there are no economic thresholds developed, but growers should aim to keep the population below 15 aphids/60 leaves. An insecticide is needed when the population starts to grow. Monitor weekly.

Thank you to the participating growers over the years. This project was funded through the Sustainable Canadian Agriculture Partnership Plant Health Project.