Myth Busting and Frequently Asked Questions

Thursday, April 21, 2022

If you’ve taken advantage of winter webinars you’ve probably soaked in an abundance of information recently. This quick Q&A post will answer your frequently asked questions, including how all of this information that you learned applies to you – locally.

Remember, if you plan on changing your practices, consult colleagues and advisors first and always try a new practice on a small area or encourage local research on the topic.

Are there reports of apple powdery mildew being resistant to fungicides in North America?

Although a region in North America might not have fully documented resistance to a fungicide, there are cases of fungicide resistance on tested farms. The 2013 National Powdery Mildew Resistance Survey showed there is resistance in some NS orchards to the group 11 products Flint and Sovran. In resistant mildew populations, Flint and Sovran will not provide adequate control. Plus, in almost half of the tested farms the mutant genotype for resistance to strobilurins (QoI – group 11) was already present. It would be only a matter of time before the majority of the surviving fungal population would be the resistant genotype.

The samples for the national survey were taken from a random sample of orchards, not only those that were sprayed for mildew on an annual basis where the concern would be greater. The survey was done back in 2013 so given that more time has lapsed, and only single site products can be used for powdery mildew management, there has been more opportunity for resistance to develop. Also, consider that when two late-season applications of Pristine/Merivon (groups 7 and 11) are being applied to the orchard, existing infections of mildew in the orchard would be sites where selection pressure could happen unintentionally.

At the time of the survey, the powdery mildew population on most tested Nova Scotia farms was still susceptible to the group 3 Nova and group 7 Fontelis, so group 3 and 7 products would be expected to give control of powdery mildew. 

Can fire blight trauma blight be managed by copper?

No, not directly. Copper could be part of a sanitation program early in the season to reduce the population of bacteria on plant tissue surfaces. The usefulness of copper is limited because it is not systemic and therefore cannot reach bacterial populations after they enter plant tissue. During a trauma event, plant tissues are damaged and bacteria enter tissues. Only streptomycin has been proven to be effective (when applied soon after trauma) due to its partially systemic activity.

How many applications of antibiotic do I need to manage fire blight blossom blight during bloom?

It is impossible to give an all-encompassing recommendation. You might have heard the claim that only one application of an antibiotic would provide protection during bloom but this is a very risky assumption!

Fire blight infection is caused by the bacteria Erwinia amylovora, which is a living organism that responds to environmental conditions. Given a lot of heat, the bacteria will grow more quickly. Given moisture, the bacteria will move from the flower and into the nectarthodes to cause infection.

Models were created to improve our ability to implement best control programs and have had great success. The Maryblyt model takes into consideration heat ‘clocks’ that govern the life of individual flowers and bacterial growth. If Maryblyt predicts an infection risk, then it is good justification for an antibiotic treatment. Even a few early or late blossom infections are a significant risk to orchards.

The claim that only one application of an antibiotic would provide protection during an entire bloom period also ignores the susceptibility of individual flowers that do not open at the same time. If a spray is applied before individual blossoms open, then the floral stigma where bacteria grow would be unprotected. The new flower would be susceptible to infection if it was open long enough for bacterial colonization, even if an antibiotic was applied to flowers that had opened previously.

From the Maryblyt manual, the writers describe, “In our experience with the Maryblyt program, one or two well-timed streptomycin sprays are sufficient to prevent blossom blight in most orchards in most years.” Again, the uncertainty in this statement is why we have the model to inform decisions in real-time.

Note that the risk of infection is especially frequent on young plantings that bloom late in hot weather. Bacterial growth can be very aggressive in such conditions.

Figure 1: Infection by fire blight under the right weather conditions at bloom on unprotected floral stigmas (left) results in blossom blight symptoms (centre) and production of bacterial ooze (right). The need for blossom protection relies on several environmental factors that are predicted by the Maryblyt model.

Should I try Blossom Protect for management of fire blight blossom blight?

Blossom Protect is a living yeast that colonizes the flower stigma and the nectary. By living in the same location as the fire blight bacteria, the healthy growth of yeast competes for nutrients and space. Colonization of flowers by Blossom Protect reduces the chance of bacterial population sizes that cause infection.

Blossom Protect is registered for control of fire blight but directly on the label it says, “A sufficient reduction of fire blight is not expected in all cases.” Biological products that are living organisms can be unpredictable so their use comes with added risk that should not be taken lightly. For example, the yeast needs sufficient time to build its own population to compete with fire blight bacteria for effective control. This may require properly timed applications during early bloom, repeat applications, and weather conditions conducive to the growth of yeast.

Also, remember that biologicals come with a lot of variability usually shown as error bars in research graphs. The error bars on streptomycin are small because it is consistent across individually tested trees. Alternatively, variable infection from biological treatments could mean that some trees in a commercial situation do not have acceptable control.

Also note that Blossom Protect can cause fruit russetting in a high humidity environment. And fungicides cannot be tank mixed with the yeast at application so you would need to apply fungicides the day before or 2 days after Blossom Protect.

Would I need to interpret Maryblyt differently if using Blossom Protect?

An EIP of 40 to 70 (as opposed to EIP 100) is theoretically the threshold for treatment with biologicals, as determined by research in other regions. We should question if this threshold applies to our regional conditions. Regardless, biologicals will only slow bacterial growth instead of reducing populations so the biologicals MUST be applied before significant Erwinia bacterial growth occurs.

Furthermore, the default for a protective spray entered into Maryblyt is that a chemical treatment is 100% effective. The model assumes that the user is spraying an antibiotic like Streptomycin or Kasumin and therefore resets the bacterial growth (EIP) to zero. Users can adjust the efficacy in the Maryblyt settings and estimate a new level of efficacy, in which case the new EIP will be proportional to the spray efficacy setting. However, I have not been able to find recommendations on if or how much the efficacy should be adjusted for biologicals. Under situations of explosive bacterial growth, we can only be sure that the antibacterial products can reduce bacterial populations.

Should I try LifeGard for management of fire blight?

LifeGard is registered for the suppression of fire blight in apples. Suppression is defined as not consistently reducing pest numbers or damage to a commercially acceptable level. Expect about 60% efficacy. In the case of fire blight, not achieving effective control can devastate orchards. If you are set on trying a biological product, try biologicals for a disease that would result in less economic damage in the case that control is not achieved.

Is Streptomycin being cancelled or do we need to stop using it?

No, we have not been informed that streptomycin will be unavailable. Streptomycin is scheduled for review in Canada by the Pest Management Regulatory Agency (PMRA) beginning in December 2023 after which we will learn if there are any new concerns.

Given the significant fire blight pressure in our region and the more predictable performance of Steptomycin and Kasumin than biologicals, the use of Steptomycin and Kasumin continues to be recommended in Nova Scotia. Using no more than three applications of Streptomycin per year has been shown to protect against resistance development. Also, following all label directions meets safety requirements.

Research into biologicals is a good and forward-thinking practice so that we understand and have alternatives available. In practice at this time, the traditional products that give reliable control for such a devastating disease are the proven practice.

Can we use the NEWA disease forecasting for apple scab and powdery mildew?

No, currently NEWA does not integrate Canadian weather stations or forecasts for their model outputs and predictions. The value in hearing about the NEWA system is that we can decide if models are a helpful approach that we want to create access to or pursue in other formats. Interest is what spurs future development. This year through funding support the NSFGA has integrated three weather stations with the RIMPro model that is known for apple scab forecasting. Stay tuned for observations.

Also, disease or insect models that are developed in other regions need to be validated for Nova Scotia before they could be reliably adopted. Modifications may need to be made based on local conditions. Until that time, models may be reviewed out of curiosity, but any decisions should be based on local knowledge and experience.

Is salicylic acid (Actigard) registered for use in tree fruit in Canada? Will it be registered in the near future?

Syngenta is the registrant for Actigard and currently they cannot expand the label in Canada even though uses are allowed on apples in the US. The risk factor calculated by the PMRA is higher than it is in the US and it prevents new crops from being added to the label in Canada. Syngenta continues to look into the possibility and welcomes interest from apple growers each year.

What is sudden/rapid apple decline syndrome? Has the syndrome been confirmed in our region?

Sudden apple decline is a mystery syndrome that researchers have been exploring and have not been able to identify a cause. An exact definition of sudden/rapid apple decline is very important since a clear definition allows researchers to study this particular category of affected trees and develop suitable treatments. Unfortunately, the most prominent symptoms can resemble a lot of other issues.

Dr. Kari Peter hosted a rapid apple decline summit in 2019 to identify similar trends and issues, and she offers a list of defining characteristics. The syndrome presents as the unusual sudden death of trees that are within 2-5 years of planting, on M9 rootstocks and clones no matter the cultivar, and on high density orchard at 6 ft or less tree spacing. Replant disease and fire blight are not present. The rootstock is always healthy, and the location of the issue is at the graft union where the stem tissues are dying. There could be horticultural or virus relationships that are currently being explored. According to this definition, no incidences of the syndrome have been confirmed in our region.

There are times in the orchard when apple trees will die suddenly but this does not confirm the syndrome. Sudden/Rapid Apple Decline Syndrome is not a synonym for trees that die rapidly. Other factors can be at play that require exploration to eliminate possibilities.

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