Considerations for the Future of Apple Scab Management

Recent Re-Evaluation Decisions

The Pest Management Regulatory Agency (PMRA) has released a number of re-evaluation decisions recently that impact apple growers. Table 1 is a summary of these changes including important dates of last use.

All information on past and present re-evaluations can be found in the Pesticide Product Information Database on the PMRA website. The PMRA’s Re-evaluation and Special Review Work Plan 2020-2025 is also available on the PMRA website and lists anticipated dates for public consultation or final decision for pest control products. However, with the on-going pandemic, there may be some delays in the release of certain documents.

Table 1. Date of last sale and use for registered apple fungicides being phased-out based on recent final re-evaluation decisions

Active Ingredient
Trade Name
Last Date of Use
captan
Maestro 80 DF Supra Captan 80 WDG
May 10, 2021
metiram
Polyram DF
June 21, 2021
thiram
Granuflo T
December 14, 2021
ferbam
Ferbam 76 WDG
December 14, 2021

Captan

A requirement of the final decision for captan was for the change of all formulations to a water soluble packet which is why Maestro 80 DF and Supra Captan 80 WDG are listed for phase-out in Table 1. The new WSP formulations have different PCP numbers. Any growers with existing stock of the old formulations can still use the product as per the label on the container until May 10, 2021. This includes a 48-hour re-entry interval, 7-day preharvest interval and no restrictions to the number of applications per year.

However, if using the new formulations, Maestro 80 WSP or Supra Captan 80 WSP, you are legally required to follow the new use patterns based on the final re-evaluation decision and stated on the label. These new patterns are summarized in Table 2. 

Table 2. New use pattern requirements for Maestro 80 WSP and Supra Captan 80 WSP.

Label Specification
High Density (tree canopy width <2m/tree)
Low Density (tree canopy width >2m/tree)
Rate (per application)
2.4 kg a.i./ha, or 3 kg/ha
2.4 kg a.i./ha, or 3 kg/ha
Maximum applications per year
10
2 (if hand thinning, max. 1 before and 1 after thinning)
Restricted entry interval
15 days (hand thinning/harvest) 6 days (hand pruning/training) 2 days (all other activities)
24 days (hand thinning) 19 days (hand harvest) 4 days (hand pruning/training) 2 days (all other activities)

Mancozeb

The group of fungicides commonly referred to as EBDCs include the active ingredients, metiram (Polyram DF) and mancozeb (Penncozeb 75 Raincoat, Manzate Pro-Stick, Dithane Rainshield). An earlier review determined the cancellation of metiram due to worker exposure with a last date of use June 21, 2021. However, at that time, the PMRA retracted a similar final decision for mancozeb and a new re-evaluation decision was released in November 2020.

While strict risk-reduction measures have been put in place for use on apples, the good news is registration has been maintained for mancozeb. Registrants and retailers will have until November 19, 2022 to transition to newly amended labels.

The risk-reduction measures include:

  • New rate – 4.5 kg a.i. / ha
  • Minimum interval between applications – 7 days
  • Maximum applications per year – 4 (full rate)
  • Restricted entry interval – 12 hours (general re-entry), 35 days (hand thinning)
  • Preharvest interval – 77 days

What is still a moving target is how half rates will be accounted for. It is widely understood that half rates are used as a resistance management strategy. However, the wording used on the final mancozeb labels will determine the number of acceptable half rates allowed. If, for instance, the label states – as proposed above – maximum 4 applications per year rather than a seasonal total (e.g., 18 kg a.i./ha), this will be the restriction regardless of whether the application was a full or half rate. The US label accounts for this by including a half rate program as a labelled use pattern. Recommendations have been provided to the Mancozeb Task Force to consider this approach on the new Canadian labels. Stay tuned.

Folpet

On a lighter note, the final re-evaluation decision released in February 2020 for another registered Group M fungicide, folpet (Folpan 80 WDG) maintained all uses on apples. The new restricted entry interval will be 6 days for hand thinning with a maximum of 6 applications per year and 1-day preharvest interval.

This product has not been widely used over the years due to russeting risk of sensitive cultivars and as it has tended to be more expensive than captan. Reports from old trials consider it’s efficacy relatively comparable to captan though there may be some reduced control under moderate to high pressure. Throughout cover sprays, this could be used as a tank-mix or rotational alternative for resistance management of other registered fungicides for summer disease control.

Be sure to read the label carefully as there are phytotoxicity risks early season on Delicious varieties. No information is available on newer cultivars such as Ambrosia.

Why Are Group M Fungicides Important?

The Group M fungicides mentioned in the previous section are important disease management tools for several reasons:

  1. Multi-site (low resistance risk), contact fungicides
  2. Broad-spectrum pest range
  3. Relatively inexpensive
  4. Critical component to resistance management for single site fungicides

This brings to question, without the use of Group M fungicides, how and when do we use the tools left to manage apple scab and summer diseases? Let’s take a closer look at management options available at times during the season. For the remainder of this article, I will not be considering metiram, thiram and ferbam as options for scab control. As well, you will also need to incorporate in-season fire blight management plans which are not discussed here.

Dormant

The efficacy of dormant copper sprays to reduce the spread of fire blight inoculum from overwintering cankers has often been debated. Its effectiveness really comes down to how it is applied and post-application weather. Copper provides an unfriendly environment over the bark and bud surfaces of the tree, preventing bacteria from getting established or spreading. Thus, it must be applied as a high-volume spray to ensure sufficient coverage.

Copper sprays applied to green tissue may also provide some protection against apple scab overwintering in buds equivalent to mancozeb applied at this stage.

Dormant copper such as Copper Spray, Copper 53W, Cueva and Parasol can safely be applied up to ¼” green  (possibly ½” green) without risk of phytotoxicity. However, the use of a softer copper registered for season-long control such as Cueva could be extended in those early spray timings to ½” green or tight cluster in blocks with low scab inoculum (ie., free of scab last year). Residual activity typically last about 7-10 days under ideal spring conditions. However, once rainfall exceeds 2” from last copper application, it should be assumed all residue has been washed off.

Using 1-2% dormant oil, unless the label states otherwise, will act as a sticker/spreader as well as provide efficacy on scale, European red mite and suppression of powdery mildew. Cueva is formulated with a fatty acid so there is little benefit to adding oil as a sticker. If using dormant oil, do not apply captan or sulphur products within 14 days of application. Oil can enhance penetration of these products into sensitive tissue, resulting in phytotoxicity issues.

Green Tip to Tight Cluster

Unfortunately, the cool temperatures we often experience in early spring do not kill scab spores. Spores will continue to mature in overwintering leaf litter and release so long as there is a wetting period long enough for infection to occur. For more information on leaf wetting hours required for infection at various temperatures, refer to Table 3-9, Relationship of Temperature and Moisture to Apple Scab Infection in the 2021 Publication 360A, Crop Protection Guide for Apples. While mature spores are released more during warm rains than in colder ones, they can still cause infection, nonetheless, if they land on green tissue.

Green tip to tight cluster is a period of extensive new growth. Keep covered with a good protectant fungicide program and re-apply fungicides every 5-7 days during periods conducive to disease development or following heavy (greater than 1”) rain.

A protectant program consists of contact fungicides such as captan or mancozeb and does not provide effective post-infection or anti-sporulant activity. That means if sprays are applied in less than ideal conditions, ie., windy, alternate rows, or washed off in rain, the risk of scab infection is increased. The redistribution characteristics of a fungicide are often considered when selecting appropriate control products. See Table 3-7. Characteristics of Apple Scab Fungicides in the 2021 Publication 360A, Crop Protection Guide for Apples for more information on redistribution of registered scab fungicides. However, it is important to also consider leaf growth since last application, scab pressure and residue wash off when determining an appropriate chemistry, dose and spray interval.

During cool, wet springs, protectant fungicides may not be enough. Consider tank-mixing with one of the products listed in Table 3 that provide post-infection activity. Syllit and Scala should be used early season only, up to tight cluster and pink, respectively. As you approach tight cluster, consider any issues with powdery mildew and/or rust in previous years. If these have been a problem, use a Group 9-containing product that also has activity on these diseases as well, such as Inspire Super or Luna Tranquility. Buran (garlic extract) which is registered for scab and powdery mildew can also be used as an eradicant, or for early post-infection activity following a rain event when protectant efficacy may be reduced. This product does not have pre-infection activity.

In warmer, dryer springs, you could also consider products such as Fontelis or Aprovia Top at the tight cluster stage, both of which have efficacy on scab, powdery mildew and rust.

Table 3. Registered fungicides for apple scab for use during cool, wet springs

Product
Properties
Timing
Max app
Tank mix req
Scab efficacy
Other disease
Syllit (U12)
retention, redistribution, anti-sporulant
Green tip – tight cluster
2
Yes
+++(+)1
No
Scala (AP)
post-infection, retention
Prebloom
2
Yes
++(+)
No
Inspire Super (DMI+AP)
post-infection, retention, redistribution, anti-sporulant
Prebloom
42
Yes
++++
Mildew, rust
Luna Tranquility (SDHI+AP)
post-infection, retention
Prebloom
42
Yes
+++(+)
Mildew
Buran (NC)
post-infection, anti-sporulant
Season long
3
No
++(+)
Mildew
++ = suppression; +++ = good control; ++++ = excellent control
1 Dodine resistance is stable in ON and some scab populations may not be sensitive to this product.
2 As stated on label. For resistance management, limit to 2 applications from fungicide group per year if possible.
3 Information is not specified on the product label.

It is important to note protectant fungicides do not control powdery mildew. Include 3-5 kg/ha sulphur (Microthiol Disperss, Microscopic Sulphur, Kumulus, Cosavet DF Edge) with the protectant sprays until tight cluster when more effective mildew products will be used. Keep in mind mildew does better in dry conditions with high relative humidity. Protectant sprays may be needed during dry periods when there is little risk from scab. During wet springs like 2019, powdery mildew escapes can happen relatively quickly during breaks in rain events if fungicide residues are washed off.

Pink to Petal Fall

By tight cluster or pink, apples are typically entering a time of critical infection period for scab and powdery mildew with higher daily temperatures, large amounts of lush growth and rapid maturation of spores. Management programs at this point should begin incorporating systemic fungicides (Groups 1, 3, 7 and 11). However, keep in mind that unlike protectant fungicides, which have multi-site activity and low resistance potential, systemic fungicides are typically single-site and are at high risk of resistance development. For resistance management:

  • Include at least half rate protectant fungicides with all applications.
  • Do not use products containing the same chemical group in consecutive applications.
  • Do not use Group 3 fungicides including Nova, Fullback and Inspire Super after bloom as they are weak on fruit scab. Trials with Cevya have indicated good efficacy on fruit scab.
  • Do not use systemic fungicides for post-infection activity.

While captan and folpet do not provide effective rust control, mancozeb does. Be sure to keep mancozeb as the protectant tank mix particularly at petal fall if rust is a concern in your orchard. It’s best to avoid the use of captan and folpet in general during this growth period to avoid phytotoxicity by complicated tank-mixes, use of adjuvants or other incompatibility issues. Instead, save captan or folpet for summer disease control.

While this section does not focus on Buran, the use of this product as a post-infection and/or tank-mix option would still be effective against scab during this timing until the first summer covers.

The remainder of this section and Table 4 look at systemic fungicides registered for control of apple scab in more detail.

Table 4. Efficacy of registered systemic fungicide groups on apple scab, powdery mildew, rust, black rot, bitter rot and fly speck/sooty blotch on apples.

Fungicide Group
Scab
Powdery mildew
Rust
Black rot
Bitter rot
1 (Senator)
+++
+++
+++1
+1
3 (Cevya, Fullback, Nova)
+++(+)2
++++
++++
3+7 (Aprovia Top)
++++
+++
+++
3+9 (Inspire Super)
++++
+++
++++
7 (Excalia, Fontelis, Kenja, Sercadis)
++++
++(+)
+++
(Fontelis only)
7+9 (Luna Tranquility)
++++
+++(+)
7+11  (Merivon, Pristine)
+++(+)
+++(+)
+++(+)
++++
11  (Flint, Sovran)
++++2
++++
+++1
++(+)1
+ = poor control or partial suppression; ++ = suppression; +++ = good control; ++++ = excellent control
1 Not registered on this disease. 2 Resistance is present in ON to some products from this group

Methyl benzimidazole carbamate (MBC) fungicides

The fungicide Senator belongs to the MBC fungicides (Group 1) and is registered for control of apple scab and powdery mildew. This product may also have efficacy on black rot, fly speck and sooty blotch if applied for scab when these pests are active.

Historically, Benlate-resistant forms of apple scab have been present in Ontario. Senator belongs to the same chemical group as Benlate. Use caution if Benlate was used in your orchard in the past. For resistance management, tank-mix with a compatible protectant scab fungicide from a different group. See label for more information on suggested tank-mix partners.

Sterol inhibitor (SI) fungicides

The SI fungicides (Group 3), now including Nova, Fullback, Cevya, Aprovia Top (Group 3+7) and Inspire Super (Group 3+9) have been a standard for control of apple scab, powdery mildew and rust for many years. Resistance to Nova has been documented in Ontario scab populations and suspected in powdery mildew populations. For this reason, SIs should not be used in orchards with resistant scab populations and should always be tank-mixed with a protectant fungicide, including the recently registered product, Cevya. The benefit to keeping Group 3 fungicides in rotation is the good powdery mildew and rust control they offer (so long as resistance is not present) as well as to help reduce resistance pressure for the other single-site fungicides.

Succinate dehydrogenase inhibitor (SDHI) fungicides

The SDHI fungicides (Group 7) are the newest group of systemic fungicides including Aprovia Top (Group 3+7), Excalia, Fontelis, Kenja, Luna Tranquility (Group 7+9), Merivon (Group 7+11), Pristine (Group 7+11) and Sercadis. These products are registered for control of apple scab, powdery mildew, rust (Fontelis, Aprovia Top only) and summer disease (Merivon, Pristine only). Some formulations within this group contain mineral oil or require a surfactant, which can result in injury under certain conditions if used in combination with captan. Always refer to the label.

At the present time, there is no known resistance to any Group 7 product in Ontario. For this reason, more applications of these products can be used compared to the Group 3 and 11 fungicides. A maximum of 4 applications of Group 7 fungicides is allowed each season, with ideal placement between tight cluster and first covers. However, Merivon and Pristine are also very effective products for the control of fruit rots, fly speck and sooty blotch. Since these pre-mix products include Group 7, it may be worthwhile to save 1-2 applications for summer use.

As with all sound resistance management practice, do not apply consecutive sprays from the same fungicide group. This is where other fungicide groups can come into play with product rotation. 

Strobilurin fungicides

Similar to the SIs, the strobilurin fungicides (Group 11) such as Flint, Sovran, Merivon (Group 7+11) and Pristine (Group 7+11) have been used for many years for control of apple scab, powdery mildew and rust. Scab resistance to Flint has also been documented in Ontario to this group so tank-mixing with a protectant fungicide is critical. Where strobilurins are working, an ideal fit may be at petal fall and into cover sprays as this group has good efficacy on summer diseases such as fruit rots, fly speck and sooty blotch.

Resistance management

One key strategy to good resistance management is rotating between products of different chemical groups. This does not just mean rotating from one product to another. Fungicides are grouped based on their mode of action, or how the product actually affects the disease. For example, all products in Group 3 have the same mode of action, so using one product is virtually the same as using all other products within that group. In pre-mix fungicides, both groups need to be considered in all rotation decisions. Figure 1 shows which fungicides belong to Groups 3, 7, 9 and 11. For instance, since Aprovia Top belongs to Group 3 and 7, it should not be followed by Cevya, Fullback, Nova, Inspire Super, Excalia, Fontelis, Kenja, Sercadis, Luna Tranquility or Pristine.

Figure 1. Schematic diagram of Group 3, 7, 9 and 11 fungicides, including pre-mixes registered for use on apples.

Summer Sprays to Harvest

The end of primary scab season can be determined through the use of the ascospore maturity degree-day model as described in the apple scab module on Ontario AppleIPM. At 418 DDC (degree-days Celsius), over 95% of the ascospores supply should be depleted if sufficient rain has occurred. After this point, wait 2 weeks and then check the trees for scab. It can take up to 14 days for lesions to appear after a scab infection period. Be sure to check the top of standard trees or anywhere else spray coverage may not have been adequate, such as a thick tree canopy. Primary scab is often overlooked in these areas and can come as a surprise when lesions suddenly show up on the fruit late season.

If primary scab was controlled, the rates of fungicides may be reduced and the interval between sprays may be lengthened for the remainder of the growing season. Following first or second cover, mancozeb is no longer an option due to the preharvest interval. Summer applications of captan or folpet, however, will only provide efficacy on scab, black rot, bitter rot, fly speck and sooty blotch if the full rate is used. Remember, if using the new captan WSP formulations, low density orchards with a canopy width of 2m will be restricted to a maximum of 2 full rates of captan at 3 kg/ha and a 19-day PHI. Rotation with alternative products effective against summer diseases such as Allegro (28-day PHI), Flint (14-day PHI), Merivon (0-day PHI but 5-day REI for hand harvest) or Pristine (5-day PHI) will be necessary to compensate for the limited captan allowance. Regardless, if summer oil or other tank-mixes that are incompatible with captan or folpet are used, rotation with the above-mentioned products will be important. Allegro will also provide mite suppression.

Organic Management

Organic or biofungicides are not necessarily just options for organic operations but can be effective alternatives to incorporate into a conventional program as well. Their precise fit, however, will depend on the product used. For instance, as mentioned above, Buran should be timed for after heavy rain or periods of prolonged leaf wetness when protectant fungicide activity may be reduced. The products listed in Table 5 provide broad-spectrum activity on a number of key apple diseases.

When used properly, biofungicides can have the following advantages in conventional disease management:

  • Lower potential for pest resistance
  • Provide protection during times of lower infection risk, allowing more effective products to be saved for critical periods
  • Provide a rotational option to help manage resistance development to other conventional products
  • Shorter re-entry intervals helpful for times of significant orchard activity (e.g., hand thinning, pruning)
  • Shorter preharvest intervals can provide disease suppression up to the day of harvest, helping to reduce potential storage issues 
  • Potentially lower toxicity to non-target organisms, including honeybees

Biofungicides typically do not provide the same high level of control as conventional fungicides and are often labelled for suppression, or partial suppression, of pests. Control can be improved, however, when their use is combined with other control practices including cover crops, sanitation, promoting tree health and canopy management to name a few.

Table 5. Organic fungicides for management of apple diseases

Product
REI
PHI
Scab
Powdery mildew
Rust
Fruit rot
Fly speck / sooty blotch
Buran
when dry
0 days
++(+)
++
Oxidate 2.0
4 hours
0 days
+(+)
++
+(+)
Regalia Maxx
when dry
0 days
+(+)
+(+)
++1
++
+(+)
Serenade OPTI
when dry
0 days
+(+)
+(+)
++1
++1
++1
sulphur
24 hours
1 day
++
++(+)
+1
+(+)1
+(+)1
+ = poor control or partial suppression; ++ = suppression; +++ = good control; ++++ = excellent control
1 Not registered on this disease.

Postharvest

Following harvest, scab management can continue by reducing the amount of overwintering inoculum and pressure you’ll be starting out with in the orchard next season. Applying urea fertilizer at 45 kg/ha (mixed with 1,000 L of water/ha) and/or shredding fallen leaves with a flail mower are two practical and inexpensive methods to minimize scab pressure. Ideally, these practices should be done in the fall and/or before green tip if possible, but there may still be some benefit if this is delayed shortly after bud break.

Summary

The following description and Figure 2 summarizes many points from this article but is only an example conventional scab management program and does not include all control options available. Select products that fit best for your operation, cultivars, resistance management program and disease pressure as well as current weather conditions. Product choices in this example were based on a hypothetical cool, wet spring followed by a warm, humid summer and used a somewhat “worst-case scenario”, or combination of 7x half rate mancozeb (77-day PHI) and 2x captan (19-day PHI).

An effective scab management program should begin with a delayed dormant copper application between silver tip and ½” green. This will have efficacy on any early season activity of overwintering fire blight cankers or scab infection periods. From green tip to pink consider a protectant fungicide program consisting of a combination of half-rate mancozeb (especially at pink for rust control) and half rate protectants (mancozeb, folpet, captan) tank-mixed with Sylitt, Scala, Inspire Super or Luna Tranquility. Over the bloom period into the first and second cover, incorporate systemic fungicides belonging to Groups 1, 3, 7 and/or 11 tank-mixed where possible with half-rate mancozeb. A rotation of Merivon/Pristine or Senator tank-mixed with half-rate captan or Folpan and Allegro as summer cover sprays will provide good control of scab, fruit rots and fly speck/sooty blotch. Allegro can be used up to 3 consecutive sprays before rotating to another fungicide group. A late season application of full-rate captan (within the required pre-harvest interval) or Folpan followed up with a Merivon/Pristine just before harvest (if not exporting) will protect against pinpoint scab and any other fungal infections that may occur. Reduce inoculum by applying urea and/or flail mowing in the fall following harvest or as soon as the orchard can be entered in the spring.

Figure 2. Example of a full-season apple scab management program during a cool, wet spring followed by a warm, humid summer. Fungicides highlighted from tight cluster through covers may also provide efficacy on other diseases.
Kristy Grigg-McGuffin
Kristy Grigg-McGuffin

Horticulture IPM Specialist, OMAFRA