Apple Scab Management in British Columbia

Apple scab, caused by the fungus Venturia inaequalis is a common disease of apple in British Columbia.

It is an annual threat in the wetter interior fruit growing districts and is even more severe in the coastal areas of B.C. and the Fraser Valley. Apple scab has also caused significant losses in the drier areas of the Okanagan and Similkameen Valleys in years with higher than normal rainfall.


Symptoms

Leaf spots are circular or irregular in shape, and brown to olive green in colour with feathery margins. Severe infections, known as sheet scab, may cause the leaves to fall prematurely.

Fruit lesions are circular and brown to black in colour. The skin ruptures around the lesions revealing a dark velvety layer of spores. Older lesions become corky in appearance. The fruit is most susceptible when young. Early infections often result in large fruit lesions and severely cracked and malformed fruit.  Late season fruit infections may not become visible until the apples are in storage. They appear as either pin-point sized lesions, or jet black spots, and are sometimes called pin-point scab. 

leaf scab apple scab on fruit
Apple scab leaf lesions Severe fruit scab

Life Cycle

Primary Infection: Apple scab overwinters in infected leaves on the orchard floor. Spores (ascospores) are produced in these dead leaves just about the time that new growth appears in the spring (green tip stage). They are then discharged during rainy periods, and can be carried long distances in air currents. Ascospores land on new leaves, blossoms and fruit, and if the leaves or fruit are wet long enough for the spores to germinate, primary infection of fruit and foliage occurs. The primary scab season lasts until all the ascospores from last year's leaves have been released. This varies from year to year but typically peaks around blossom and ends by mid to late June.  

Secondary Infection: Once primary infections have taken place, lesions develop on the leaves and fruit which produce secondary spores (conidia). Conidia are spread short distances mainly by splashing rain, and cause additional spread and build-up of the disease. The time needed for a new infection to begin producing a new crop of spores (incubation period) varies from 9 to 17 days, depending on the temperature. 

Conidia require a similar or slightly longer period of foliage wetting as needed for a primary spring infection. However, unlike ascospores, they do not require an initial period of rainfall to initiate spore release. Early morning dew can be important in initiating wet periods that may lead to infection, particularly if followed by showers. Secondary scab lesions can remain active for about a month, producing large numbers of conidia.

Fruit becomes more resistant to apple scab as it matures, requiring increasingly longer wet periods for infection (see Table 3). With prolonged rainy weather, fruit may be infected right up until harvest.

Monitoring Infection Periods: The length of time required for infection depends upon the temperature during the time period when the leaves or fruit are wet. Table 1 illustrates the relationship between leaf wetness duration and average temperature, and can be used as a guideline to determine whether or not an infection period has occurred. The average temperature may be calculated by taking the average of the high and low temperatures during the wet period. When trees are dry less than 24 hours between showers, the periods the leaves are wet should be added together in calculating the infection period if the relative humidity is above 85%. As the length of wet periods increase at a given temperature, the severity of the scab infection also increases.  

Table 1. Minimum number of hours of leaf wetness required for infection of apple leaves by ascospores and conidia.  Source: Stendsvand et al., 1997

Temperature (°C) Minimum number of hours of leaf wetness required
Ascospores Conidia
1 40.5 37.4
2 34.7 33.6
3 29.6 30.0
4 27.8 26.6
5 21.2 23.4
6 18.0 20.5
7 15.4 17.8
8 13.4 15.2
9 12.2 12.6
10 11.0 10.0
11 9.0 9.5
12 8.3 9.3
13 8.0 9.2
14-15 7.0 9.2
16 6.1 9.0
17 6.0 8.8
18 6.0 8.5
19 6.0 8.2
20 6.0 7.9
21-22 6.0 7.8
23 6.0 8.3
24 6.1 9.3
25 8.0 11.1
26 11.3 14.0

 

Fruit Infection:  

Fruit becomes more resistant to apple scab as it matures. Increasingly longer wet periods are necessary for fruit infection as the season progresses.  Table 2 shows the length of wet period required for secondary apple scab infection on the fruit at various intervals after full bloom. Fruit of several varieties was evaluated, and all reacted similarly.

Table 2. Hours of wetting for 2% fruit infection.

Average Temperature (°C) Weeks after full bloom
1 5 10 15
10
12
26
37
45.5
12
10
21.5
31
38
14
8.5
18.5
26.5
32.5
16
7.5
16
23
28.5
18
6.5
14.5
20.5
25.5
20
6
13
18.5
23

Adapted from Schwabe et al., 1984.

Apple Scab Management

Resistant Varieties

Commercial apple cultivars vary in their susceptibility to apple scab.  Highly susceptible cultivars include Gala, McIntosh, Empire, Mutsu and Summerred. Moderately susceptible cultivars include Red Delicious, Golden Delicious, Spartan, Fuji, Honeycrisp and Jonagold.

Numerous apple cultivars with resistance to scab have been developed in breeding programs around the world, and there are many promising varieties under development.  Currently none of the mainstream varieties grown in the B.C. commercial market are highly resistant.  Many of the resistant varieties evaluated in the past were not suitable for long term storage or had other issues with quality or marketing.  Most currently available resistant varieties have a single gene that governs apple scab resistance. As a result, it is not difficult for the apple scab fungus to overcome the resistance (similar to fungicide resistance). Failure of resistant varieties has occurred in many areas of Europe and the United States, and has been detected in at least one Lower Mainland B.C. orchard.

Resistant varieties are suitable for backyard gardeners, and will eliminate the need for fungicide sprays for apple scab control. Resistant varieties may also be of interest to organic growers or direct farm marketers.

When planting resistant varieties:  

  • Isolate the resistant trees from sources of apple scab inoculum.  Don't interplant with susceptible trees.  Constant exposure to apple scab will eventually result in a breakdown of resistance.
  • Consider using fungicide sprays on resistant trees that are exposed to inoculum during the peak period of scab susceptibility in the spring
  • Practice other cultural control methods, such management of leaf litter to reduce inoculum pressure. 

Cultural Control

Water management: Do not run over-tree sprinklers longer than the time needed to cause an infection period. Infection can occur with as little as 6 to 9 hours of leaf wetness at optimum temperatures. The use of over-tree sprinklers can also prolong leaf wetness periods initiated by rain or dew, creating an infection period. When rain ends, shut off over-tree sprinklers until trees have been dry for at least 10 to 12 hours. Do not start the sprinklers when wet weather is predicted.

Orchard design & pruning: Good air movement can reduce the number of infection periods because the foliage dries more quickly. An open canopy also improves spray coverage and light penetration. To improve air movement in the orchard, avoid overcrowding and keep trees well pruned.

Reducing primary inoculum: Several strategies will help to reduce the amount of overwintering scab. Primary inoculum is produced by infected leaf litter on the orchard floor. Any practices that reduce the amount of leaf litter will help to reduce the amount of inoculum. Note that these control methods do not replace the need for fungicides in commercial orchards. Reducing the initial inoculum will reduce the disease pressure in the orchard, making apple scab easier to control. It will not, however, eliminate the disease, and fungicidal protection will continue to be necessary. Low apple scab pressure is also valuable in helping to prevent or delay the development of fungicide resistance.

  • Remove fallen leaves by raking, vacuuming, or using leaf blowers (may not be practical except in small plantings).  Home gardeners who remove all fallen leaves from under their apple trees should experience good control of apple scab. Leaves may be safely composted.
  • Foliar application of a 5% urea solution in autumn just before leaf drop will speed natural decomposition of leaves and help to deactivate the scab fungus. Research has shown large reductions in spore production as high as 80 to 90% following application. Care should be taken that urea is not applied too early in the fall.  This approach may be equally effective when applied to leaves on the orchard floor after they have dropped in the fall. Spring application to the leaf litter is less likely to have the same impact as fall application. The effectiveness of urea may vary from year to year.
  • Fall application of dolomitic lime to leaf litter on the orchard floor has been shown to have similar effects to urea application in Oregon, however urea is considered to be more effective.
  • Chopping or shredding the leaf litter in the spring or fall using a flail mower can also dramatically reduce the amount of scab inoculum. Chopped leaves will decompose much faster. Leaf chopping can also be done in combination with urea application.

Chemical Control

Protectant fungicides (see Table 3) are applied to protect new growth during rainy periods or in anticipation of a rainy period. These chemicals provide a film of fungicide which prevents spores from germinating. Protectant sprays are usually applied on a calendar basis. Fungicides with eradicant properties also have protectant properties, and are generally more effective when used in protective programs.

Eradicant fungicides (see Table 3) prevent or reduce further growth of infections that have already started, if the sprays are applied soon after the start of a rainy period. The advantage of eradicant sprays is that they are applied only when necessary; i.e. when infection periods occur. In most seasons this will reduce the number of sprays required.  A disadvantage is that repeated use of eradicant fungicides is likely to lead to resistance.  Also growers must have spray machinery available to cover the trees quickly after an infection period, and winds must be low enough to permit good coverage.

Table 3. Fungicides registered for apple scab control.  
Fungicides with the same resistance group have similar modes of action. Additional rates and uses may be listed on the product label. Always follow label instructions and precautions when using pesticides.

Resistance Group*
Fungicide
Rate/ha
Days to Harvest
Comments and Limitations
Protectant Fungicides
M3
Polyram (metiram)
6.0 kg
45 Use preventatively.  Low risk of resistance.
M3 Manzate DF or Dithane DG (mancozeb)
6.0 kg
45
Use preventatively. Low risk of resistance.
M3 Granuflo T (thiram) 1.5 to 2.25 kg/1000 L water 28 Use preventatively. Low risk of resistance.
M3
ziram
5.0 kg
14 For pinpoint scab (late season spray).  Low risk of resistance.
M4
Captan 80% or Maestro (captan)
3.75 kg
7 Use preventatively.  Do not use during bloom. Low risk of resistance.
M1 Cueva (copper octanoate) See label 1 Do not exceed 1% rate for varieties susceptible to russeting. Repeat every 7-10 days. 
M2
Kumulus DF or Microthiol Disperss (sulphur)
7.0 kg
1 Use preventatively. May harm predatory mites. Shorter spray intervals required. May cause damage under certain weather conditions (see label).
29
Allegro 500F (fluazinam)
0.75-1.0 L
28 Use preventatively and repeat at 7 to 14 day intervals. Use the highest rates and shortest intervals for high disease pressure. Allegro will also suppress European red mite, apple rust mite and two-spotted spider mites.
Eradicant/Protectant Fungicides (may be used as eradicants or protectants)
3 + 7 Aprovia Top (difenoconazole + benzovindiflupyr) 386-643 mL 30 Apply alone or tank-mix with a protectant fungicide (see label). Do not apply more than 2 consecutive sprays. Rotate with other non-Group 3 or 7 fungicides. 
3+9 Inspire Super (difenoconazole + cyprodinil) 560 - 836 g 14 Use on a preventative schedule at 7-10 day intervals. Tank mix with a protectant fungicide (e.g. Allegro, Dithane, Manzate, captan, Polyram) for improved control and resistance management. For curative sprays, apply within 48 hours of the onset of an infection period. Inspire is more likely to be effective in orchards where resistance to group 3 fungicides is developing than other group 3 products.  Limit sprays of group 3 fungicides (Nova, Inspire, Fullback) to 2 per season. 
3
Nova (myclobutanil)
340 g 
14

Best used on a protectant schedule at 10 day intervals in a tank mix with a protectant fungicide (e.g. Dithane, Manzate, captan, Polyram)Provides up to 4 days of post-infection activity.  If used as an eradicant, follow with another eradicant in 7 days or a protectant in 4 to 5 days.  Apple scab resistance to Nova has been detected in a few B.C. orchards.  Limit sprays of group 3 fungicides (Nova, Inspire, Fullback) to 1 or 2 per season. 

3 Fullback 125 SC (flutriafol) 950 mL 14 Tank mix with a protectant fungicide (e.g. Dithane, Manzate, captan, Polyram). Best used on a protectant schedule at 7-10 days through petal fall and 10-14 days after petal fall.  Limit sprays of group 3 fungicides (Nova, Inspire, Fullback) to 1 or 2 per season. 
11
Flint (trifloxystrobin)
140-175 g
14 Best used on a protectant schedule at 7-10 day intervals. Provides up to 4 days of post-infection activity for scab control. Apple scab resistance to group 11 fungicides has been detected in a few B.C. orchards. Limit sprays of class 11 fungicides (Sovran, Flint, Pristine) to 2 per season. Alternate with other fungicides from a different resistance group.
11 Sovran (kresoxim-methyl) 180 - 360 g 30

Best used on a protectant schedule at 10-14 day intervals, using a 10 day interval during periods of rapid shoot growth. Provides 4 days of eradicant activity. Use the high rate for post-infection sprays or high disease pressure. Apple scab resistance to group 11 fungicides has been detected in a few B.C. orchards. Limit sprays of group 11 fungicides (Sovran, Flint, Pristine) to 2 per season. Alternate with other fungicides from a different resistance group. Caution: Sovran drift may injure cherries. 

7+11
Pristine (boscalid + pyraclostrobin)
1.0-1.2 kg
5

Best used on a protectant schedule at 7-10 day intervals. Use the higher rate and shorter interval during periods of rapid growth or when disease pressure is high. Limit sprays of group 11 fungicides (Sovran, Flint, Pristine) to 2 per season. Alternate with other fungicides from a different resistance group.  

7
Fontelis (penthiopyrad)
1.0-1.5 L
28 Broad spectrum fungicide that provides pre-and post-infection activity against apple scab. Best used on a preventative schedule, on a 7-10 day interval. Use the higher rate and shorter interval when disease pressure is high. Limit use to 3 applications/season, and rotate with fungicides from other groups. 
7 Sercadis (fluxapyroxad) 167-333 mL 0 Apply preventatively. Use higher rate when disease pressure is high. 
7 + 9 Luna Tranquility (fluopyram +pyrimethanil)
800 mL
14 Broad spectrum fungicide that provides pre-and post-infection activity against apple scab. Best used on a preventative schedule, on a 7-14 day interval. Provides up to 72 hours of post-infection control of scab. Limit to 1 application per season when the total number of fungicide sprays in a program is 1 to 2, and 2 per season when the total number of sprays is 3 or more. 
9
Vangard (cyprodinil)
370 g
72 Vangard is recommended at the full label rate for pre-bloom sprays. Do not apply more than 2 applications of Vangard per season. Tank mixes using a reduced rate of Vangard (see label) have not been fully evaluated in B.C. Best used as a protectant at 7 to 10 day intervals. Provides 48 hours of post-infection activity. Caution: Vangard drift may injure some cherry varieties.
9
Scala (pyrimethanil)
0.75-1.0 L
72
Best used as an early season protectant. Do not use after bloom. Good activity during cool temperatures. Apply at 7-12 day intervals, from green tip to petal drop. Do not apply within 72 days of harvest for apple scab. However Scala may be used 14 days to harvest for management of post-harvest rots. See label. 
1
Senator (thiophanate-methyl)
See label
1 Not recommended for apple scab management in the Okanagan due to presence of resistance. May harm predatory mites.
U12
Equal, Syllit (dodine)
See label
7 Do not use in areas with resistance. Do not use under slow-drying conditions.

*Fungicide resistance management group. M=multisite. U=unknown

Fungicide Resistance Management

To help prevent resistance from developing:

  • Use recommended tank mixes at label rates
  • Alternate between different groups of fungicides. Do not use more than 2 back-to-back sprays of fungicides from the same resistance group.
  • Limit use of fungicides that are at high risk of resistance to 2 sprays/season. See apple fungicide table for risk of resistance for each fungicide.
  • Rely on protectant programs with good coverage rather than eradicant or “rescue” sprays.
  • Limit use of high risk fungicides to the early primary scab season. Switch to low risk (protectant) fungicides after mid-June if scab sprays are still needed.
  • Maintain good scab control to reduce the probability that a resistant strain may develop.

Apple scab resistance to group 3 and group 11 fungicides has been detected in several orchards in the Okanagan and Creston Valleys in 2011-2012.  Limit use of group 3 and 11 fungicides to prolong effectiveness.  

Apple scab resistance to thiophanate-methyl (Senator) and dodine (Equal, Syllit) was present in the Central and North Okanagan in the 1980's-1990's, and was also found in other interior fruit growing districts. Because of the extent of resistance, these chemicals are not recommended for scab control in B.C.  Growers in areas where resistance was known to be low may choose to use these chemicals with caution. Resistance to Senator develops more quickly and easily than resistance to Equal/Syllit.  

Switch to fungicides from other groups if resistance is suspected in your orchard, and contact your field advisor. Commercial testing services for resistance to some fungicides may be available.  Also consider other factors that may lead to fungicide failure, including spray timing and coverage.

Spray Timing and Scheduling

First Spray: The risk of apple scab infection may begin as early as the green tip stage. If using a protectant program, and scab was a problem the previous year, apply the first spray at the green tip stage. If scab was well controlled last year, it may be safe to delay the first spray until the tight cluster stage. Sprays may also be delayed if there is no possibility of rain in the forecast.

If using an eradicant program, apply the first spray immediately after the first infection period. Begin monitoring infection periods at the green tip to tight cluster stage, depending on severity of scab the previous year.

Subsequent Sprays:  A protectant schedule is highly recommended during the critical pink to petal fall period, both for improved scab control and resistance management. During this time there is abundant scab inoculum, the weather is often rainy, and tree growth is rapid and highly susceptible. Protectant schedules are also recommended for orchards with high levels of overwintering apple scab inoculum.

To successfully use an eradicant schedule, the grower must be able to determine whether or not an infection period has occurred after every rainfall. Packinghouse field staff issue scab warnings to packinghouse members. Alternatively, growers could install weather monitoring equipment in their own orchards, or hire orchard consultants who are monitoring infection periods.

For protectant schedules, fungicides should be applied every 7 to 14 days during the primary scab season. A 7 day interval should be used from the pink to calyx stage of tree development. Following the calyx stage, protectants such as Kumulus are best used on a 7 day interval, while Polyram, mancozeb, or captan may be used at 10 to 14 day intervals during most years. This interval should be shortened to 7 days following heavy rain. With an eradicant schedule, fungicides are applied shortly after an infection period has occurred. Often protectant and eradicant schedules are combined. For example, eradicant schedules are often backed up with protectant sprays during the critical pink to petal fall period. 

In late June growers should carefully check leaves and fruit for the presence of scab lesions. If primary scab has been well controlled, such that only a few lesions can be found per tree, the interval between sprays may be lengthened for the remainder of the growing season. If scab lesions are not found, the grower could decide to eliminate fungicide applications entirely after the end of the primary season. However, this should be done with caution because it takes only a small number of lesions to produce a major epidemic later in the season if leaves and fruit are not protected, and if an extended period of wet weather occurs. If sprays are suspended, the orchard should be periodically monitored for the development of scab lesions for the rest of the season.

Pin-point or late season scab infections can develop only when there are visible leaf or fruit infections in the tree. For this reason, it is important that spray programs be continued throughout the summer if scab is present in the orchard. In the Okanagan, spray programs are often relaxed during dry summer weather. Be aware that this may result in fruit infection if wet weather occurs while the trees are unprotected and scab lesions are present in the orchard. Captan and ziram are used as late season protectants. Do not use eradicant fungicides as protectants for pin-point scab, and observe pre-harvest intervals for all late-season sprays.

Action Thresholds:  Techniques to assess the "potential ascospore dose", or PAD in an orchard are used in some areas. If the PAD is determined to be low enough, it is deemed safe to delay the beginning of the spray program until the pink fruit bud stage or until after 3 infection periods, whichever comes first. To determine PAD, it is necessary to do an assessment of scab levels on leaves in the fall. The use of PAD as a threshold has not been evaluated in B.C. For more information refer to Apple Scab: Improving Understanding for Better Management

Weather monitoring equipment:  To forecast infection periods and properly time fungicide applications, it is necessary to have access to weather data. Okanagan packinghouses operate a network of electronic weather stations. Downloaded weather data is run through computerized apple scab models to determine when infection periods have occurred. Growers can check their email bulletins from field service, or call into their packinghouse code-a-phones to find out the latest recommendations for scab control.

Because microclimates differ between orchards, it is always preferable to collect weather data as close to your own orchard as possible. Growers with larger acreages of apples may consider purchasing their own weather stations. There are several on the market that include software for forecasting apple scab. For more information on weather stations, contact your field advisor.

Further Information

Updated November, 2016