Skip to content.
Français

Some features of this website require Javascript to be enabled for best usibility. Please enable Javascript to run.

Ministry of Agriculture, Food and Rural Affairs

Apple Scab

Young apple scab lesions appear velvety brown to olive green with indistinct marginsYoung apple scab lesions appear velvety brown to olive green with indistinct margins The olive green apple scab lesions eventually turn dark brown to black Lesions on older leaves are raised and appear dark green to gray brown with distinct margins A small scab lesion on a fruit Older scab lesion on fruit becomes brown, corky and scabby Scab lesions cause fruit to become deformed and cracked when infected at an immature stage Flail mowingFlail mowingMonitor weather conditions

Click to enlarge.

Beginner

Scientific Name
Venturia inaequalis

Identification
Leaves:

  • Lesion development occurs on both sides of leaves, usually developing first on the lower side
  • On leaves, young lesions are velvety brown to olive green with indistinct margins and are often not noticeable until after petal fall
  • With time, olive green lesions turn dark brown to black
  • Lesions on older leaves are typically raised, dark green to gray brown with distinct margins, and cause cupping on the underside of the leaf
  • Leaves that are heavily infected with scab will curl, shrivel and fall from the tree

Fruit:

  • Small black velvety spots develop on fruit
  • As spots grow and become older, the centre loses the velvety appearance and becomes brown, corky and scabby
  • Heavily infected fruit becomes deformed and cracked
  • Fruit infections occurring late in the summer may not be visible at harvest to the naked eye, but enlarge in storage to pinhead size (pinpoint scab) and tend to cluster around the calyx end of the fruit

Often Confused With

  • Blister spot - Blister spot on fruit appears as small raised blisters that affect only Mutsu and lesions penetrate the skin of the apple. Apple scab affects many different varieties and the small scab lesions do not penetrate the skin. However as the lesions grow the lesion will crack the skin.
  • Sooty blotch - Sooty blotch appears only on the fruit as brown to olive green blotches on the surface of the apple skin. Apple scab lesions can be on both fruit and leaves. The lesions on the fruit are green and velvety and as they mature they become black and corky.

Period of Activity
Apple scab activity occurs from the emergence of green tip until leaf drop in the fall.

Scouting Notes
From green tip to harvest leaves and fruit should be monitored for signs of apple scab and results should be recorded. Temperature and hours of leaf wetness should be recorded in order to determine ascospore maturity, primary and secondary infection periods.

Thresholds
Fungicides should be applied to prevent infection during periods when weather facilitates the spread of apple scab. See OMAFRA Publications 310 and 360 for a chart depicting the relationship of temperature and moisture to apple scab infections. Using the ascospore maturity degree day model (see publication 310) when 125 DDC is reached the risk of primary infection is high. When 418 DDC is reached, the primary infection period is over if enough precipitation has fallen. If no scab is observed following the end of primary infection period, the fungicide regime may be diminished accordingly. Secondary infections occur once lesions are present and produce conidia and weather conditions are conducive to infections. 

Advanced

Scientific Name
Venturia inaequalis

Identification
Infection and lesion development occur on both sides of leaves, usually developing first on the lower side. On leaves, young lesions are velvety brown to olive green with indistinct margins and are often not noticeable until after petal fall. With time, olive green lesions turn dark brown to black. Lesions on older leaves are typically raised, dark green to gray brown with distinct margins, and cause cupping on the underside of the leaf. Leaves that are heavily infected with scab will curl, shrivel and fall from the tree.

Small black spots develop on fruit, enlarging more slowly than on leaves. As these spots grow and become older, the centre loses the velvety appearance and becomes brown, corky and scabby. Heavily infected fruit becomes deformed and cracked when infected at an immature stage. Fruit infections occurring late in the summer may not be visible at harvest to the naked eye, but enlarge in storage to pinhead size. These lesions are “pin point scab” and tend to cluster around the calyx end of the fruit.

Often Confused With

  • Blister spot - Blister spot on fruit appears as small raised blisters that affect only Mutsu and lesions penetrate the skin of the apple. Apple scab affects many different varieties and the small scab lesions do not penetrate the skin. However as the lesions grow the lesion will crack the skin.
  • Sooty blotch - Sooty blotch appears only on the fruit as brown to olive green blotches on the surface of the apple skin. Apple scab lesions can be on both fruit and leaves. The lesions on the fruit are green and velvety and as they mature they become black and corky.

Biology
Apple scab overwinters in infected apple leaves on the orchard floor. During the winter and early spring, small black pseudothecia develop in the infected leaves on the orchard floor. By early spring, ascospores, which serve as the primary inoculum for early season infections, are formed inside the pseudothecia.
 
Maturation of the ascospores in the dead leaves on the orchard floor usually occurs at the same time the apple tree is emerging from dormancy. Mature ascospores are present and ready to infect the first green tissue in spring. The percentage of mature ascospores in the orchard generally peaks when apples are at the late pink to early bloom stages of bud development.

Once the tree breaks dormancy and green tissue is present, a primary infection occurs if the following three conditions are met:

  • Mature ascospores are present in leaf litter in the orchard.
  • Weather conditions favour ascospore discharge and infection.
  • Fungicide protection is inadequate to prevent infections.
Mature ascospores are discharged from the pseudothecia by rain and carried up to emerging green tissue in the trees by wind currents. Moisture – dew or rain – is necessary for ascospore discharge and germination, as well as subsequent infection of apple tissue. Olive green, velvety lesions appear 10-28 days after infection by an ascospore. The lesions initiated by ascospores result in primary infections, and in turn, produce spores called conidia.

Conidia are spread from primary lesions by splashing raindrops and wind, and initiate further infections when the combination of temperature and leaf wetness enables them to germinate and become established. These are called secondary infections, and generally occur within a tree or between adjacent trees rather than at a long distance. 

The secondary cycle can be repeated many times during the growing season. With frequent rainfall, the control of apple scab becomes extremely difficult, particularly if the disease becomes established from primary infections in the spring.

Leaves are most susceptible to infection until they are fully expanded. Old leaves may again become susceptible to the fungus in late season, and previously inhibited mycelia inside the leaf tissues may resume growth, resulting in new visible lesions. This phase of epidemics in autumn has significant implications for disease management because it provides additional primary (ascospores) inoculum next spring.

Period of Activity
Apple scab activity occurs from the emergence of green tip until leaf drop in the fall.

Scouting Notes
From green tip to harvest leaves and fruit should be monitored for signs of apple scab and recorded. Temperature and hours of leaf wetness should be recorded in order to determine ascospore maturity, primary and secondary infection periods.

Thresholds
Fungicides should be applied to prevent infection during periods when weather facilitates the spread of apple scab. Using the ascospore maturity degree-day model when 125 DDC is reached the risk of primary infection is high. When 418 DDC is reached, the primary infection period is over if enough precipitation has fallen. If no scab is observed following the end of primary infection period, the fungicide regime may be diminished accordingly. Secondary infections occur once lesions are present and produce conidia and weather conditions are conducive to infections. 

Management Notes

Monitoring weather
Accurate weather data provides daily maximum and minimum temperatures, a necessary component in using the degree-day model for assessing ascospore maturity and primary infection (described below). Reliable weather monitoring equipment in the orchard to measure temperature, relative humidity, leaf wetness is needed to indicate ascospore maturity and when infection periods have occurred.

Determining the inoculum level
The incidence of fruit scab at harvest does not necessarily reflect the disease pressure in the orchard, since foliar infections continue to occur after harvest. To predict the amount of inoculum in the orchard in spring, assess the leaf scab present in the fall using Potential Ascospore Dose (PAD) method.

In late September or early October, before leaf fall, assess for foliar scab using the following technique.

Select a sampling scheme to sample every 10-30 trees, making sure trees are selected throughout the block. Be sure the total number of shoots examined equals 600. Here are three sampling schemes:

  • 20 shoots on each of 30 trees (a block of 300 to 900 trees) or
  • 15 shoots on each of 40 trees (a block of 400 to 1,200 trees) or
  • 10 shoots on each of 60 trees (a block of 600 to 1,800 trees)

Use the following procedure to sample:

  • On each selected tree, examine shoots at random from the top, bottom, inside and outside of the canopy. It is particularly important to include shoots near the top of the tree. This is where scab is more likely to occur due to greater chance of poor spray deposition. If sucker shoots are common, include one sucker shoot per tree.
  • On each selected shoot, scan the upper and lower surfaces of the leaves and record the number of leaves with one or more scab lesions, including spots where scab lesions is suspected.
  • Total the number of scabbed leaves recorded.
  • If the number of scabbed leaves is 50 or less: Assume the orchard has a low inoculum level for the next season. The first fungicide spray for apple scab can be safely delayed until as late as tight cluster.
  • If the number of scabbed leaves is 50 to 100: Where inoculum assessments in autumn show less than 53 infected leaves per 600 shoots, green tip spray can be omitted in most years. This strategy is risky for orchards where apple scab is resistant to fungicides such as dodine and DMI fungicides because no fungicides with pre-symptom activity are available to compensate for any errors. Select a sanitation program, perform the selected practices and assume the orchard has a low inoculum level for the next season.
  • Caution - if the number of scabbed leaves is greater than 100: The amount of inoculum in the orchard is high. Maintain a protectant fungicide program beginning at green tip the following season.

Do not use PAD under the following conditions:

  • If DMI fungicides have been applied that season, particularly when used for after infection activity. Scab lesions are inhibited, but not killed, by these fungicides and this inhibition has shown to break down in the fall. These lesions cannot be detected in the fall when doing PAD sampling.
  • If there are scabby, wild trees which contribute inoculum within 200 m of the orchard perimeter.

Reducing apple scab inoculum
While growers have little control over weather conditions, they can reduce inoculum and ensure adequate fungicide protection. The following methods will reduce inoculum in orchards.

  • The application of urea (45 kg of agricultural urea per 1,000 L of water per ha) to the orchard floor after about 95% leaf drop (November) or in the spring (April) before bud break reduces the number of ascospores. The urea works by directly inhibiting the development of ascospores, and stimulating the growth of naturally occurring organisms that are antagonistic toward the apple scab fungus. In most years, the spring treatment is more effective and results in fewer leaf and fruit infections. However, when snow cover remains until bud break, there is little time for the urea to work, and the treatment is less effective.
  • Shredding overwintering leaves can also reduce primary inoculum. Rake or blow leaves from under trees and shred them using a flail mower. Shredded leaves decay more quickly.

Primary infection periods
Mature ascospores are always present in infected overwintering leaves at bud break. The number of ascospores present depends on the amount of inoculum in the overwintering leaves. The rate at which ascospores mature in the overwintering leaves on the orchard floor is mainly determined by temperature.

Researchers at Cornell University have developed a model that relates temperature to ascospore maturation. The model is expressed in degree-days Celsius (DDC) and allows you to predict the percentage of the season’s ascospores that have matured. For further information, see Figure 1. Cumulative Percentage of Ascospores Matured at Various Degree-Day Accumulations. Daily accumulated degree-days are calculated as follows:

DDC = ((Daily max °C  +  Daily min °C) ÷ 2) - min base °C

The minimum base temperature for development of apple scab is 0°C.
For example, on a day with a high of 10°C and a low of 2°C, the accumulated degree-days are 6 DDC. If either of the daily temperatures is below 0°C, i.e., a negative number, then use a value of zero for that temperature. Begin degree-day accumulations at bud break, which is defined as the day on which at least 50% of the fruit buds on McIntosh are between silver tip and green tip.

Use the degree-day accumulations to estimate the percentage of mature ascospores from the central curve in Figure 1. Cumulative Percentage of Ascospores Matured at Various Degree-Day Accumulations. The upper and lower curves are the upper and lower 90% confidence limits around the estimates on the graph. The estimate should fall within this range 90% of the time.

Figure 1. Cumulative Percentage of Ascospores Matured at Various Degree-Day Accumulations

Figure 1. Cumulative Percentage of Ascospores Matured at Various Degree-Day Accumulations

Two points of particular interest on this graph are:

  • At 125 DDC, there is a rapid maturation of ascospores that indicate higher risk of infections.
  • At 418 DDC, over 95% of the ascospore supply should be depleted if sufficient rain has occurred. This marks the end of the primary infection season. With this model, growers can obtain a site-specific forecast of ascospore maturity and discharge for their own orchard(s).

Ascospores are released when there is rain to wet the pseudothecia in the dead leaves. Most of the available mature spores are discharged within 2 hours after the start of rain. Ascospore release is strongly light-dependent and only a small percentage of available ascospores are released at night from 7:00 p.m. to 8:00 a.m. Eastern Daylight Savings Time (EDST).

  • In low-inoculum orchards, ascospores are not released to any significant extent during night-time hours.
  • In high-inoculum an orchards, although the percentage of ascospores released is small at night, the total number of ascospores released is large and can result in a significant primary scab infection.
  • If scab was present in the orchard the previous season, consider the orchard to have high inoculum.

In a low-inoculum orchard, calculate the length of the wetting period using the following method:

  • When rain begins during the day, between 8:00 a.m. and 7:00 p.m. EDST, count the hours of leaf wetness from when the first hour rain was recorded until the leaves are dry.
  • When rain begins at night, between 7:00 p.m. and 8:00 a.m. EDST, count the hours of leaf wetness from 8:00 a.m. in the morning until the leaves are dry.

For a high-inoculum orchard, calculate the length of the wetting period from the start of the rain until the leaves are dry, regardless of the time of day. In high-inoculum orchards if a rain event starts during the day, continue counting the hours of leaf wetness even after nightfall.

Rainfall is needed for the release of ascospores and the leaves and fruit must be wet for infection to occur. The length of the wetting period required for infection varies with temperature. The relationship between hours of wetting and temperature is outlined in Table 1. Relationship of Temperature and Moisture to Apple Scab Infection.

Once the length of the wetting period is determined, calculate the average temperature during this interval and check Table 1. Relationship of Temperature and Moisture to Apple Scab Infection, to see if leaves were wet long enough for an infection to occur.

Once a primary infection occurs and primary lesions are present, then secondary infections can occur. Initially lesions can be very difficult to see. As a result, scouting and maintaining adequate fungicide coverage are very important.

Table 1. Relationship of Temperature and Moisture to
Apple Scab Infection

Average temp.
(°C)

Minimum number of hours of leaf wetness required

Primary season
(ascospore infection)

Secondary season
(conidia infection)

1

40

37

2

34

33

4

27

26

5

21

23

6

18

20

7

15

17

8

13

15

9

12

13

10

11

12

11

9

10

12

8

9

13

8

9

14

7

9

15

7

9

16

6

9

17

6

8

18

6

8

19

6

8

20

6

7

21

6

7

22

6

7

23

6

8

24

6

9

25

8

11

26

11

14


Adapted from: Stensvand, A., Gadoury, D. M., Amundsen, T., Semb, L., and Seem, R. C. 1997. Ascospore release and infection of apple leaves by conidia and ascospores of Venturia inaequalis at low temperatures. Phytopathology 87:1046-1053.

Secondary infection periods
Secondary scab infections occur when splashing rain spreads conidia, developed from primary scab lesions, on leaves. Secondary infections only occur if moisture from rain is present for long enough at a given temperature. Since secondary infections can occur day or night, calculate from the beginning of the wetting period, regardless of time of day.  See Table 1. Relationship of Temperature and Moisture to Apple Scab Infection.

Effect of intermittent rain and dew on infection periods
Periods of dew or high humidity (over 90%) also contribute to a wetting period, but are significant only if preceded by rain. Add together wet periods caused by intermittent rain to determine the length of an infection period, unless wet periods are separated by 10 hours or more of dry, sunny weather.

Fruit infection
As the fruit matures, it takes a longer wetting period for infection by apple scab to occur. Table 2. Relationship of temperature, hours of wetting and weeks after full bloom to secondary apple scab infection of fruit, outlines this relationship.

Table 2. Relationship of temperature, hours of wetting and
weeks after full bloom to secondary apple scab infection of fruit

Average temperature °C

Hours of wetting for 2% fruit infection

1 week after full bloom

5 weeks after full bloom

10 weeks after full bloom

15 weeks after full bloom

10

13.0

26.0

37.0

45.5

12

10.0

21.6

31.0

38.0

14

8.5

18.5

26.5

32.5

16

7.5

16.0

23.0

28.5

18

6.5

14.5

20.5

25.5

20

6.0

13.0

18.5

23.0

Adapted from: Schwabe, W.F.S., Jones, A.L., and Jonker, J.P. 1984. Changes in Susceptibility of Developing Apple Fruit to Venturia inaequalis. Phytopathology 74:118-121. Cultivars tested were: Golden Delicious, Starking Delicious, Starkrimson Delicious, and White Winter Pearmain.

The wetting periods required for infection of fruit by apple scab are longer than those required for leaf infection. For orchards with only light primary leaf infection - and where only fruit scab is a concern - base spray decisions on the longer infection times required for fruit. However, if unchecked during the growing season, leaf infection could create large amounts of overwintering scab inoculum and heavy ascospore release the next year.

Management of apple scab with fungicides
Fungicides are used to control scab in most commercial orchards. See OMAFRA Publication 360, Fruit Crop Protection Guide  for information about the activity of different fungicides available for apple scab. 

Early season management
Management programs for primary apple scab infection are based on the timing and type of fungicide used. There are two general approaches: 1) Apply fungicides as a protectant program before infection occurs, or 2) apply fungicides after infection but before symptoms develop.

Protectant fungicides - Residues inactivate and kill fungal spores and prevent infection. These products are formulated to stick to foliage if it rains after the spray has dried. If it rains, a small proportion of residue goes back into solution and redistributes on tissue that was sprayed, but does not redistribute to unsprayed leaves or fruit. Coverage with these products must be complete, on both upper and lower leaf surfaces and the entire surface of fruit. Apply fungicides to expanding foliage when infections are imminent.

Pre-symptomatic (after infection) fungicides - An after-infection spray is applied within a specific time after the start of an infection period. Time available to apply the fungicide effectively depends on the product used and average temperature. Do not rely on pre-symptomatic or after-infection fungicide programs to control scab. To use this spray program successfully, well-calibrated equipment that provides thorough spray coverage of leaves and fruit under adverse conditions is necessary.

  • Fungicides are applied to expanding foliage before infection periods occur. Apply fungicides when conditions for infection are imminent.
  • Begin fungicide programs between silver tip and green tip. Continue at 7-10 day intervals, using shorter interval if frequent heavy rainfall or rapid shoot growth occurs.
  • From first cover until early August, apply fungicides at 10–14-day intervals depending on rainfall frequency.
  • New growth and expanding fruit must be protected and residues washed off by rain must be replaced. The exception to this is when using fungicides in the DMI group (Nova, Fullback, Inspire Super). Use a 5–7-day spray schedule regardless of rainfall.
  • DMI fungicides are weak on fruit scab, so do not start a DMI program after bloom.
  • Some fungicides are not readily washed off and the residues are redistributed by rainwater. See OMAFRA Publication 360, Fruit Crop Protection Guide for information on retention and redistribution of apple scab fungicides.

Control of apple scab under adverse weather conditions
Several consecutive days of continually wet conditions and mild temperatures are common in the spring. These conditions encourage apple scab infection. It is often difficult to maintain sufficient fungicide on the trees to provide protection for the rapidly expanding foliage and fruit under such conditions.

  • If there is concern about adequate protection, it is better to apply a protectant-type fungicide during a break in the rain than to not spray at all. Be sure to use a fungicide with good retention properties. For example, products with mancozeb such as Dithane, Manzate and Penncozeb have good retention activity.
  • After-infection sprays may be required when protectant fungicides become too diluted to prevent the apple scab fungus from establishing itself on the foliage or fruit.
  • If an infection exists, scab lesions become visible on the foliage and fruit in about 9 days at 20°C or in about 14–21 days if the average temperatures are 12°C or lower. Once this occurs, avoid using fungicides with high potential for resistance (DMIs, strobilurins, SDHIs) and focus on protecting uninfected tissue from further infections.
  • Strong winds can hamper spray operations by causing pesticides to drift away from the intended target. Spray at night or early morning to help reduce spray drift. You can also lower the spray trajectory into the wind to minimize drift, but take extra care to ensure the treetops get good spray coverage.

Secondary scab management
The end of primary season can be determined through use of the degree-day model. At 418 DDC, over 95% of the ascospores are mature, and after a significant rain all of the ascospores have been discharged for the season. 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.

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. If scab lesions are found, maintain a fungicide program for the remainder of the season.

Fungicide resistance
Several fungicide families available to apple growers are prone to the development of resistance, so fungicide programs must be developed to minimize this risk. Implement these strategies to avoid or manage the development of fungicide resistant scab populations in apple orchards.

  • Always rotate among chemistries in a fungicide program.
  • Never exceed the maximum number of applications listed on the label. 
  • Always use full rates of single-site fungicides (DMI, strobilurin, SDHI) with a half to full rate of protectant fungicide.
  • Do not miss fungicide sprays at green tip.
  • Be prepared to spray, even in the rain if necessary, with protectant fungicides but keep in mind residual control will be lost.
  • Always apply fungicides preventatively before infection, not curatively after infection. The curative application of fungicides may encourage survival of fungicide-resistant populations. 
  • Always use full label rates of fungicides. Reduced rates increase the number of resistant individuals present in an orchard.
  • Ensure complete coverage with the appropriate volume of water. Do not spray alternate rows. 
  • Do not extend spray intervals beyond those listed on the label. Shorten spray intervals in case of rain.
  • Employ sanitation procedures to reduce overwintering inoculum.
  • Do not apply DMI, strobilurin or SDHI  fungicides when scab lesions are present as this encourages resistance to develop.

Scab-resistant cultivars 
Several cultivars are now available with high resistance of field immunity to apple scab. Some of these cultivars also have resistance to other diseases. Table 3. Resistance of cultivars to apple scab and other diseases, is a partial list of available scab-resistant apple cultivars, and additional selections are presently being developed.

Before planting of large number of disease-resistant apples, evaluate diseases resistance, fruit quality, and most importantly, marketability of the cultivar. There is a wide range of growing conditions in Ontario apple districts, and the best way to evaluate these cultivars is to establish a small planting of cultivars of interest.

Table 3. Resistance of cultivars to apple scab and other diseases

Cultivar

Apple Scab

Powdery Mildew

Fireblight

Cedar Apple Rust

Quince Rust

Belmac 

MR

U

U

U

Britegold

R

R

R

S

S

Dayton

R

MR

MR

S

S

Enterprise

R

MR

R

R

U

Freedom

R

MR

R

MR

U

Florina (Que-rina®)

R

T

MR

U

U

Goldrush

R

MR

MR

S

U

Jonafree

R

MR

MR

S

MR

Liberty

R

R

R

R

MR

Macfree

R

MR

MR

R

S

McShay

R

S

U

U

U

Moira

R

S

S

R

S

Murray

R

R

MR

R

S

Nova
Easygro

R

R

S

R

S

Novamac

R

MR

U

R

S

Novaspy

R

MR

U

S

S

Prima

R

MR

MR

S

S

Primevere

R

U

U

S

S

Priscilla

R

R

R

R

U

Pristine

R

R

MR

S

MR

Redfree

R

MR

MR

R

S

Richelieu

R

MR

MR

S

S

Rouville

R
susceptible to race 5

U

U

U

S

Sir Prize

R

MR

MR

S

S

Trent

R

MR

MR

R

S

William's
Pride

R

MR

MR

R

S

* Not all cultivars listed have been evaluated in Ontario. Where this is the case the information given is taken from the originating source.

Disease resistance ratings:

R - resistant (no control necessary)
MR - moderately resistant (control may be necessary under severe disease pressure)
S - susceptible (control necessary in growing areas where the disease is prevalent)
U - unknown