Notes on Apple Diseases: Apple scab

Identification

Apple scab, Venturia inaequalis, is the most serious fungal disease of apples in Ontario. The fungus attacks foliage, blossoms and fruit, defoliating trees and causing fruit to be unmarketable. In addition to cultivated apple, apple scab also attacks flowering crab apples, mountain ash and firethorn.

There are two types of apple scab infection. Primary infection is caused by air-borne ascospores that overwinter on infected leaves on the orchard floor. These ascospores infect leaves and developing fruitlets. Lesions caused by primary infection form conidia, which are rain-splashed to new tissue and cause secondary infection.

Primary Infection Periods

Once apple trees break dormancy and there is green tissue present, a primary infection can occur if the following three conditions are met:

  • mature ascospores are present
  • ascospores are discharged
  • moisture is available

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 determined mainly by temperature.

Researchers at Cornell University have developed a model, which relates temperature to spore maturation. The model is expressed in degree-days Celsius or 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.

Cumulative Percentage of Ascospores Matured at Various Degree-day Accumulations

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 - 0°C

For example, on a day with a high of 10°C and a low of 2°C, the accumulated degree-days are six DDC. If either of the daily temperatures are 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. 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.

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. See Table 1. Percentage of Mature Ascospores Discharged Under Various Environmental Conditions. 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).
Table 1. Percentage of Mature Ascospores Discharged Under Various Environmental Conditions
Type of rain event % of mature ascospores discharged
Night rain only
5%
Day rain < 0.25 cm; < 10°C
25%
Day rain > 0.25 cm; < 10°C
50%
Day rain < 0.25 cm; > 10°C
50%
Day rain > 0.25 cm; > 10°C
90%

Ascospores are released when there is rain to wet the pseudothecia in the dead leaves. Most of the available mature spores are discharged within two 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 p.m.- 8 a.m. Eastern Daylight Savings Time [EDST].

Ascospores are not released to any significant extent during night-time hours in low inoculum orchards. In an orchard where the ascospore inoculum is high, although the percentage of ascospores released is small, the total number of ascospores released is large and can result in a significant primary scab infection.

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 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 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.

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. This relationship between hours of wetting and temperature is outlined in Table 2. Relationship of Temperature and Moisture to Apple Scab Infection.

    Table 2. Relationship of Temperature and Moisture to Apple Scab Infection
      Minimum number of hours of leaf wetness required
    Average temp.
    (°C)
    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, Gadoury, Amundsen, Semb and Seem, 1997 Phytopathology 87: 1046-1053.

Once the length of the wetting period is determined, calculate the average temperature during this interval and check Table 2 to see if leaves were wet long enough for an infection to occur.

Secondary Infection Periods

Secondary scab infections occur when splashing rain spreads conidia, developed from primary scab lesions, on leaves. As with primary infections, secondary infections only occur if moisture from rain is present 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. Continued infections through the summer result from lesions caused by conidia.

    Table 3. Relationship of Temperature, Hours of Wetting and Weeks After Full Bloom to Secondary Apple Scab Infection of Fruit
      Weeks after full bloom
      1 5 10 15
    Average temp. °C Hours of wetting for 2% fruit infection
    10
    13
    26
    37
    45.5
    12
    10
    21.6
    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
    10
    6
    13
    18.5
    23

Adapted from Schwabe et al., 1984. Cultivars tested were: Golden Delicious, Starking Delicious, Starkrimson Delicious, and White Winter Pearmain.

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 3. Relationship of Temperature, Hours of Wetting and Weeks After Full Bloom to Secondary Apple Scab Infection of Fruit, on this page, outlines this -relationship.

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 of concern, spray decisions may be based 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. Table 4. Characteristics of Apple Scab Fungicides, outlines registered fungicides and their effectiveness on scab and other apple diseases.

Here are some definitions of fungicide activity:

  • Protectant activity is the ability of fungicide residues to inactivate and kill fungal spores and prevent infection.
  • Pre-symptomatic or after infection activity is when fungicide applied within the period given in Table 4, prevents scab lesion development once the infection has occurred.
  • Post-symptomatic or curative activity is the fungicide's ability to prevent or greatly inhibit the further production of secondary spores (conidia) when applied to sporulating scab lesions. Generally, applications of fungicides with this activity must be repeated for best effect.

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: apply fungicides on a protectant program before infection occurs or apply fungicides after infection but before symptoms develop.

Protectant fungicides

Fungicides are applied to expanding foliage before infection periods occur. Apply fungicides when infections are imminent.

  • Begin the program just before the first anticipated primary infection period between silver tip and green tip.
  • 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 DMI fungicides. Use a seven day spray schedule regardless of rainfall.
  • Some fungicides are not readily washed off and the residues are redistributed by rainwater. See Table 4.

Pre-symptomatic (after-infection) sprays

An after-infection spray is one applied within a specific time after the start of an infection period. The time available to apply the fungicide effectively depends on the product used and the average temperature. When you calculate after-infection activity, count the time interval from the beginning of the infection period. For example, at 6°C apply Captan within 48 hours from the beginning of the infection period.

Do not rely on pre-symptomatic or after-infection fungicide programs to control scab. To use this spray program successfully, equipment that can provide good spray coverage under adverse conditions is necessary. Reliable and accurate temperature and leaf wetness monitoring equipment for each orchard is essential.

Use of demethylation inhibitor (DMI) fungicides (sterol-inhibitors)

Nova 40 W and Nustar are members of Demethylation Inhibitors (DMIs), a group of fungicides loosely known as sterol inhibitors. These products must be used differently than protectant fungicides such as captan or mancozeb.

The main strength of the DMI group of fungicides is their relatively long post-infection, pre-symptom activity. These products can cure new scab infections if applied within 72-96 hours after infection. The DMI fungicides, however, generally have only three to five days of protectant ability.

DMI fungicides must be absorbed by the leaf and fruit tissue to be effective. They are absorbed within one hour of application and their efficiency is not reduced if rain occurs more than one hour after -application.

DMI fungicides do not redistribute well after application. Therefore uniform spray coverage is essential for good disease control.

Do not use Nova 40 W or Nustar earlier than tight cluster. Apply in combination with a protectant fungicide in blocks of two applications, seven to ten days apart. DMI fungicides are weak on fruit scab so do not start a DMI program after bloom. Apply Nova 40 W, or Nustar plus a protectant fungicide (e.g. Captan), within 72-96 hours after the beginning of a scab infection period. Do not stretch consecutive applications past ten days. Apply at least two but no more than four sprays per season, preferably between tight cluster and first summer spray.

After a DMI plus a protectant fungicide application, leaves and fruit are protected for only five days. Apply a protectant fungicide before the next scab infection period. Follow up with a protectant fungicide program as required for the rest of the season. Also see Fungicide resistance.

Use of strobilurin fungicides

Sovran and Flint 50 WG are members of a new family of fungicides called strobilurins. These products have good protectant and post-infection activity on apple scab and powdery mildew. Do not use Sovran or Flint 50 WG earlier than tight cluster and apply in blocks of two applications, ten days apart. Due to of the mode of action, it is important to follow resistance management strategies carefully. Use another chemical family for two applications before you return to either Sovran or Flint 50 WG. After an application of Sovran or Flint 50 WG, tissue is protected from infection for five to eight days. In conditions of rapid growth, shorten intervals between applications.

Some strobilurin fungicides are phytotoxic to other fruit crops, such as Sovran for sweet cherries and Flint for concord grapes. Always check the label for precautions.

Control of apple scab under adverse weather conditions

Several consecutive days of continuous 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 nine days at 20°C or in about 14-21 days if the average temperatures are 12°C or lower.

Control these infections as soon as they become visible by following the instructions outlined in the section on Deactivation of established scab -lesions.

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.

Deactivation of established scab lesions

Apple scab lesions visible on the foliage and fruit can produce many spores capable of disease spread through secondary infection. Several fungicides have good post-symptomatic activity. See Table 4. Characteristics of Apple Scab Fungicides, for examples. Continuous use of these fungicides as eradicants promotes resistance. For example, thiophanate-methyl (Senator) and dodine (Equal) have lost their efficiency against apple scab because resistance to these fungicides has developed in Ontario orchards.

If fungicide-resistant scab is not present in your orchard and apple scab lesions are visible, try to deactivate these tissue infections. Apply two sprays of a post-symptomatic fungicide at the full rate five to seven days apart as soon as scab lesions are observed. These materials reduce the number of spores produced on the lesions and help to slow the spread of the disease. Follow with a regular season-long protectant program.

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. See Table 1. After this point, wait two weeks 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, for example 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. Some growers have successfully eliminated all use of fungicides during the summer months where primary scab was controlled. If scab lesions are found, maintain a fungicide program for the remainder of the season.

    Table 4. Characteristics of Apple Scab Fungicides
    Product Protectant activity
    (pre-infection)
    Max. after-infection1 activity (hr) Post-
    symptom activity
    Retention
    (50 mm rain)
    Re-distribution (12 mm rain)
        6°C 12°C 17°C      
    Captan 50 WP
    VG
    48
    24
    18
    N
    VG
    G
    Dikar WP
    VG
    48
    28
    18
    VG
    E?
    VG?
    Dithane M 45/80 WP
    VG
    48
    24
    18
    N
    E
    VG
    Equal 65 WP
    VG
    48
    24
    18
    VG
    VG
    VG
    Flint 50 WG
    E
    963
    963
    963
    E
    E
    F?
    Manzate 200
    VG
    48
    24
    18
    N
    E?
    VG
    Nova 40 W 2
    F
    96
    96
    96
    FG
    F?
    F?
    Polyram 80 WP
    VG
    48
    24
    18
    N
    E?
    VG
    Nustar
    F
    96
    96
    96
    E
    E
    Sovran
    E
    963
    963
    963
    E
    E
    F?

Data is adapted from New York (Geneva) from work done by Szkolnik et al, using conidia. These values are adapted from greenhouse tests on Golden Delicious. The after-infection activity of these fungicides may not be adequate to control primary scab in commercial orchards. Do not rely solely on after-infection activity for the control of apple scab.

These results are formulation specific. Newer formulations of some products have not been evaluated (e.g., Dithane DG, Polyram DF, Manzate 200DF). Contact the manufacturer for more information.

1 Maximum after-infection activity is calculated from the start of the infection period.
2 Data on Nova is adapted from 1993 Pest Management Recommendations for Commercial Tree Fruit Production, Cornell Cooperative Extension, New York.
3 After-infection activity of DMI (Nova and Nustar) and QoI (Flint and Sovran) fungicides may be reduced in orchards where scab populations have shifted toward resistance to DMI fungicides
KEY: E = excellent; VG = very good; G = good; F = fair; P = poor; N = none. Values followed by '?' are based on field observations.

    Table 5. Examples of Apple Cultivars Susceptible to Key Diseases
      Very susceptible Least susceptible
    Apple scab McIntosh, Cameo, Sunrise, Shizuka, Gala and Golden Supreme, Cortland Pristine, Enterprise, GoldRush
    Fire blight Fuji, Gala, Ginger Gold, Idared, Jonagold Red Delicious, Liberty, Enterprise, Freedom
    Powdery mildew Cortland, Idared, Paulared Red Delicious, McIntosh, Empire, Northern Spy, Freedom, Jonafree
    Blister spot Mutsu, Golden Delicious, Red Delicious, Jonagold, Cortland all others

    Table 6. Effect of Fungicides on Apple Diseases

    Ratings in bold indicate the disease is listed on the product label for control or suppression. Please see the product label or crop calendars for registered uses. Use fungicides only for diseases listed on the product label for the crop and for the disease. Additional information is provided in this table to assist the grower in choosing the best fungicide for control of diseases listed on the product label.

    Fungicides Apple scab Powdery mildew Cedar apple rust/quince rust Black rot Bitter rot Sooty blotch Fly speck
    captan (Supra Captan 80 WDG, Maestro 80 DF)
    +++
    0
    0
    +++
    ++
    +++
    ++
    copper
    +
    +
    +
    +
    +
    +P
    +P
    cyprodinil (Vangard 75 WG)
    ++
    +
    0
    0
    NA
    0
    0
    dithane+ karathane (Dikar)
    +++
    +++
    ++
    +
    NA
    +++
    +++
    dodine (Equal, Syllit)
    +++R
     
     
    0
    NA
    0
    0
    flusilazole (Nustar)
    +++R
    ++
    +++
    0
    0
    0
    0
    kresoxim-methyl (Sovran)
    +++
    ++
    +
    +++
    ++
    +++
    +++
    mancozeb (Dithane DG, Manzate DF, Penncozeb 75 DF)
    +++
    0
    ++
    0
    NA
    +++
    +++
    metiram (Polyram DF)
    +++
    0
    ++
    0
    NA
    +++
    +++
    myclobutanil (Nova 40W)
    ++R
    +++
    +++
    +
    0
    0
    0
    pyrimethanil (Scala)
    ++
    0
     
     
     
    NA
    NA
    sulphur
    +
    ++
    +
    0
    0
    0
    +
    trifloxystrobin ( Flint 50 WG)
    +++
    ++
    +
    +++
    ++
    +++
    +++

P = Phytotoxic, causes russetting.
R = resistance has been reported in individual orchards
0 = no control.
NA = not applicable because the fungicide cannot be applied during late summer when bitter rot infections usually occur.
+ = poor to fair control, ++ = good control, some limitations, +++ = excellent control, few if any limitations, ? = efficacy unknown.

by McSmith Agricultural Research Services.

Fungicide resistance

Apple scab resistance to fungicides (Senator, Equal) has been documented in most apple growing areas in the province.
Preliminary research from samples collected from a few orchards during 2004, suggests that populations of apple scab resistant to the DMI fungicides, also known as sterol inhibitors like Nova and Nustar, may be present in some Ontario orchards. These fungicides are still very effective for the management of apple scab in most Ontario orchards.
Implement these strategies to avoid or manage the development of fungicide-resistant scab populations in apple orchards:
  • Always rotate between chemistries in a fungicide program.
  • Never apply more than four applications of sterol inhibitors or strobilurins per season.
  • Always use full rates of DMI fungicides with a half rate of protectant fungicide.
  • Don't miss fungicide sprays at green tip.
  • Be prepared to spray, even in the rain if necessary, with Supra Captan, Maestro, Dikar, Manzate, Dithane, Penncozeb, Polyram but keep in mind you will lose the residual. Don't spray Nova, Sovran, Flint, Scala or Vangard in the rain.
  • 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 of fungicides may increase the number of resistant individuals present in an orchard.
  • Don't spray alternate rows.
  • Don't extend spray intervals beyond those listed on the label. Shorten spray intervals in case of rain.
  • Urea sprays may be applied to leaves on the orchard floor in November and/or the spring to increase leaf decomposition and reduce the apple scab inoculum the next spring. Flail mowing has also been shown to reduce inoculum levels.
Do not apply DMI (sterol-inhibiting) or Qol (strobilurin) fungicides when scab lesions are present because this encourages resistance to develop.

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