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About Enviroweather's Apple Scab Report

By Bill Shane, MSU Extension

More information on apple scab at IPM Resources

About the models:

Three models are used in the Apple Scab Report--infection, symptom expression, and ascospore maturity.

  • Primary infection model: Scab infection prediction helps the grower to determine his/her fungicide control options. Some fungicides can eradicate an infection if applied within a specified time of the start of an infection period. Infection is predicted by the Mills table system for apple scab, as modified and adapted by A. L. Jones of Michigan State University. Once green tissue of apples is exposed in the spring, apples are vulnerable to scab infection. The apple scab infection model predicts if a wetting period was sufficient to initiate an infection.

    A rain of at least 0.01 inches is needed to soak overwintering leaves to initiate release of ascospores. Depending on the temperature and duration, the wetting period may result in infection or no infection, as given by the Mills/Jones table.

    For primary scab: a wetting period ("mother" wet period) begins when measurable precipitation is collected (i.e. 0.01 or more inches). This infection period lasts until the RH drops below 90% and leaf wetness proportion for the hour is less than 0.25. However, the spores do not die immediately and can resume if moisture becomes available soon enough. This is how the scab model handles "interrupted" wetting periods.

    A "daughter" wet period begins if measurable precipitation occurs or leaf wetness proportion for the hour is at least 0.25 or RH is at least 90% within 8 hours from the time the last wetting period ended. A daughter wet period ends for the same reasons as a mother period (RH drops below 90% and leaf wetness proportion for the hour is less than 0.25). More than two wetting periods can be linked together into a single infection event (multiple daughters). The "span" contains both wet and dry hours. When a "mother" and "daughter(s)" wet periods are jointed together, both the hours and the temperatures of the intervening "dry" hours are not included when using the Mills table to see if infection has occurred.

    Note: infection by conidia is not covered by this model.  Conidia are produced in the tree leaves and fruit by successful ascospore infections.  The time for infection by conidia is less than for ascospores because conidia are washed to a new infection site immediately by a rain, whereas ascospores are released only after the overwintering leaves on the orchard floor have absorbed sufficient water.

    Table 1. Approximate wetting period required for primary apple scab infection at various air temperatures and time required for development of conidia.
    Average Temperature (F) Wetting duration (hr) Incubation period (days)
    78 13 -
    77 11 -
    76 9.5 -
    63-75 9 9
    62 9 10
    61 9 10
    60 9.5 11
    59 10 12
    58 10 12
    57 10 13
    56 11 13
    55 11 14
    54 11.5 14
    53 12 15
    52 12 15
    51 13 16
    50 14 16
    49 14.5 17
    48 15 17
    47 15 -
    46 16 -
    45 17 -
    44 19 -
    43 21 -
    42 23 -
    41 26 -
    40 29 -
    39 33 -
    38 37 -
    37 41 -
    33-36 48 -

    Adapted from Mills, 1944; modified by A.L. Jones. The infection period is considered to start when rain begins. The incubation period is the estimated number of days required for conidial development after the start of the infection period.

  • Scab symptom appearance model: Once infection has been established for a wetting period, symptoms will show up after an incubation period of one to several weeks, depending on the temperature during incubation.  Symptoms are expected 330 degree days base 32 following infection. If 330 DD is not reached in the forecast period, the last 3 days of the forecast are repeated to estimate when symptoms are first expected.

  • Apple scab ascospore maturation model: The supply of ascospores on the orchard floor can be predicted by degree day models developed by Gadoury and MacHardy. Their model initiates at the start of green tip growth stage of apples and uses a base of 32 F to estimate ascospore maturity. The purpose of the model is to predict when growers can relax their fungicide program for scab. Warning: the ascospore maturity model is less reliable in unusually dry and/or hot years. Under such conditions, ascospore catch has been detected in spore traps in significant numbers beyond 1000 degree days base 32 F.

    Table 2. Cumulative Percentage of Ascospores Matured at Various Degree-Day Accumulations ( 32 F Base)
    Cumulative Degree-Days [1] Cumulative Ascospores Matured (%) 90% Confidence Interval for Estimate [2]
    35 1 0-7
    110 3 0-14
    145 5 1-19
    215 10 2-32
    325 25 7-55
    450 50 21-80
    575 75 46-94
    685 90 69-98
    740 95 79-99
    790 97 86-100
    865 99 93-100
    • [1] Degree-Days should be recorded from the date when 50% of McIntosh fruit buds are between silver tip and green tip. Data of Gadoury and MacHardy, 1982.
    • [2] The width of the 90% confidence interval is a statistical measure of the precision of estimated maturity. It is the range within which the estimate should fall 90% of the time. Source: 1998 Cornell Pest Management Recommendations for Commercial tree-Fruit Production

Use of the models:

Typically, the apple scab models start at green tip and are discontinued by the end of June. The infection-prediction model can be used as a guide to determine the need for eradicative sprays if a protectant coverage was weak during a rain episode. The model should not be used as a routine guide for eradicative spray applications-a protectant spray program is the most reliable approach for control of apple scab. The incubation period model is used to predict when scab symptoms should appear on unprotected apple tissue. The model is useful for comparing predicted apple scab infection activity between years.

To read the Apple Scab Spore Maturity and Discharge Report, locate the date of McIntosh Green Tip across the top of the chart. Follow that column down to today's date to find the estimates of spore maturity and discharge. The ascospore maturation model has not been particularly useful in recent seasons in Michigan.


  1. Gadoury and MacHardy, Phytopathology, 72:7, 1982.
  2. A.Stensvand, T. Amundsen, L. Semb, D.M. Gadoury, and R.C. Seem. 1997. Ascospore release and infection of apple leaves by conidia and ascospores of Venturia inaequalis at low temperatures. Phytopathology 87:1046-1053.
  3. Jones, A. L., Lillevik, S. L., Fisher, P. D. and Stebbins, T. C. 1980. A microcomputer-based instrument to predict primary apple scab infection periods. Plant Dis. 64:69-72.
  4. Biggs, A. R. "Apple Scab" in Jones, A. L., and Aldwinckle, H. S. (editors). 1990. Compendium of apple and pear diseases. APS Press, St. Paul, MN.
  5. More information on apple scab at IPM Resources
  • MSU Extension
  • Michigan State University Ag Bio Research
  • Project Greeen