Average Temperature (°C)1 Minimum Number of Hours of Leaf Wetness Required - Primary (Ascospore) Infection2
Minimum Number of Hours of Leaf Wetness Required - Secondary (Conidia) Infection2 Lesion appearance (days)3
Light Moderate Heavy
 1  40  69  93  37  -
 2  34  69  93  33  -
 3  30  52  65  30  -
 4  27  42  57  26  -
 5  21  34  50  23  -
 6  18  27  44  20  17
 7  15  23  37  17  17
8  13  21  34  15  17
9  12  17  27  13  17
 10  11  16  26  10  16
11  9  14  22  9.5  15
12-13  8  12  20  9  14
14-15  7  11  19  9  12-13
15.5  6.5  10  17  9  10-11
16-24  6  9  16  7.0-9.0  9-10
24-25  6.5  9  16  9-11  -
25  8  11  18  11  -
25.5  10  14  23  13  -

1Add lowest and highest temperatures during wet period and divide sum by 2 to get average temperature.

2Calculate hours of wetting by either (1) beginning the count at the time leaves first become wet and ending the count when the relative humidity drops below 90%, or (2) adding consecutive wet periods (hours) if the leaves are again wetted within 8 hours from the time relative humidity dropped below 90%.

3Number of days required for lesions to appear after infection has been initiated. 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 and Carisse, O. 2006. Apple Scab: Improving Understanding for Better Management. Agriculture & Agri-Food Canada, Publication 10203E.