Mills Table – Apple scab infection periods

 

Average
Temp.
Hours of leaf wetness
Relative infection level
 Days till first
lesions
Low Moderate High
78 10 14 23
77 8 11 18
76 6.5 9 16
61-75 6 9 16 9-10
60 6.5 10 17 11
57-59 7 11 19 12-13
55-56 8 12 20 13-14
54 8.5 13 21 14
52-53 9 14 22 15
51 10 15 24 16
50 11 16 26 16
49 11.5 17 27 17
48 12 17 27 17
47 14 20 32 17
46 16 (13) 21 34 17
45 17(15) 23 37 17
44 19 25 40 17
43 21(18) 27 44 17
42 23 30 47 17
41 21 34 50 ?
40 29 38 53 ?
39 33(28) 42 57 ?
38 37 47 61 ?
37 41(30) 52 65 ?
36 48(35) 69 93 ?
34 48(41) 69 93 ?

Adapted from the table of W. D. Mills, revised by W. E. MacHardy. Numbers in parenthses are the minimum hours of continuous leaf wetness that resulted in scab lesions on apple leaves inoculated with ascospores at controlled temperatures from 36 to 46 (reported by Gadoury et al. in the New York Fruit Quarterly.)
Low identifies the minimum hours for infection. Moderate and high identify the hours of leaf wetness required for noticeale increases in scab lesions. The average temperature during wetness should be based on hourly temperatures.