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Apple scab infection periods

Identifying Apple Scab Infection Periods

The key to managing scab is preventing primary infections. By successfully preventing or limiting the development of primary lesions, the threat of continued infection by conidia is reduced. Since scab infections are invisible for at least 9 to 17 days after infection, is important to understand the conditions that cause a scab infection period to know whether preventative action is needed, or post-infection treatment required. 

Use the Scab Infection Table and measure the length of wetting periods to determine infection periods during the primary scab season.

  • When rain begins during the day (between 8:00 a.m. and 7:00 p.m. in New England under Daylight Savings Time), count the hours of leaf wetness from the first hour rain is recorded until the leaves are dry.
  • Scab ascospores are not released when it is dark, so when rain begins at night (between 7:00 p.m. and 8:00 a.m., DST), count the hours of leaf wetness from 8:00 in the morning until the leaves are dry.
  • The average temperature during leaf wetness should be calculated using hourly temperatures.
  • NEWA Infection Hours are the minimum hours of leaf wetness used by NEWA, as modified in 1989 by MacHardy and Gadoury, from the Jones version of Mills original table. They are adjusted to account for night wetting, and are the most conservative intervals.
  • Low is the minimum hours of wetness required for any infection determined by Jones and Sutton (Diseases of Tree Fruits, North Central Reg. Ext. Pub. 45, 1984).
  • Moderate and High indicate the hours of wetness that will generate significant increases in the amount of infection.
  • Additionional days may be required if conditions are unfavorable for lesion development such as a prolonged period above 80 F or very dry weather.

The process of determining the risk of scab infection can be greatly simplified by using a web-based decision support system, such as Ag Radar or NEWA. For details, contact coordinators through the DSS web sites.

Ag Radar

NEWA

Scab Infection Table

Average

TempErature
oF

newa

Infection
HOURS

Low
infection
hours

Moderate

Infection
HOURS

High

Infection
HOURS

Days

to First 

Symptoms

78

10

13

17

26

 

77

 8

11

14

21

 

76

6.5

9

12

19

 

61 - 75

6

9

12

18

9 - 10

60

6.5

9

13

20

11

57 - 59

7

10

14

22

12 - 13

55 - 56

8

11

15

23

13 - 14

54

8.5

11

16

24

14

52 - 53

9

12

17

25

15

51

10

13

18

27

16

50

11

14

19

29

16

49

11.5

14

20

30

17

48

12

15

20

30

17

47

14

15

23

35

17

46

14

16

24

37

17

45

14

17

26

40

17

44

15

19

28

43

17

43

18

21

30

47

17

42

20

23

33

50

17

41

21

26

37

53

?

40

21

29

41

56

?

39

28

33

45

60

?

38

29

37

50

64

?

37

30

41

55

68

?

36

33

48

72

96

?

35

35

48

72

96

?

34

40

48

72

96

?