1996-1997 New England Apple Pest Management Guide


APPLE SCAB BIOLOGY AND EPIDEMIOLOGY

1996-1997 New England Apple Pest Management Guide, pp. 7-10

Apple scab is the most important disease of apples in New England. This is because our humid temperate climate and frequent rains, particularly in spring, provide conditions for infection by the fungal pathogen causing scab and the major cultivars are highly susceptible to scab (Table 3).

Table 3 Disease susceptibility of selected New England apple cultivars to apple scab, rusts, and powdery mildew.*

Cultivar

Scab

Quince Rust

Cedar Rust

Powdery Mildew

Cortland

4

3

3

4

Crispin(Mutsu)

4

2

3

4

Delicious

3

3

1

2

Empire

3

2

2

3

Golden Del.

3

3

4

3

Idared

3

2

3

3

Jonagold

4

3

3

3

Macoun

4

2

2

3

McIntosh

4

1

2

3

Paulared

4

1

3

3

Spartan

3

2

2

2

Key to ratings:
1 = Resistant. No management needed for the disease.
2 = Slightly susceptible. Normal management may be reduced.
3. = Moderately susceptible. Normal management adequate
4 = Highly susceptible. May require extra management.
*Adapted from: Management Guide for Low-Input Sustainable Apple Production. 1990. USDA et al.

The apple scab fungus overwinters on fallen leaves that had been infected during the growing season. Fungal reproductive structures (pseudothecia) are produced in scabbed leaves after the leaves fall in autumn. In spring, ascospores are produced in the pseudothecia, usually beginning around green tip and usually continuing until the first soaking daytime rain that occurs around or soon after petal fall. This period is referred to as the primary scab season.

Ascospores are discharged from the pseudothecia into the orchard air every time there is sufficient rain (at least 0.01 inches) to thoroughly wet the leaves, with nearly all of the ascospores discharged during daytime. During the day, two to three times more ascospores are discharged when the temperature is above 50 F and there is more than 0.1 inches of rain than at temperatures below 50 F and less than 0.1 inches of rain. The ascospores cause initial infections (called primary infections) on susceptible apple tissues (sepals, young leaves, and young fruit).

Whether or not the ascospores discharged during a rain event will infect (cause scab lesions) depends on the temperature and how long the leaves and fruit remain wet (see Table 4). If conditions are favorable for infection, scab lesions will become visible in 9 to 17 days, depending on the average temperature during the incubation period (see Table 4).

The importance of scab lesions in the epidemiology of scab is that they produce spores (conidia) that are splash dispersed to susceptible tissue, usually on the same tree, and can cause additional infections (called secondary infections). Each scab lesion can produce conidia for four to six weeks.

Thus, after ascospore production has been completed, scab lesions are the sole source of spores responsible for the continued buildup of scab on the leaves and fruit for the remainder of the growing season.

IDENTIFYING PRIMARY INFECTION PERIODS

The key to managing scab is to prevent primary infections, because the fewer primary lesions that develop, the less the threat of additional scab lesions caused by conidia after ascospores are no longer being discharged. It is important to know that an infection period has occurred, and a decision to apply a fungicide is often based on that knowledge.

Use Table 4 and the procedure described below for determining an infection period during the primary scab season:

When rain begins during the day (between 8:00 a.m. and 7:00 p.m., DST): count the hours of leaf wetness from the first hour rain is recorded until the leaves are dry
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.


DETERMINING THE IMPORTANCE OF A PRIMARY INFECTION PERIOD

The procedure explained above will tell you if the minimum hours for infection have occurred, but it will not indicate how much infection will occur, i.e., how many lesions will develop. The number of lesions that will develop is dependent mainly on:

1. the amount of inoculum (ascospore density) in the orchard,
2. the amount of susceptible tissue on the trees,
3. the level of susceptibility of the cultivar(s),
4. how long the leaves and fruits remain wet after the minimum hours for infection in Table 4.

Information about each of these factors will help to categorize the severity of an infection period and to determine the risk of not having preventive or retroactive (kickback) fungicide protection against scab infection.

Table 4 Levels of noticeable differences in scab development as a function of the hours of leaf wetness at different temperatures 1

                Relative Infection Level 2

Average T (F)

Low

Moderate

High

Days until first lesions3

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

26(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

?

1 Adapted from the table of W. D. Mills, revised by W. E. MacHardy. This table can be used only in conjunction with the criteria for determining an infection period described in this guide.
2 Low identifies the minimum hours for infection. Moderate and high identify the hours of leaf wetness required for noticeable increases in scab lesions. The average temperature during wetness should be based on hourly temperatures. Numbers in parentheses 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).
3 Additional days may be required if conditions are unfavorable for lesion development (a prolonged period above 80 F or very dry weather)


Ascospore density
.
There is no direct method to trap ascospores in the orchard air during a rain event and determine the density of airborne ascospores, but it is possible to indirectly obtain a reasonable prediction of the threat of infection due to ascospores in the orchard. The amount of ascosporic inoculum predicted for an orchard is referred to as potential ascospore dose (PAD), which is the predicted production of ascospores per square meter (about 1 square yard) of orchard floor during the primary scab season. The most important input to determine PAD is an assessment of foliar scab in autumn, and a procedure for making this assessment is presented below (See Scab Management Strategies: Predicting next year's scab potential). A general idea of the percentage of the season's ascospores that may be matured and ready for discharge when it rains can be obtained using the procedure described below and referring to Figure 1.

The rate at which ascospores mature in the pseudothecial population in an orchard is determined mainly by temperature, and a model (Figure 1) has been developed that relates temperature to maturation. The temperature is expressed as degree-days (DD), and the model allows you to predict the percentage of the season's ascospores that have matured based on the accumulated DD. A maximum/minimum thermometer, available in most hardware stores, is all that is needed to determine DD. Alternatively, farm-specific DD accumulation reports are available as a commercial weather service, and values for your general area may be published in Extension newsletters. Each day beginning at silver tip/green tip, the maximum and minimum temperatures are added together and then averaged. The number of degrees the average temperature is above 32 F (referred to as degree-days) is then determined according to the equation:

DD = [(max T + min T) / 2] - 32

Figure 1. A model for tracking the maturation of ascospores during the primary scab season.

For example, if the low and high temperatures identified on a maximum and minimum thermometer on May 5 were 45 and 65, respectively, the DD calculated for May 5 would be (110/2) - 32 = 55 - 32 = 23.

The accumulated daily DD is used to predict the percentage of the season's ascospores that have matured (Figure 1). The two outermost curves identify the variation from the predicted maturity of ascospores that can be expected 90% of the time. For example, at 300 DD, the model predicts that approximately 18% of the season's ascospores will have matured. What you should be aware of is that although 18% is the best prediction of ascospore maturity at 300 DD, 90% of the time the computed percentage of matured ascospores would be expected to fall anywhere between 4% and 48%.

What is the most practical way to use this information? You should think of the ascospore maturation curve as consisting of three phases:

A lag phase, extending from 1 DD to approximately 300 DD, when approximately 20% of the season's ascospores mature. Ascospores mature slowly because of cool weather: the daily average DD is only 15. Thus, matured ascospores do not accumulate rapidly in the pseudothecia between rain events. For example, during a 5-day dry period, only about 5% of the season's ascospores would mature. This phase usually ends about 20 days after bud break.

An accelerated phase, extending from approximately 300 DD to 700 DD. This phase occurs in warmer weather, when the daily average DD is 22. A high percentage of the season's ascospores accumulate rapidly during dry weather. For example, during a 5-day period, about 18% of the season's ascospores would mature. This phase usually lasts about 18 days.

A final phase, extending from approximately 700 DD to 800 DD. The daily DD accumulation averages 25. This phase lasts only about 4 days, and the first daytime rain after 760 DD usually signals the end of the primary scab season. At this time, approximately 95% of the ascospores have matured and discharged.

Decomposition of the leaf litter, the continued growth of groundcover that will intercept some of the discharged ascospores before they reach the open orchard air, the buildup of microorganisms on the leaf litter that injure pseudothecia or somehow interfere with ascospore discharge, and, perhaps, deterioration of pseudothecia associated with aging combine to render the remaining ascospores in a well-managed commercial orchard insignificant for management purposes.

Relationship between the model and the amount of susceptible tissue. The slow buildup of matured ascospores during the lag phase plus the limited amount of exposed green tissue make this phase relatively easy to control with fungicides. Because of lower infection risk and typically slower infection progress due to lower temperatures, the lag phase provides the best situation for scheduling fungicides according to a "postinfection" strategy. In orchards that were wellmanaged for scab last year, fungicide may not be necessary during this phase.

The accelerated phase usually lasts only a little more than two weeks, but it identifies a period in which the crop is potentially at high risk if proper control is not exercised. This is because there is much more susceptible tissue present (it often occurs during pink and bloom) and the percentage of the season's ascospores that are matured and ready for discharge builds up rapidly. It is not important to know precisely that 28% or 63% of the ascospores have matured: what is important is to know that you are in the accelerated phase. A protectant schedule is recommended if scab was not managed well last year.

The final phase is significant in that it identifies the end of the primary scab season. If thorough inspection finds that scab lesions do not develop in the two weeks following the end of this phase (but see footnote c under Table 4 for exceptions), then it would appear that primary scab has been controlled. In this case, fungicides should be selected and timed to control other diseases such as sooty blotch and flyspeck.

Relationship between cultivar susceptibility and risk assessment. Apple cultivars differ in susceptibility to scab (see Table 3). Fungicide label specifications are based on controlling scab on highly susceptible cultivars. What this means is that it may be possible to adjust the fungicide schedule or fungicide dose in a block planted with a more resistant cultivar, e.g. Empire.

Relationship between the length of an infection period and the amount of infection. The percentage of ascospores deposited on the leaves and fruits that infect depends on how long the leaves and fruits remain wet after the minimum hours of wetness needed for infection have been met (Table 4). That is, the longer the leaves and fruits remain wet, the greater the likelihood that a spore will infect. Thus, what must be kept in mind when referring to Table 4 is that the number of lesions that will actually develop at each infection level is determined by the number of ascospores that have been deposited: a long wet period does not necessarily mean there will be severe infection. For example, at 50 F and 30 hours of leaf wetness (high infection level), fewer lesions may develop in an orchard with low ascosporic inoculum than in an orchard with a high ascosporic inoculum in which the leaves remained wet for 12 hours (low infection level).


SUMMARY

Table 4 will be most useful in reaching a decision to apply fungicide if it is used in conjunction with other information: the ascospore maturation curve (Figure 1), a prediction of the level of ascosporic inoculum, tree growth stage, and a consideration of cultivar susceptibility (Table 3).


DETERMINING AN INFECTION PERIOD AFTER SCAB LESIONS DEVELOP

Count the hours of leaf wetness from the first hour rain is recorded until the leaves are dry, regardless of the time of day rain began, and refer to Table 4.


DETERMINING AN INFECTION PERIOD WHEN TWO RAIN EVENTS ARE SEPARATED BY A SHORT DRY PERIOD

There is no one rule strongly supported by research that is best for deciding if two leaf wetness periods separated by a short dry period should be considered as separate or combined. For that reason, one apple production guide may state that two rain events should be considered one continuous wetness (infection) period unless the two wetness periods are separated by at least 12 hours of dry weather (i.e., relative humidity below 90%). In another apple guide the minimum hours of dryness may be as few as four hours or as many as 32 hours. The following rules appear to best account for research on "split wetting periods":

Go to Apple Scab Management


1996-1997 New England Apple Pest Management Guide