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Scab

Venturia inaequalis

Scab, caused by Venturia inaequalis, is one of the most important diseases of apples in the mid-Atlantic and Northeastern United States. However, it is generally not as important in apple orchards in the Southeast because the warm weather during the summer is unfavorable for secondary spread.

Symptoms

Young scab infections on leaves are characterized by velvety olive green lesions (Fig 1) that often become necrotic in their centers as they age. Early season infections frequently occur on the lower leaf surface, but lesions can be found on the upper surface as well. Extensive infections can cause early defoliation that weakens the tree and often reduces return bloom for the next years crop. Late season infections often occur on the underside terminal leaves after the final spray and before leaf fall and are often dark in appearance (Fig 2).

Fig 1 - Young leaf infections

Fig 1. Young leaf infections

Fig 2 - Late season leaf infections

Fig 2. Late season leaf infections

First infections on fruit often appear as small, dark lesions on the sepals or near the calyx end (Fig 3) . Apple scab infections do not rot the fruit but early-season infections may cause cracking and distortion as the fruit enlarge (Fig 4). Fruit become more resistant as they mature and infections occuring after mid-May in the Southeast usually result in smaller lesions (Fig 5). If infections are numerous, infected fruit may abscise.

Fig 3 - Early fruit infections

Fig 3. Early fruit infections

Fig 4 - Fruit distorted from early infections

Fig 4. Fruit distorted from
early infections

Fig 5 - Later fruit infections

Fig 5. Later fruit infections

Disease Cycle

Venturia inaequalis overwinters primarily on fallen leaves on the ground, although some conidia can overwinter in the bud scales. In the early spring, fruiting structures known as pseudothecia mature in the leaves (Fig 6), producing 2-celled spores called ascospores. The spores are discharged during rain and blow to the leaves and the developing fruit. Ascospore discharge usually starts about the time bud swell occurs and continues through petal fall. Leaf infections can occur in as few as 8 hours under favorable temperature and wetting conditions. Recently expanded leaves are more susceptible than older ones. Spores produced on these primary lesions are washed onto fruit and leaves, causing secondary infections that can continue into the early summer if the weather is cool and wet. Leaves and fruit become more resistant to infection as they mature.

Fig 6 - Pseudothecia

Fig 6. Pseudothecia

Control

The need to control apple scab is usually greater in North Carolina and Virginia than in other growing areas of the Southeast. Where scab is a problem, fungicides should be applied from green tip through petal fall or first cover. Additional applications may be needed in cool, wet seasons. Fungicide applications can be made on either a protectant or postinfection schedule.

Protectant sprays should be made on a 5- to 7-day interval beginning at silver tip or early green tip and continuing through petal fall. In most years the peak period of ascospore maturity occurs from tight cluster to bloom. However, during some years, spores mature by silver tip, and if conditions are favorable for infection, sepal infection can occur and result in misshapen fruit.

In the postinfection approach, fungicides are applied only after an infection period and rely on the back action of certain fungicides to eradicate infections once they have occurred. Fungicides used in the postinfection approach typically have 24 to 100 hours of back action, or eradicant activity. Whether or not an infection period has occurred can be determined by monitoring the temperature and hours of wetting (see Mills chart modified by Jones below). Growers using eradicant schedules need to consider other early season diseases such as black rot, powdery mildew and the rust diseases that may be a problem.

In addition to chemical control, the inoculum levels in the orchard can be lowered by reducing leaf litter by shredding leaves with a flail mower or with applications of urea made to leaves just prior to leaf fall or to leaves on the orchard floor.

More than 50 cultivars resistant to apple scab have been released worldwide; however, most are not adapted to the growing conditions in the southeastern United States.

"Mills' Chart": Temperature And Moisture Requirements For Apple Scab Infection Periods
as Determined By Mills And Modified By A.L. Jones.

Hours of Wetting Required for Infection from Primary Inoculum (Ascospores)
Avg.Temp. (°F) Light Infection Moderate Infection  Heavy Infection  Secondary Inoculum (Conidia)
78 13 17 26 8.7
77 11 14 21 7.3
76 9.5 12 19 6.3
63-75 9 12 18 5.9
62 9 12 19 5.9
61 9 13 20 5.9
60 9.5 13 20 6.3
59 10 13 21 6.6
58 10 14 21 6.6
57 10 14 22 6.6
56 11 15 22 7.3
55 11 16 24 7.3
54 11.5 16 24 7.7
53 12 17 25 7.9
52 12 18 26 7.9
51 13 18 27 8.7
50 14 19 29 9.3
49 14.5 20 30 9.7
48 15 20 30 9.9
47 15 23 35 11.3
46 16 24 37 12.6
45 17 26 40 13.3
44 19 28 43 14.6
43 21 30 47 16.5
42 23 33 50 19.9
41 26 37 53  
40 29 41 56  
39 33 45 60  
38 37 50 64  
37 41 55 68  
33-36 48 72 96  
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Format updated March 29, 2011