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DISEASES


Apple Scab



Apple scab is the most consistently serious disease of apples in Kentucky. This disease has the potential to cause serious economic losses every year, and should, therefore, be monitored closely.

SYMPTOMS: Scab infections are most prominent on the leaves and the fruit. On the leaves, early infections appear velvety, olive-green and become puckered with time. Severely diseased leaves may become distorted or turn yellow and fall from the tree. Spots initially appear on the underside of leaves, so be sure to look there; later, spots also appear on the upper leaf surface, where they are somewhat easier to detect.


Apple Scab


Fruit spots resemble leaf spots when young, but eventually become brown or black, develop a corky ("scabby") appearance, and frequently become cracked. Infections may be more prevalent near the blossom end.

DISEASE CYCLE: The scab fungus overwinters in diseased leaf litter on the ground. In spring, the fungus produces windblown spores, which can blow to susceptible apple tissue and cause an initial (primary) infection, given the proper temperature and moisture conditions. If primary infection occurs, new spores are produced; this is the velvety growth on leaf and fruit spots. These new spores can then be splashed by raindrops to nearby tissues and cause repeating (secondary) infections the rest of the season when the weather is conducive. The cycle is completed when diseased leaves eventually fall to the ground.

As can be seen from the accompanying Mill's table (page 27), infection and incubation periods are predictable, being based upon average temperatures and the number of hours that leaf surfaces remain wet. A knowledge of recent weather, consequently, will indicate when new scab infections might be observed. Also, note that secondary infections cannot develop unless primary infections occur first, during the early part of the season.

SCOUTING: Lesions (spots) can first appear about 10-14 days after bud break. Early infections are most likely to be noticed on the flower bud leaves (sepals). Recall also that the undersurface of leaves may become spotted before the top surface. Examine 20 leaves on each of 5 limbs per tree and record the number of leaves showing any scab lesions. After fruit has set, but in addition to leaf observations, also examine 20 fruit on each tree showing any scab lesions. Use this information to better manage scab in the future.

MILLS' TABLE

Approximate number of hours of wetting required for primary apple scab infection at different air temperatures and the length of time required for secondary spore development.

Avg. Temp FDegree of Infection (hrs)aDays Incubationb
LightModerateHeavy
78 13 17 26 -
77 11 14 21 -
76 9.5 12 19 -
63-75 9 12 18 9
62 9 12 19 10
61 9 13 20 10
60 9.5 13 20 11
59 10 13 21 12
58 10 14 21 12
57 10 14 22 13
56 11 15 22 13
55 11 16 24 14
54 11.5 16 24 14
53 12 17 25 15
52 12 18 26 15
51 13 18 27 16
50 14 19 29 16
49 14.5 20 30 17
48 15 20 30 17
47 17 23 35 -
46 19 25 38 -
45 20 27 41 -
44 22 30 45 -
43 25 34 51 -
42 30 40 60 -


a The infection period is considered to start at the beginning of rain.
b Approx. no days required for secondary spore development after primary infection.
Data are incomplete at low temperatures.
From W. D. Mills, Cornell University.

Disease prediction, computer programs and weather monitoring instruments, when placed in the orchard, use Mills Table to compute the disease prediction. Such instruments offer the grower and IPM scout a precise means of managing apple scab diseases. The advancement of new fungicides capable of "eradicating" already progressing infections up to 4 days after a wetting event makes apple scab management using predictive instruments a practical activity.



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