Online Publications

PPA-24

Apple Scab

Apple scab is the most consistently serious disease of apples and flowering crabapples in Kentucky. The most noticeable losses result from the reduced quality or premature dropping of infected fruit, but the disease also causes a general weakening of the tree when infected leaves are shed prematurely. For flowering crabapples, summer defoliation from scab invariably results in fewer flowers the next spring. The best control of scab is provided by use of scab-immune cultivars. A good spray program also can provide excellent control of apple scab, but failure to follow such a program can lead to substantial losses on susceptible varieties, especially in a wet year. The apple and flowering crabapple scab fungus also attacks hawthorn and mountain ash; closely related fungi cause scab diseases of pear and pyracantha.

Symptoms

Leaves

Indefinite olive-green to brown spots (lesions) appear on either surface of infected leaves, although those on the upper leaf surface may be easier to see. Spots range in size from that of a pinhead to slightly smaller than a dime. As the disease progresses, the lesions develop a more definite outline and become covered with a greenish black, velvety growth (Figure 1). Eventually, tissue near the lesions thickens and bulges upward. If heavily infected, leaves may become disfigured and/or turn yellow and drop prematurely.

Figure 1.—Mature, well-outlined apple scab leaf lesions, covered with a dark, velvety growth.

Fruit

Symptoms on the fruit are similar to those on the leaves, although on fruit the spots may be more distinctly outlined. Older lesions turn dark brown to black, develop a corky (“scabby”) appearance, and frequently become cracked as the fruit enlarges (Figure 2). Uneven growth in the vicinity of the “scabs” can cause fruit to become severely deformed if infections occur while the fruit is still small. Heavily infected fruit may drop prematurely.

Figure 2.—Typical, cork-like apple scab fruit lesions.

Disease Cycle

Table 1. Approximate Number of Hours of Continuous Wetness Required for Primary (Ascospore) Apple Scab Infection at Different Air Temperatures
Average Temperature Degrees F	Degree of Infection
	Light hours1	Moderate hours	Heavy hours
78	13	17	26
77	11	14	21
76	9½	12	19
63 to 75	9	12	18
62	9	12	19
61	9	13	20
60	9½	13	20
59	10	13	21
58	10	14	21
57	10	14	22
56	11	15	22
55	11	16	24
54	11½	16	24
53	12	17	25
52	12	18	26
51	13	18	27
50	14	19	29
49	14½	20	30
48	15	20	30
47	17	23	35
46	19	25	38
45	20	27	41
44	22	30	45
43	25	34	51
42	30	40	60
1The infection period is considered to start at the beginning of the rain.

If primary infection occurs, thousands of secondary spores (conidia) are soon formed in established scab spots. How soon these spores appear after primary infection is also dependent on the weather (Table 2). Once formed, conidia can be washed or splashed to adjacent fruit and leaves. Here they are capable of causing secondary infections after wet periods approximately three hours shorter than required for ascospore (primary) infection. Thus, if the average temperature was 63 to 75 degrees F (see Table 1), then light, moderate, and heavy degrees of secondary scab infection would require wetting periods of six, nine, and 15 hours, respectively. Additional infective conidia are then produced throughout the season each time a new lesion develops. Conidia are thus able to function as “repeating spores,” multiplying a few initial primary infections into hundreds or thousands of secondary infections as the year progresses.

Table 2. Time Required for Production of Secondary Scab Inoculum at Different Temperatures
Temperature	Days Required for Conidia Development Following Primary Infection
Degrees F	Days
30-40	18
41-45	16
46-50	14
51-55	13
56-60	12
61-65	10
66-70	08
71-75	07

Control

1. A regular fungicide spray program is the principal means of controlling scab on most apple varieties. (See UK Extension publication ID-92, “Commercial Tree Fruit Spray Guide,” or ID-21, “Disease and Insect Control Program for Home Grown Fruit in Kentucky” for specific recommendations.) Remember from the preceding discussion of the disease cycle that secondary scab infections can only occur if primary infections occur first. Also remember that primary infection can only occur from bud break until two to four weeks after petal fall. This means a strong, conscientious control program during the relatively short primary scab “season” greatly aids in control of secondary scab through the rest of the year.

   Be aware that most spray programs are designed to leave a constant, adequate spray residue on susceptible tissue before an infection period occurs. If Tables 1 and 2 indicate that an infection period occurred before you were able to apply a scheduled spray, you may still obtain control by applying a fungicide capable of eradicating new infections. Some standard fungicides (and several new ones) have this eradicative (or “kickback”) activity if applied within 18 to 72 hours after the start of an infection period, depending on the material. See ID-92 or consult your county Extension agent for specific product information.

   Growers with special weather monitoring equipment such as Envirocaster or Metos may spray apples on an “as needed” basis as determined by recognized infection periods. The weather monitors contain a computer chip which can process the weather information and, using Table 1, alert the grower electronically that an infection has occurred. Fungicides with eradicative or “kickback” activity are then used for scab control only when needed. This approach is often incorporated into integrated pest management (IPM) programs for apples.

   Another IPM approach makes use of eradicant fungicides applied at scheduled intervals during the primary scab infection period. This technique reduces the total number of spray applications required for scab control. Details are presented in Extension publication ID-92.

2. Apple varieties vary considerably in their susceptibility to apple scab. Several new varieties—Enterprise, Freedom, Gold Rush, Jonafree, Liberty, Macfree, Nova Easy, Prima, Priscilla, Redfree, Sir Prize, and William’s Pride—are highly resistant or immune to scab and are particularly well suited to homeowners wishing to avoid the expense and/or effort associated with chemical control. Among the more standard varieties, Jonathan is moderately resistant, Golden Delicious is moderately susceptible, and Red Delicious, McIntosh, Cortland, and Rome Beauty are very susceptible. A number of crabapple varieties are also highly resistant or immune to apple scab.
3. Prune trees to allow good spray penetration and air circulation, thereby promoting rapid drying of fruit and leaves. If control breaks down and defoliation occurs early in the year, fertilize and water trees to promote new bud growth. This will help reduce the weakening effects of premature leaf loss. Do not fertilize or water trees late in the growing season as this practice prevents trees from properly hardening off for the coming winter.