Fruitful Land

Apple Scab (Venturia ineaequalis) is the most common fungal disease in apples, It infects the fruit, leaves, petioles, blossoms, and fruit, but the most commom and damaging areas are the leaves and fruit. In order to control Apple scab you must understand the live cycle and conditions that favor infections. You do not need to become an expert, but a little time spent learning the facts can save your crops year after year.

Apple scab fungus spore overwinter on dead leaves on the ground. When moisture (leaf wetness) and temperatures are right, the leaves release spore that reinfect trees as they grow in the spring.

HOMEOWNER TIP: By simply raking and mulching your apple leaves in the Fall, or by mowing the leaf litter to smitherines around your apple trees (including any flowering crab apples) you can reduce the innoculum of scab spores  in your trees. This is not a 100% control, but it is definitely helpful.

There are two seasons for Apple Scab infection, the first is called PRIMARY SCAB and the other is called SECONDARY APPLE SCAB.

PRIMARY SCAB is the first infection(s) that are caused by overwintering ascospores on last year's leaves. This occurs in early spring after the new leaves are 1/4"-1/2" until all the ascospores reach maturity and release their spores. This season usually lasts through petal fall till the apples are about 10 mm in diameter. The actual severoty and timing primary applescab infection can vary significantly from year to year because the release of spores and susceptibility of the new growth is determines by leaf wetness and temperatures.

Detailed  Information available here

SECONDARY SCAB: Secondary scab infections come from Primary Scab Leasions that come to maturity and release new generation of spores to re-infect leaves and fruit. As with primary scab the frequency and severity of infections is driven by leaf wetness and temperatures. the most important thing to know about Secondary Scab is that if you eliminate any primary infections, you will not have to be concerned about the secondary scab because there are no living scab lesions to release spore after all the primary scab ascospores have  matured. Thus you need to scout (observe with knowledge and detail) your apple trees in mid through late June to determine whether you have controlled Primary Scab. If scab is present, you will need to follow some  scab disease control practice thrughout the summer until harvest to prevent destructive infections, If you find no lesions on the fruit or foilage by the end of June you can be fairly certain that you have no scab for that season and you do not need to do further control measures.

SYMPTOMS - Both Primary Scab and Secondary Scab infections are infections of the same disease. No matter when they occur, they produce the same symptom

LEAF LESIONS:  Leaf lesions start out as small olive green somewhat fuzzy spots. As they grow, they become darker. As infections multiply they become larger and more numerous. Severe infections can cause significant leaf crop thus weakening the tree and contributing to crop falure.

FRUIT LESIONS: Fruit lesions resemble leaf lesions but become brown to black and corky in texture. If left untreated these lesions will grow and crack. If infected early in the season, apple scab can severely deform the fruit. Some fungicide treatments will kill growing lesions, these lesions will turn brown and corky. If the lesion develops a black ring around the edge, it is a sign that the scab has become active again.

MANAGEMENT

Apple Scab is primarily controlled with fungicides applied from green tip through mid summer. Contol of leaf litter as mentioned above is always helpful. You can also plant Scab Resistent Varitiey and avoid the issue altogether. These varietes are usually good, but they may not be the commonly marketed varities you have come to know and love. the choice is yours.

Note:  Here are some common vareties according to susceptibility

High Susceptibility: Cortland. McIntosh, Paula Red, Crispin (Mutsu)
Moderate Susepttibility: Red Delicious,Empire, Gala, Golden Delicious, Ida Red, Spartan
Resistent: Goldrush, Enterprise, Florina, Liberty, Jonafree, Macfree, Novamac, Nova,Easy-gro,Prima, Priscilla, Sundance, Scarlet O'Hara, Williams Pride

Be sure to confirm that these resistant varieties are cold tolerant and suitable to your geographic area. Yes, I know its a bit of homework, but it is alot less work than a freeze uot failure 4-5 yearsafter planting.

Discuss: Ask you questions, give us your solutions, send us your pictures on our forum.

Find More Info: View and download selected fact sheets, and Apple Growing Ebooks

Recommended Refer