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Disease Management

Chemical Control
Management of apple scab on susceptible trees is focused on the prevention of primary infection by ascospores in the spring. Early infection of trees may result in poor fruit set, and will result in more secondary inoculum being produced throughout the season. The initial fungicide sprays are therefore timed to coincide with the spring release of primary inoculum. Later sprays are often targeted at other fungal diseases, in addition to scab, but also are effective against apple scab secondary inoculum. In commercial orchards, airblast sprayers (Figure 15) are typically used to apply fungicides.


Figure 15	Figure 16

Chemical fungicides for apple scab control are generally of two types, preventive (protectant) and curative. Most growers use a combination of the two for maximum effectiveness. Preventive fungicides are applied to the leaves and fruit before infection. When ascospores or conidia are present on susceptible plant surfaces and leaf moisture and temperature are suitable, the fungicide prevents the fungal spores from germinating or penetrating the host tissue. Good spray coverage and uniform deposition are essential (Figure 16). To ensure coverage of newly emerging tissues and to replace the chemical as it is lost to weathering, fungicides must be re-applied on a regular schedule. Usually this consists of spraying every 5 to 7 days in the early part of the season, and every 7-10 days later in the season.

Curative fungicides have limited systemic activity and if applied soon after infection, are capable of penetrating the leaf and stopping further development of the fungus, thus preventing symptom development. These fungicides are especially useful when growers use a Mills table-based weather monitoring system (Figures 17, 18) that tells them when an infection has occurred. Prompt application of the fungicide will stop further disease development. Growers using these systems spray fungicides only when needed, and in a season when few infections occur (in a dry spring for example), they may apply fewer fungicide treatments than a grower spraying preventively. By the time apples are harvested in summer and fall, fruit fungicide residues are either absent or present at extremely low safe levels.


Figure 17	Figure 18

Genetic Resistance
Scab-resistant apples and crabapples are widely available. Fungicide applications to these trees are not needed for scab management, although they may be necessary to control other fungal diseases. Cultivars such as 'Enterprise', 'Freedom', 'Gold Rush', 'Jonafree', 'Liberty', 'Pristine', and 'Redfree' are gaining acceptance by both farmers and consumers (Figure 19). Varieties susceptible to scab, such as 'Delicious', 'Fuji', 'Gala', 'Golden Delicious', 'McIntosh', and 'Winesap' are still widely sold because consumers like them and because they can be grown easily in drier regions where apple scab is not a problem. There are good reasons to plant scab-resistant flowering crabapples such as 'David', 'Harvest Gold', 'Mary Potter', and 'Prairiefire'. These are beautiful landscape plants and provide greater bloom potential because there is no risk of defoliation by apple scab (Figure 20). Another advantage is that application of fungicides for scab management in the landscape will not be needed if resistant cultivars are grown.


Figure 19	Figure 20

Cultural Practices
Other controls for apple scab in addition to those mentioned above may also be useful. Sanitation can be effective (picking up and disposing of fallen leaves in the fall) but is usually not practical for large orchard operations. Applications of urea to trees just before leaf drop or to leaves on the ground in the fall, tilling the fallen leaves into the soil, or chopping them into small pieces, are all used to speed up leaf decomposition thereby decreasing the amount of ascospore inoculum the following season. Trees should be pruned regularly to enhance air movement and to allow sunlight to penetrate, which speeds up drying of leaves and fruit. Overhead irrigation should be avoided, especially when weather conditions are favorable for scab development.

Biological control, such as utilizing natural microbial antagonists of the apple scab fungus to keep it in check, may be a viable option in the future.

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