How to Control Apple Scab (Continued from page 5)
alone or 6 lb/A of Captan 50W alone are acceptable alternatives, but both alternatives have disadvantages. Using the higher rate of mancozeb during pre-bloom triggers a label restriction against any mancozeb use after bloom, and mancozeb may be needed after bloom as a substitute for captan if insecticide+oil sprays are planned for early summer. The higher rate of captan alone is effective, but captan usually is more expensive than mancozeb.
3) Use Scala or Vangard to work around pre-bloom oil sprays or when 48-hr post-infection activity is essential. Both of these fungicides work best in cool weather. They have the advantage of providing 48 hours of post-infection activity, but as protectants they are no more effective than the less expensive mancozeb fungicides.
4) Consider Flint or Sovran at tight cluster and pink or at pink and bloom, but keep the spray interval at 7 days. These fungicides often give slightly better control of scab than can be achieved with mancozeb or captan sprays. Sovran and Flint can be applied alone; tank mixing with captan or mancozeb has not improved control in field trials. If oil sprays are applied at tight cluster, then Sovran or Flint can be used as a substitute for Captan or mancozeb+Captan when the oil is applied. Using Sovran and Flint at pink and bloom provides two benefits: they will provide protection against early powdery mildew infections, and they will suppress sporulation of any primary scab lesions that may have become established at green tip, thereby slowing secondary spread of scab during the period around bloom and petal fall and fruitlets and early terminal leaves are at peak susceptibility. Note, however, that Sovran and Flint will not completely arrest development of primary scab lesions in the way that DMI fungicides did, so using Sovran or Flint at pink and bloom is not an acceptable substitute for a green tip spray. In an "easy" scab year, when protectant scab fungicides were in place ahead of all pre-bloom infection periods, it may be more cost-effective to continue with a mancozeb+captan program until petal fall rather than using Flint or Sovran during bloom.
5) Use DMI-captan or DMI-mancozeb sprays at petal fall and first cover. The DMI fungicides still provide the best available mildew control, and using them at petal fall and first cover optimizes their usefulness against mildew and against cedar apple rust infections on terminal leaves.
Managing Secondary Scab:
What course of action is recommended if primary scab lesions start appearing on leaves? This is a difficult question because of all the variables that must be considered. For example, a little bit of scab showing up on late terminal leaves in the latter half of June poses less of a threat than a little bit of scab showing up at petal fall, because by late June fruit will be more resistant to infection than they are at petal fall. Similarly, a little bit of scab on Empire, Honeycrisp, or Red Delicious is of less concern that a similar scenario for scab susceptible cultivars such as McIntosh, Ginger Gold, or Silken. Finding scab on leaves just ahead of a predicted heat wave is less threatening than finding scab just before a week of cool wet weather. Finally, the fungicide resistance status of the orchard must be considered when deciding what to do. Following are a few general principles: