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:

1) Depend on captan; pray for hot, dry weather.  If the fungicide resistance status of the orchard is uncertain, then the best defense against fruit scab will be to apply the full label rate of captan on a 7 to 14 day interval (depending on weather) until terminal buds are set or until hot weather intervenes to slow scab development.  Several days with maximum temperatures above 85º F will reduce viability of scab conidia produced in new lesions.  Hot weather also seems to increase the effectiveness of captan.  During cool wet summers, protection with captan will need to be maintained throughout summer and to within two weeks of harvest.  Lower rates of captan and 14-day spray intervals should suffice during July and August, but coverage will need to be renewed at shorter intervals if rain removes fungicide residues. 


2) Sovran and Flint can help because they reduce sporulation in lesions that are visible when sprays are applied.  However, many growers have found that Sovran and Flint used alone are not satisfactory for stopping well-established scab epidemics.  Therefore, I recommend that Sovran and Flint should always be used in combinations with the full rate of captan if they are applied in orchards with visible scab lesions.  Using Sovran and Flint in combination with captan where scab lesions are already present will also limit selection pressure for resistance to Sovran and Flint. 


3) Syllit could be an option in orchards where it is still effective.  Syllit is very effective for shutting down scab epidemics in the absence of dodine resistance.  However, using Syllit alone in orchards where there is dodine resistance could result in complete crop loss.  Therefore, even where Syllit resistance is not suspected, Syllit should be used in combination with at least 3 lb/A of mancozeb or Captan 50W (or equivalent).  For effective pre-symptom and anti-sporulant activity, Syllit 400F must be used at a minimum rate of 12 fl oz/100 gal (or 36 fl oz/A for medium-sized trees).


4) Beware of late-summer under-leaf scab and the potential for late-season fruit infections.  Sovran, Flint, and Captan can protect new leaves and foliage during summer, but they usually will not completely eradicate scab from existing lesions.  Some of these old lesions can become active again in late summer or fall.  Conidia from older scab lesions can infect the undersides of leaves in late summer.  If the harvest season is exceptionally wet, then inoculum from these late-summer under-leaf infections can contribute to fruit infections that appear as pinpoint scab or storage scab.  If scab is evident on the undersides of leaves in early September, then an additional fungicide spray may be needed to protect fruit against pinpoint scab.



All of the options noted above for controlling secondary scab will prove extremely expensive as compared to adding one or two sprays of protectant fungicide during the pre-bloom period. Thus, in this era of fungicide resistance and failing fungicides, the importance of controlling primary scab cannot be overemphasized.





Acknowledgements:
The concepts and guidelines presented in this document were derived from discussions and collaborative work with Dr. Wolfram Koeller and Dr. Bill Turechek, both from the Department of Plant Pathology at the N.Y. State Agricultural Experiment Station in Geneva.