terminal leaves reach their period of peak susceptibility around petal fall, and the huge numbers of conidia produced by a single lesion can overwhelm protectant fungicides in a wet season.



Misconception #5: I can still rely on dodine in a pinch.  Perhaps!  In New York, however, lab tests showed high levels of dodine resistance in many orchards where growers had assumed that dodine should still be effective.  Most growers have relatively poor memories for what was actually sprayed in their orchards in the 1960s, and dodine resistance can persist a long time after it becomes established in the orchard.



Misconception #6: Switching to protectant fungicides should be an easy transition.  Perhaps!  Maintaining scab control with protectant fungicides like mancozeb and captan is relatively easy in low-inoculum orchards.  High inoculum and wet pre-bloom weather are a deadly combination, however, and the difficulties of controlling scab under those conditions should not be underestimated.  In large operations where equipment and pesticide applicators were already stretched to the limit when applying DMI fungicides, it may be necessary to buy another sprayer and hire another applicator so as to ensure that all trees can be sprayed on a weekly basis during the pre-bloom intervals.





Essentials for pre-bloom scab control in the era of fungicide resistance: 




1) In high inoculum orchards, consider applying urea before bud break to reduce ascospore production.  Applying 40 lb/A of urea to the orchard floor in late winter or early spring can reduce ascospore production by more than 75%.  In a high-inoculum orchard, that could spell the difference between effective scab control and a control failure, especially if wet weather between green tip and petal fall favors scab development and hinders fungicide application.



2) Start protectant fungicide sprays at green tip.  It is absolutely essential to have the first fungicide applied BEFORE the first apple scab infection period.  That means that prunings in the orchard must be chopped or removed well in advance of green tip, and sprayers should be up and running by silver tip.

3) Use full rates of protectant fungicides.  Remember that

1 lb of mancozeb fungicide or 1 lb of Captan 50W per 100 gal of dilute spray (3 lb/A for medium-sized trees) is actually a half-rate of fungicide that was initially recommended as a complement for Benlate, Topsin M, or DMI fungicides.  Using mancozeb fungicides at 3 lb/A on a 7-day spray interval can result in a control failure in a high-inoculum orchard.

4) Use shorter spray intervals.  Where DMI fungicides are no longer working, forget about 10-day spray intervals.  Plan on a 5 to 7 day spray interval instead.  Fungicide protection might need to be renewed after 5 days following heavy rains or to ensure protection ahead of slow-moving weather fronts that might impede spraying for several days.  If mancozeb fungicides or Captan 50W are applied at 2 lb/100 gal (6 lb/A for medium-sized trees), then residual activity should hold up through 1.5 to 2 inches of rain (other captan formulations would be equally effective when applied at similar rates of active ingredient).  If mancozeb fungicides or Captan 50W are applied at only

1 lb/100 gal., then fungicide protection will often be exhausted after only an inch of rainfall.

5) Spray in the rain if necessary to protect new foliage during infection periods that last more than 2 or 3 days.  If fungicide protection is removed by heavy rains at the beginning of a wetting period, and rains are predicted to continue for several more days, then protectant fungicides should be re-applied during the rain to protect against ascospores that will mature as the wetting period continues.  Sulfur, captan and mancozeb fungicides that are applied in the rain will provide 3-4  days of protection against scab infection.  Sovran, Flint, Vangard, Scala, Topsin M, and DMI fungicides should never be applied in the rain, however, because all of these fungicides must dry on the leaf to be fully effective.



6) Be wary of alternate row spraying on an extended interval.  Alternate row spraying often leaves a shadow of unprotected foliage on the back sides of tree trunks.  Missing a few leaves here and there was not very important when DMI fungicides applied 7-10 days later from the opposite sides of the trees could arrest scab development on the few leaves that may have become infected.  Where DMIs are no longer working, it is imperative that all leaves be protected every 7 days.  If in doubt about spray coverage, use water-sensitive paper to evaluate coverage on the back sides of trees.  Attempting to judge spray coverage based on visual analysis of the spray plume can be misleading because the spray mist that refracts the most light carries a relatively small proportion of the fungicide load.



7) Where DMI resistance is suspected, do not use any DMI sprays before petal fall.  Application of DMI+protectant sprays to a fully DMI-resistant scab population may actually stimulate scab growth and result in less scab control than would occur if a low rate of protectant fungicide were used alone.  However, even where scab is resistant to DMI fungicides, the DMIs may still be needed to control powdery mildew and rust diseases.  Delaying DMI sprays until petal fall will minimize risks of stimulating scab problems because most ascospore release will be completed by petal fall, and there should be no secondary scab inoculum if appropriate pre-bloom sprays were applied.





Suggestions for a conservative scab control program:




1) Use a copper spray or mancozeb at silver tip to green tip.  In high inoculum orchards, this could be the most critical scab spray for the entire season!  Copper is recommended for orchards with a history of fire blight.  Otherwise, mancozeb fungicides will provide the most cost-effective protection.  Neither copper nor mancozeb will provide any post-infection activity.  Note, however, that protectant fungicides (copper, mancozeb, captan, sulfur) will usually be effective as long as they are applied before a Mills period is completed.  For example, approximately 40 hours of wetting are required for light Mills infection period at 37ºF, so a protectant fungicide could be applied up to 40 hours after the start of a wetting period if the mean temperature for the wetting period was 37ºF. 



2) Consider mancozeb-captan combinations from half-inch green through tight cluster.  In high inoculum orchards, a combination of 3 lb/A of a mancozeb fungicide plus 3 lb/A of captan 50W (or the equivalent amount of another captan formulation) may be the best option.  Using 6 lb/A of mancozeb

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