Scab Management Fungicide Programs

At this time of year apple growers are starting to think and implement their early season apple scab programs. Apple scab is an unforgiving pest of apples and can result in significant crop loss if left unmanaged. Here are a few tips on how to manage apple scab effectively.

Tips on managing apple scab effectively

1. Fungicides should be applied throughout the pre-bloom stage when rains are predicted and the residue from the earlier sprays is not considered to be adequate. Sprays should be applied prior to a rain event. Remember that early in the season the leaves are expanding quickly and fungicide residues may not last as long. EBDC fungicides and Scala are generally considered to have a 5-7 day residues at this time.
   
2. Captan generally provides better scab control than mancozeb; however, captan's usage is limited by its higher pricing, its incompatibility with oil sprays and its lack of activity against rust diseases.
   
3. Excellent scab control can be achieved by using only mancozeb or captan sprays if the fungicides are applied just ahead of predicted rains. Higher rates are needed in very large and/or poorly pruned trees or when fungicides are applied on a weekly schedule irrespective of rain events.
   
4. Programs involving only protectant fungicides (e.g., copper, mancozeb, Polyram, captan) can be inexpensive, especially in dry years, but they are unforgiving because they offer no post-infection or anti-sporulant activity. As a result, lapses in spray coverage will almost always result in at least a few scabby fruit. Coverage lapses may occur due to stretched spray intervals, wash-off during heavy rains, or spraying in windy conditions. Furthermore, if summer weather remains cool and wet, any scab that becomes established during April, May or early June will continue to spread during summer and will increase the potential for late-season fruit infections that develop into pinpoint scab during storage. In wet years, attempting to control scab with only contact fungicides can be both frustrating and expensive.
   
5. Growers planning to use only contact fungicides through the entire scab season will need to be more conservative in covering ahead of predicted infection periods than growers who plan to use SI or strobilurin fungicides during the peak scab season. Those planning to switch to SI's or strobilurins can afford more risk because the SI and strobilurin fungicides will cover minor lapses in coverage that may occur with prebloom applications of contact fungicides.
   
6. Scala is systemic within the tissue that is sprayed so leaves that are exposed will be protected as they expand. However, it does not redistribute to unsprayed leaves.
   
7. Scala belongs to a group of fungicides that is prone to resistance development so combinations of dithane and Scala are recommended.
   
8. Scala usually provides similar control to mancozeb; however, Scala provides 48-60 hr of post infection activity against apple scab (counting from the start of the wetting period) while mancozeb provides only 18-36 hr of kickback activity counting from the start of the wetting period, with the longer duration limited to colder infection periods Table 7-14 page 124 of publication 360.
   
9. The key to minimizing scab control costs is knowing exactly when to switch from a mancozeb or captan program to the extra protection provided by SI's or strobilurins. In a dry year when contact fungicides can easily be applied ahead of infection periods, contact fungicides alone may suffice for all sprays up to petal fall, especially in blocks where mildew is a minimal problem. However, in years with heavy rains and extended wetting periods, using SI's or strobilurins beginning at tight cluster may be the most cost-effective approach.
   
10. Do not start an SI or strobilurin program prior to tight cluster.
    
11. The application of SI or strobilurin fungicides in combination with captan or mancozeb at petal fall and first cover will help suppress powdery mildew and provide post infection activity against any scab infections that may have slipped through during the prebloom and bloom sprays. SI's will also suppress rust infections. Using 2 applications of SI's or strobilurins after bloom help minimize selection pressure for resistant scab while maximizing the benefits that these fungicides provide for disease management programs. For more information refer to page 125 of publication 360.
    
12. Although fungicides programs can be reduced after the end of primary infection period. It is important that orchards are clean of scab before moving to a reduced fungicide program. Under some environmental conditions (ie. dry springs) ascospore discharge may be delayed, resulting in scab lesions appearing in orchards later then expected. Careful inspection of the leaves for up to 2 weeks after the primary infection period has ended, is necessary to determine if the orchard is scab free.
    
13. Apple scab should always be managed preventatively. Attempting to control apple scab after lesions are present in the orchard can result in the development of fungicide resistance. Protective fungicides have limited or no kickback activity, but even if they are applied too late for an infection…they will protect against the subsequent infections. A fungicide with kickback is required to stop the development of scab lesions after infections have taken place. Use a protectant spray timing even when SI's or strobilurin fungicides are included in the schedule.
    
14. It is also critical that post-infection fungicides (SI's, stobilurins, Vangard/Scala) not be applied once sporulating lesions are found in the orchard as this will rapidly promote the development of resistance.
    
15. If you have had problems with black rot in your orchard be aware that SI fungicides do not provide control of Black rot. The best options for managing black rot is the application of Flint, Sovran, or a full rate application of Captan starting at pink.

Although there has been a lot of buzz about resistance in apple scab to SI fungicides in other regions, currently there are no documented cases of SI resistance here in Ontario. A survey of orchards in 2000-2001 by Dr. Wendy McFadden-Smith did not find significant shifts in resistance in apple scab. Furthermore, no reports of fungicide failures have been reported so for the time being we are assuming that this isn't an issue here in Ontario. This does not mean that we should be complacent about the spectre of fungicide resistance. Growers should always use resistance management programs to reduce the chance of developing fungicide resistance.

Apple Scab Fungicide Resistance Management Strategy:

• Rotate between chemistries in a fungicide program
  
• Avoid the application of more than 4 applications of Sterol Inhibitors (SI's) or strobilurins/season. 2 applications/season of each is preferable.
  
• Always use full rates of SI fungicides with ½ rate of protectant fungicide.
  
• Don't miss fungicide sprays at green tip. Be prepared to spray even in the rain if necessary (with Supra Captan, Maestro, Dikar, Manzate, Dithane, Penncozeb, Polyram), but keep in mind that you will lose your residual. Don't spray Nova, Sovran, Flint, or Vangard or Scala in the rain.
  
• Always apply fungicides preventatively (before infection) not curatively (after infection). The curative application of fungicides may select for fungicide- resistant populations.
  
• Always use full label rates of fungicides. Reducing the rates of fungicides may increase the number of resistant individuals present in an orchard.
  
• Complete coverage is imperative. Don't spray alternate rows.
  
• Read label to determine if product can be applied using tree row volume applications.
  
• Don't extend your spray intervals beyond those listed on the label. Shorten spray intervals in case of rain.
  
• Urea sprays may be applied to leaves on the orchard floor in November and/or the spring to increase leaf decomposition and reduce the apple scab inoculum the following spring. Flail mowing has also been shown to reduce inoculum levels.

Learn more

• An Apple Scab Primer For Primary Infections