Apple Scab

By , On , In Best Practices

We are well into the growing season throughout the Okanagan. We’ve seen a cooler Spring this year, with less growing degree days year to date. We’ve had many questions circulating around the topic of apple scab, what is it, how to control it and what products to use to ensure you’re fully protected so we’d like to address some important topics in this blog. 

Firstly, let’s look at what apple scab is. Apple scab is a relatively common disease that occurs in apples. It is caused by the fungus Venturia inaequali. Apple scab spreads easily from tree to tree, so it’s important to apply protectants against it and to limit the spread as much as possible. Scab is an annual threat and affects wetter regions more often. It can occur in drier regions during years of heavier rainfall.  

Apple scab symptoms: brown spots on the leaf
Apple scab symptoms: scabs/lesions on the fruit

What are the symptoms?

Scab is, luckily, relatively easy to identify in your apple trees and there are a few different symptoms that you can look out for: 

  • Spots on leaves that are circular or irregular in shape and are brown or light/olive green in colour. They are identifiable by their irregular shapes and feathered edges. These start to appear early in the Spring. 
  • Dark brown to black leaf spots will form, as the fruit ages. Covered leaves often turn yellow and fall off in the Summer. 
  • Scabs/lesions on the fruit are circular and are brown or black in colour.  
  • Cracked or raptured skin forms around the lesions. When cracked, they reveal a dark layer of spores. Older lesions appear “corky” in texture. 
  • Cracked and deformed fruit occurs if the fruit was infected very young. 

Apple scab should be carefully monitored and its entire life cycle, from primary infection to secondary infection, all the way to reinfection should be well understood. This equips you with the best knowledge on how to handle and prevent it. The entire life cycle is explained in the BC Tree Fruit production guide.

What can I do to treat apple scab?

Curing apple scab is difficult and preventative measure are always better. Here’s a quick list of control measures you can take, to ensure your trees are protected against scab: 

  1. Choose a resistant apple variety where possible, although this is not always easy. 
  2. Pruning and tree design. This is crucial in managing apple scab. Pruning results in better air movement and flow through the canopy, provides better spray coverage and offers better light, meaning that the foliage dries quicker. This helps prevent both initial and reinfection.  
  3. Irrigation management. Irrigate in the early hours and don’t over wet. Dry leaves will not host the fungus. Monitor rainfall and wet periods and ensure that your irrigation systems are shut off during these times. 
  4. Clean up and rake. If you had a scab infection resulting in leaves or fruit symptoms the previous year, raking and mowing leaves in the fall after harvest might help in reducing the amount of inoculum at the beginning of the following year. This will not, however, eliminate the need to protect against the disease year to year. It simply makes it easier to control. 
  5. Sign up for a free BC-DAS account, to assist you with tracking infection periods. 

Cultural Control methods, although important, will not eliminate apple scab completely. Spraying your apples with protectants from as early as green tip is important in preventing infections. 

  • Spray protectant fungicides on emerging leaves and developing fruit to prevent scab. Sprays aim to protect trees during wetter periods. These should be sprayed annually, at the same time. 
  • Spray eradicant fungicides to prevent or reduce spreading existing infections. These are applied when needed and make sure you rotate between fungicide FRAC groups to avoid resistance developing. 
  • All sprays should be used correctly, in the correct quantities, with the correct tank mixes at label rate. 
  • It is better to rely on protectant sprays, rather than eradicant sprays, which should only be used when needed. For spray scheduling, applications and timing; refer to the BC tree Fruit production guide here. and here. 

Apple scab, although serious and easily spread, is relatively easy to prevent and control. We’ve selected some apple scab protectant sprays this month (Penncozeb, Captan, Dithane and Manzate) to get you started but we’re here to help. For any assistance relating to apple scab, product selection and product use please reach out to your Growers Supply Co. grower support team.