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Late Season Apple Scab in 2009

For many Ontario apple growers 2009 has been a very challenging year with high levels of inoculum, heavy rains, and frequent rainfall resulting in apple scab infections on fruit in many orchards.

As fruit matures, it becomes increasingly resistant to scab. However, infection can occur at any time up until harvest during prolonged wet weather. Late season infections may not become visible until the apples are in storage. They appear as either pin-point sized lesions, or jet black spots. Fortunately, scab will not spread from diseased to healthy apples in storage.

Growers who have scab in their orchard should be aware the there is potential for developing pinpoint scab or storage scab on fruit. In orchards with late season scab, it is likely that more infections will develop on the undersides of leaves during September as fungicide residues are depleted. These infections can spread to fruit prior to harvest as well as increase the overwintering scab populations.

The best way of preventing under-leaf scab infections is by continuing fungicides into early September. Pinpoint scab can develop in orchards where there is abundant scab pressure, and where a wetting period of more than 24 for 48 hours occurs after the fungicide residues on the fruit are fully depleted during the pre-harvest interval.

Keeping a tight fungicide program into the fall can help reduce pin point scab, and reduce inoculum into the fall. Do not use eradicant fungicides as protectants for pin-point scab, and observe pre-harvest intervals for all late-season sprays.

Another option for growers looking at reducing inoculum for next spring is the use of fall or spring urea sprays. In research trials conducted by Dr. Bill MacHardy's lab at the University of New Hampshire, urea (45 kg of agricultural urea per 1000 L of water/ha) was applied to the orchard floor after about 95% of the leaves had fallen (November) or in the spring (April) before bud break.

The urea works in two ways: it directly inhibits the development of ascospores and it stimulates the growth of naturally occurring organisms that are antagonistic against V. inaequalis. Both treatments reduced the number of ascospores, but the spring treatment was more effective and resulted in fewer leaf and fruit infections in most years. In the years when it didn't work, the snow cover remained almost until bud break so there was not a lot of time for the urea to work.

Shredding the leaf litter in November or April will also reduce the risk of scab by 80 to 90% if all of the leaf litter is shredded. If 10 to 35% of the leaf litter cannot be shredded because of the limited offset of the flail mower and spread of the tree canopy, then the risk of scab is reduced by 50 to 65%.

In summary for growers with late season apple scab, continuing fungicides programs into September is important to avoid pinpoint scab at harvest and reduce inoculum for next year. Use urea ground sprays in fall or spring to reduce inoculum in orchards with high scab pressure. Growers with orchards with high levels of scab in 2009, should be very conservative with their fungicide programs in 2010, and always apply fungicides preventatively.