Apple Scab and Cold Weather

Has there really been an infection? It’s too cold, right? There has been a lot of discussion lately around the risk of apple scab infection during cold weather. As if there isn’t enough to worry about already with freezing temperatures, unfortunately, this does not kill scab spores. Spores will continue to mature and release so long as there is a wetting period long enough for infection to occur, says Dr. Kari Peter (Penn State Fruit Research and Extension Center) and Dr. David Rosenberger (Cornell University Hudson Valley Lab). While mature spores are released more during warm rains than in colder rains, they can still cause infection nonetheless if they land on green tissue.

Many regions in the province are beginning to show green tip, which means a protectant spray program should begin to keep growing tissue covered. This is particularly important in orchards with historically high scab pressure. Keep in mind, the level of infection risk for this season is dependent on the overwintering inoculum from last year. If you were chasing scab in your orchard and did not do any sanitation practices (urea spray, leaf mulching) in the fall, the inoculum levels are likely high enough that mature spores could be released with these early wetting events.

The long-term forecast seems to suggest warmer and dryer weather is on its way, which could make for a good spray window to apply fungicides before the next rain. As growing degree-days continue to accumulate and temperatures become warmer, the rate of ascospore maturity will increase. This could result in large amounts of spores being released during infection periods and those without adequate fungicide protection could find themselves in a bad situation.

The following are some key points to consider for effective scab control this time of the year:

  • As there is still the risk for potentially freezing temperatures, EBDC fungicides (Manzate, Penncozeb, Dithane, Polyram) should be used. In his recent blog article Adding Insult to Injury: Scab Warning, Rosenberger warns that applying captan, Syllit, copper or oil to cold injured leaves could make the injury worse due to uptake of these products into the tissue.
  • Like other protectant fungicides, EBDCs are contact fungicides and do not provide effective post-infection or anti-sporulant activity. This means if sprays are applied in less than ideal conditions (ie., windy, alternate rows, or washed off in rain), the risk of scab infection is increased.
  • In prolonged wetting events, it is better to re-apply fungicides during a break in the rain to provide temporary protection, than to not spray at all. Apply again once conditions are finally dry to replace residues washed off by rain.
  • If there is concern about coverage during a potential infection period, combining an EBDC with a product with good kick-back, such as Scala should provide post-infection activity up to 72 hours in cooler temperatures from the start of the infection period. Refer to Table 3-13. Characteristics of Apple Scab Fungicides in the new 2016-2017 Publication 360, Guide to Fruit Production for details on pre- and post-infection activity, retention and redistribution properties of registered scab fungicides.
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