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Growth stages still range across the province from mouse ear to tight cluster in the earliest regions to silver tip in the cooler areas. With signs of green tissue, 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.
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But it’s too cold, right?
Has there really been an infection when it’s snowing? 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 University) and Dr. David Rosenberger (Cornell University, retired). 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.
More information on temperature and leaf wetness requirements for scab infection can be found in Table 3-10. Relationship of Temperature and Moisture to Apple Scab Infection of the 2020-2021 Publication 360, Crop Protection Guide for Apples or on Ontario AppleIPM.
What’s in the scab forecast?
The long-term forecast seems to suggest warmer weather is on its way. 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.
Early season control of primary scab is vital to managing infection later in the season during periods of rapid growth. As shown in the figure below, green tip infections can cause significant economic loss because the expanding tissue is highly susceptible. Early season infections will produce more secondary infection throughout the season and will be harder to keep under control. In other words, don’t risk those early infection events if scab has been active in your orchard before.
Early season scab management
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. 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.
- With heavy or prolonged rain during the cool, early spring, a protectant program may not be enough to stay covered. Consider including a post-infection product such as Scala, Inspire Super, Luna Tranquility or Buran.
Note: Unfortunately, Syllit will not be available in Ontario for the 2020 season. Supplies are expected to return in 2021.