It just wouldn’t be an Ontario spring anymore without warm, dry weather one day and cool, wet weather the next! Last week’s unseasonably warm temperatures put growth in overdrive with many areas now near or at half-inch green. The earliest regions are seeing tight cluster to even early pink in some cultivars!  

Looking at the short-term forecast, the province may be back to cooler, rainy or potentially even snowy days again. Some regions have reached ascospore maturity where they could see the first apple scab infection period currently.

It’s too cold, right?

Even with the cool temperatures, it is possible for ascospores to continue to mature. Afterall, they started maturing under snow cover earlier this spring.

We’ve had some very warm daytime temperatures recently over 20C to push ascospore maturation, ready for release with the current cool rains. While the time required for maturation and infection to occur is much longer in lower temperatures, it is still possible to reach this critical timing so long as the leaves stay wet. And 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 during these wet periods.

If you’re looking for more information on how temperature and leaf wetness affect scab infection, check out the table, Relationship of Temperature & Moisture to Apple Scab Infection on the Ontario Crop Protection Hub.

Dealing with cold temperatures

A reminder to be careful not to add insult to injury when it comes to applying pesticides shortly after a cold event. The damaged tissue can be susceptible to the uptake of spray materials for 24-48 hours after a freeze before leaves have a chance to heal. This damage can serve as entry points allowing some products such as oil, copper, captan or folpet to be easily absorbed into the plant causing phytotoxicity.

Keep covered during rainy stretches

Apply protectant fungicides (e.g., mancozeb, captan, folpet) now if not already covered. Most of the available mature spores are discharged within 2 hours after the start of a wetting event. If using protectant fungicides alone, remember these are contact fungicides and do not provide effective post-infection or anti-sporulant activity.

If heavy rains greater than 2″ occur, residues will have likely been washed off. Reapply at any break in the rain over the next few days if that is the case. Be prepared to cover again once things dry up.

Consider including a post-infection, or kickback product if you are concerned about coverage. Cooler weather fungicides that provide kickback include Syllit (until tight cluster), Scala, Inspire Super, Luna Tranquility and Buran.

By tight cluster, apples are typically entering into a time of critical infection period for scab and powdery mildew with higher daily temperatures, large amounts of lush growth and rapid maturation of spores. Management programs at this point should begin incorporating systemic fungicides (Groups 3, 7 and 11). However, for resistance management, tank-mix with a protectant fungicide (e.g., Group M, biological, etc) wherever possible.