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To say we’ve seen it all in this last week would be an understatement. True to Ontario spring fashion, we’ve gone from snow to rain and scab infection events to 25C temperatures. Looking at the short-term forecast, some areas may be back to cooler, rainy or potentially even snowy days again.

Regionally, most areas are still around green tip to half-inch green. Though the earliest areas are seeing very early tight cluster in some cultivars.

By the looks of things, temperatures will be returning more seasonable later this week which could move growth along quickly as well as pest development.

It’s too cold for disease, right?

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

We’ve had some relatively warm daytime temperatures recently over 12C 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, take a look at the table Relationship of Temperature and Moisture to Apple Scab Infection below.

Relationship of Temperature and Moisture to Apple Scab Infection

Average Temperature (°C)1	Minimum Number of Hours of Leaf Wetness Required – LIGHT Primary (Ascospore) Infection2	Minimum Number of Hours of Leaf Wetness Required – MODERATE Primary (Ascospore) Infection2	Minimum Number of Hours of Leaf Wetness Required – HEAVY Primary (Ascospore) Infection2	Minimum Number of Hours of Leaf Wetness Required – Secondary (Conidia) Infection2	Lesion appearance (days)3
1	40	69	93	37	–
2	34	69	93	33	–
3	30	52	65	30	–
4	27	42	57	26	–
5	21	34	50	23	–
6	18	27	44	20	17
7	15	23	37	17	17
8	13	21	34	15	17
9	12	17	27	13	17
10	11	16	26	10	16
11	9	14	22	9.5	15
12-13	8	12	20	9	14
14-15	7	11	19	9	12-13
15.5	6.5	10	17	9	10-11
16-24	6	9	16	7-9	9-10
24-25	6.5	9	16	9-11	–
25	8	11	18	11	–
25.5	10	14	23	13	–

What’s in the scab forecast?

The long-term forecast seems to suggest warmer weather is on its way with the potential of scattered showers. 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. With temperatures above 15C, only 6 hours of leaf wetness are required for germination. Those without adequate fungicide protection could find themselves in a bad situation.

Early season scab management

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

• Keep an eye on the forecast for freezing temperatures.
  • Use fungicides from the EBDC group like mancozeb (Manzate, Penncozeb, Dithane) or cool weather fungicides that provide some kickback activity like Scala, Inspire Super or Luna Tranquility.
  • Applying captan, copper or oil to cold injured leaves could make the injury worse due to uptake of these products into the tissue.
• We’re hitting a critical infection period as areas approach tight cluster, with increasing daily temperatures, large amounts of lush growth and rapid maturation of spores.
• Be sure leaves are covered before any potential infection event. 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 if infection risk continues or be prepared to cover again once things dry up.

What about mildew?

High powdery mildew pressure last year followed by the mild winter has many growers concerned about management of this disease this season (Figure 1). Be sure to consider a mildew protectant such as a low rate (3-5 kg/ha) of sulfur in your early season scab program, especially on susceptible cultivars or orchards with a history of powdery mildew. Continue this until tight cluster when more systemic fungicides with mildew activity like the Group 3, 7 and 11s start to be used. Some orchards with historically low powdery mildew pressure are using early season and summer oil sprays to help with mildew suppression.

Peak infection is going to kick off from tight cluster to terminal bud set. While a number of systemic fungicides applied at this time for scab have good efficacy on powdery mildew, keep in mind that dry, warm weather is conducive to powdery mildew unlike the wet weather needed for scab infection. So coverage is still important during those dry spells.

Upcoming events

Looking for a chance to refresh your knowledge of apple IPM? New to growing? Join me for an Apple IPM workshop on May 6^th from 10 am – 1:00 pm where we will discuss orchard scouting including monitoring techniques for various insect and disease, tools of the trade, safety protocols and tips for success. Time will also be available to answer any pest-specific questions regarding the pre-recorded presentations available on the ONhortcrops YouTube channel.

You can join this workshop in person at the Simcoe Research Station in Norfolk County or virtually via Zoom.

Pre-registration is required. Click here to register.

Other IPM workshops are also being offered over the coming weeks. Check out ONfruit for more information.

Registration Now Open! 2022 IPM Workshops