Early apple scab lesions on top side of leaves
Apples Diseases Pears Pest Management

Apple Scab And Fire Blight Infection Risk With Warm, Wet Weather

Prolonged rainfall expected over coming days puts apples at risk for scab and/or fire blight infection.

Most areas of the province have started (or are starting) into the period of rapid ascospore maturity based on the scab degree day model. With the rain expected over the coming days, we will likely see a primary scab infection period. As well, reports have begun to come in that earlier regions are seeing some bloom in certain varieties. Warm, wet weather increases the risk of blossom blight infection. Make sure to keep expanding tissue covered!


Based on the Mills Table, which can be found in Table 3-9. Relationship of Temperature and Moisture to Apple Scab Infection of 2021 Publication 360A, Crop Protection Guide for Apples, primary infection requires a minimum of 11 hours of leaf wetness at 10 degree Celsius. The minimum number of hours decreases with increasing temperature. This requirement will be met in many regions of the province if rain continues into the weekend.

Apply protectant fungicides such as metiram, mancozeb or captan now if not already covered. 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. For more information on apple scab fungicide characteristics, refer to Table 2-7. Characteristics of Apple Scab Fungicides in Publication 360A, Crop Protection Guide for Apples.

Fire Blight

The Ontario fire blight risk predictions maps have been updated for April 28-May 4. High to extreme infection risk is predicted for earliest regions in the southwest over the coming days with rain in the forecast. If bloom is present, management may be needed during this time particularly if there is active fire blight in your orchard or neighbourhood.

It is important to note that these risks are based on accumulated temperature over a period of days. At this point in the season, we are still hovering around the temperatures that would really increase risk. Precipitation also seems quite variable with scatter storms across the province. Areas that see more temperature moderation by lake effects may see a lower risk predicted for their region.

If antibiotic materials are not being used, blossom protection with other materials should be initiated 1-2 days prior to entering a high infection risk period.

Oxidate is a surface sterilant. Do not apply prior to rain.

Continue appropriate protective sprays until the infection risk drops below the HIGH threshold. Streptomycin provides kickback activity if an infection event was missed but should be applied within 12-24 hours after.

Stay dry!

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