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Apple Pest Update: May 27, 2019

Apple pest activity continues to increase in orchards across the province
 
Crop Stage
 
Dry, sunny weather has been a welcoming sight in many areas of the province this past week. Unfortunately, the forecast doesn’t sound like the sun will be sticking around for much longer as we are in for more soggy weather.
 
Earlier regions such as Essex, Kent, Middlesex, Elgin, Norfolk, Brant and Niagara are now seeing petal fall in some – if not all – varieties. Other areas of the province such as Grey, Durham, Northumberland, Quinte and east to Ottawa valley are pink to early bloom.
 
Disease 
 
Fire Blight
 
Conditions for fire blight infection were high to extreme in most areas across the province this last week according to the fire blight prediction maps. These maps use the Cougar Blight forecasting model to determine infection risk. According to this model, some of the most damaging fire blight epidemics have occurred under these optimum conditions, including in orchards that have never experienced fire blight.
 
Growers running the Maryblyt model also saw the epiphytic infection potential (EIP) exceed the spray threshold of 100. It is important to remember with this model, the EIP resets back to zero when a bactericide has been applied. However, with high temperatures, it does not take long for the EIP to jump back up above 100. A benefit to this model is that it also predicts when fire blight symptoms will appear.
 
If forecasting models are not being followed, bactericides should be applied from first bloom to petal fall when temperatures exceed 18⁰C with high humidity (>65%), heavy dews, rain or when spraying fungicides. According to Dr. George Sundin from Michigan State University, temperatures conducive for growth can result in fire blight populations multiplying to one million cells per flower within 1-2 days. Protective sprays are critical.
 
It seems as though the cooler temperatures will bring regional infection risk down for the remainder of the week. However, a cool forecast does not mean you are out of the woods entirely for fire blight blossom infection. The bacteria are capable of multiplying in temperatures ranging from 4⁰-32⁰C and will remain on the blossom until a wetting event occurs. As well, there is a possibility the day will be warmer than what was forecast or may be warmer in your orchard than elsewhere. In terms of fire blight infection risk, a few degrees can make a significant difference in disease pressure. 
 
Despite some of the earliest regions seeing petal fall, secondary, or “rat-tail” blossoms are still possible. These late blooms are very susceptible to fire blight infection. Generally, continuing a protective program for a couple of weeks following petal fall will help ensure any late blooms are covered.
 
Regions that will be reaching bloom over the next week as infection risk potentially goes back down could consider the use of biopesticides such as Blossom Protect, Double Nickel or Serenade OPTI during extended periods of caution in early bloom. This will help establish a baseline protection on open blossoms should conditions change quickly. These products form a protective layer on the blossom and prevent the fire blight bacteria from invading. In cooler temperatures, while infection may not be occurring, the fire blight bacteria can still multiply and remain on the blossom. If temperatures climb above 18⁰C, the bacterial population can then increase exponentially in a very short period of time. Consider biologicals as a safety net for situations like this. Note, trials in the northeast have found an increased incidence of russeting when using Blossom Protect during extended wet weather particularly on sensitive varieties like Golden Delicious.
 
Apogee has started to be applied. Typical application is at king bloom petal fall for management of fire blight shoot infection; however, some growers are applying Apogee earlier between pink to bloom if there is terminal growth. This product works by:
  • Reducing vegetative growth of susceptible shoots and limiting spread of fire blight infection
  • Thickening the cell wall to provide a physical barrier from bacterial infection
  • Stimulating plant defense system and production of antimicrobial compounds to reduce infections.
Scab
 
The spring continues to be challenging for apple scab management. The continuous rainfall in recent weeks have resulted in several extended primary apple scab infection periods. While the cooler temperatures slowed new growth down, many growers have been concerned about maintaining protectant fungicide coverage with the heavy rains. Systemic fungicides have been a go-to for some looking for good kickback activity particularly in cooler weather. However, post-infection activity is calculated from the start of the infection period so may not provide full coverage in cases of extended infection periods.
 
Most areas are still within – though nearing the end of – the peak scab ascospore maturation period. Orchards have had a few rain-free days to help dry things up. However, that means there will be a significant build-up of mature ascospores ready to be released during the next rainfall. Very little moisture – dew, rain or spray – is required to trigger spore release during this time. Protectant fungicide coverage is critical as most of the available mature spores are discharged within 2 hours after the start of the wetting event.
 
At the present time, there have been no reports of scab lesions.
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