Apple fruit bud in tight cluster growth stage
Apples Diseases Insects Pest Management

Apple Pest Update: May 15, 2020

The latest update on insect and disease activity in apple orchards across the province

Listen to the audio article here:

The unseasonably cool stretch of weather we’ve had lately has made for what feels like a long start to the season. Many are reporting to be a few days to a week behind last year, which was also a later season in terms of growth. Regionally, growth stages are quite spread out depending on variety and proximity to water:

  • In Essex, Chatham-Kent, Lambton, Middlesex, Elgin, Norfolk, Brant and Niagara, early varieties are at pink. However, many remain at tight cluster.
  • In Grey, most varieties are at half inch green with some in very early tight cluster.
  • In Durham, Northumberland and Quinte, most varieties are tight cluster.
  • Further along the St. Lawrence and into the Ottawa Valley, apples are half inch green to very early tight cluster.

Dealing with cold temperatures

With the freeze on everyone’s mind lately, 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 or captan to be easily absorbed into the plant causing phytotoxicity.

Several growers have also mentioned small, narrow spur leaves or crinkling caused by cold damage. Be mindful to ensure mite and disease pressure such as powdery mildew are kept at bay since the spur leaves provide much of the energy required for fruit set.  



Regions that have seen some decent precipitation have had multiple potential scab events since the start of the season. Early areas have reached the period of rapid ascospore maturation, at about 30-50% of ascospores (or more) matured.

Even with the freezing temperatures, it is possible for ascospores to continue to develop. After all, they started maturing under snow cover earlier this spring. We’ve also had some relatively warm daytime temperatures of 10-12C. While the time required for an infection to occur is much longer in lower average temperatures, it is still possible to reach this critical timing so long as the leaves stay wet.

The length of leaf wetness required for infection will be greatly reduced with the higher temperatures predicted in the next week. Refer to Table 3-10. Relationship of Temperature and Moisture to Apple Scab Infection in Publication 360A, Crop Protection Guide for Apples for more information. Make sure to keep covered every 5-7 days during this time, especially ahead of rain and be sure to tank mix single-site fungicides with protectant Group M fungicides for resistance management.

Scab lesions have not been found yet. However, the cooler weather has likely slowed down the formation of conidia on lesions (secondary scab) so symptoms may not be present for up to 17 days following infection. These can develop quickly should weather warm up (Figures 1 & 2).

Fire blight

Putting the cold temperatures behind us, with early areas approaching bloom, growers will need to start considering their fire blight management program. The Ontario fire blight prediction maps are available again this year to help with that decision making.

The risk of fire blight infection is possible if the following conditions are met:

  1. Flowers are opened and/or are opening – There is no risk of infection if bloom is not present in the orchard despite the fire blight maps stating caution to extreme; however, keep an eye on any early signs of bloom as it can happen quickly.
  2. Enough heat units have accumulate based on forecasting models – The fire blight maps use Cougarblight but this applies to MaryBlyt as well. Similar to degree days, daily temperature is accumulated and must reach a certain point before risk increases. With the cool weather, this is why the prediction maps have been staying on low or caution.
  3. A wetting event is predicted – Any type of wetting event will wash the bacteria into the open bloom including rain, dew or even the artificial rain shower you create with the sprayer during critical infection periods.

Consider alternatives to antibiotics during the early bloom period such as:

  • Biologicals – eg., Blossom Protect, Buran, Serenade, Regalia or Double Nickel
  • Surface sterilant – eg., Oxidate
  • Copper labelled for in-season use – eg., Cueva

This will allow you to save your limited uses of antibiotics for later bloom (50-80% bloom) when fire blight risk is greatest.

Powdery mildew

There have been no reports of powdery mildew which is not surprising given the cool spring. However, if you are not already using low rates of sulphur, now is the time to consider selecting scab fungicides with powdery mildew activity. Peak infection often occurs from tight cluster to terminal bud set. Most systemic fungicides from Group 3, 7 and 11 applied at this time for scab have good efficacy on powdery mildew.


In general, the cooler weather has delayed activity for many insect pests.

Spring feeding caterpillar

Small caterpillars have been found feeding on new terminal growth (Figure 3) and developing fruit buds. While not a stand alone control tactic, supporting beneficial populations in the orchard will help suppress some pests such as spring feeding caterpillars (Figure 4). Consider this when deciding if a prebloom insecticide is worthwhile.

Tarnished plant bug

There have been reports of tarnished plant bug and associated feeding on buds from a few regions. Damage appears as clear to amber ooze from developing buds. Typically, affected buds abort. Fruitlets stung after calyx may fall during June drop, but those that hang on until harvest have a dimple or a deeply sunken conical area. Management usually isn’t targeted for this pest unless it’s a perennial issue, especially if your orchard is adjacent to susceptible crops such as alfalfa and strawberries or there is a history of high weed pressure.

Figure 5. First flight of apple leafcurling midge.

Apple leafcurling midge

Adult catch in pheromone traps has begun in the regions that are monitoring for apple leafcurling midge. While the freezing temperatures slowed activity, it was only temporary as flight and signs of egg laying has picked back up again late this week. Pressure is extremely high already in some orchards, as can be seen by the pheromone trap in Figure 5 after only 3 days of catch.


European red mite egg hatch has begun in early regions, though activity is relatively low. Those that didn’t get an oil spray on earlier are considering a delayed dormant application once temperatures warm up. Be sure to avoid applying oil if temperatures will be below 4C.


Trying to find a window for dormant oil targeting scale timing was difficult in many areas this year with freezing temperatures. Several growers that were unable to apply oil have opted for a tight cluster to pink application of Sivanto Prime or Closer to target the maturing scale. The high rate of Closer at this timing will also have efficacy on woolly apple aphid, tarnished plant bug and mullein bug.

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