Apples Diseases Insects Pest Management Vertebrates

Apple Pest Update: May 20, 2019

Crop Stages

The cool, wet spring 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 late season in terms of growth. In all regions, 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 to bloom. However, tight cluster can still be found in late varieties.
  • In Grey, Durham, Northumberland and Quinte, most varieties are tight cluster to early pink.
  • Further along the St. Lawrence and into the Ottawa Valley, apples are at tight cluster.

A big issue this spring has been the above average rainfall. Snow melt was long and the soil didn’t have much drying time before the snow turned to rain. Orchards across the province have stayed water-logged. Growers are trying to avoid ruts and getting equipment stuck; some by using partial tanks when spraying to lighten the load.



With the wet weather this spring, some areas have seen as many as 6 potential scab events since Easter weekend including events that have lasted 3 or more days. Most areas have reached the period of rapid ascospore maturation, at about 30-50% of ascospores matured. The forecast this weeks looks to have dryer weather. However, that means there will be a significant build-up of mature ascospores ready to be released during the next rainfall.

The length of leaf wetness required for infection will also be greatly reduced with higher temperatures. Refer to the revised Mills table in Table 1 below 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 (Figure 1 & 2). With the frequent rains and reports from growers that were having difficulty keeping good coverage, I imagine we will start hearing of lesions being observed in susceptible varieties shortly.

Table 1. Revised Mills table relating leaf wetness duration and temperature to predict infection time and symptom development of apple scab.

Average temperature (⁰C)a

Wetness (hours)b

Lesion appearance (days)c















15 17


13 17


12 17


11 16










7 12 – 13


7 12 – 13


6 9 – 10



9 – 10


6 9 – 10


9 – 10


6 9 – 10



9 – 10
22 6

9 – 10


6 9 – 10



25 8



a Add lowest and highest temperatures during wet period and divide sum by 2 to get average

b Calculate hours of wetting by either (1) beginning the count at the time leaves first become wet and ending the count when the relative humidity drops below 90%, or (2) adding consecutive wet periods (hours) if the leaves are again wetted within 8 hours from the time relative humidity dropped below 90%.
c Number of days required for lesions to appear after infection has been initiated.
Adapted from the table of W. D. Mills, revised by W. E. MacHardy

Fire Blight

While the cool temperatures kept the risk of fire blight infection low last week, the forecasted weather has had most areas across the province with orchards in bloom at high to extreme infection risk from May 17-21 based on the fire blight prediction maps. Management will be critical during this time particularly if there is active fire blight in your orchard or neighbourhood.

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

  1. Flowers are opened and 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 until this warmer weather over the long weekend.
  3. Dew or rain event is predicted – Any type of wetting event will wash the bacteria into the open bloom including the artificial rain shower you create with the sprayer during critical infection periods.

Frequent applications of antibiotics (Streptomycin, Kasumin) may be required during periods of high infection risk for the following reasons:

  • Fire blight bacteria multiply rapidly in a very short period of time when temperatures are above 18⁰C and relative humidity above 90%.
  • Bacteria can be transferred to new blooms by rain, wind or insect. Bees and other pollinators are very active in the orchard right now and can easily spread the bacteria around.
  • Antibiotics degrade rapidly in sunlight so are only active for 2-3 days MAXIMUM. Apply these products as close to a wetting event as possible.
  • Streptomycin will provide kickback following an infection, but only within the first 24 hours from the start of infection.
  • New blossoms are opening daily and will not be protected by sprays made while these blooms were closed.
  • ALL blossoms are at risk of infection including secondary (rattail) bloom or delayed bloom on newly planted trees.
  • Any wetting event, including rain, dew and even your sprayer will wash bacteria into the floral cup and initiate infection.

For resistance management with antibiotics, it is important to rotate between Streptomycin and Kasumin. Kasumin has been found to kill Streptomycin-resistant bacteria. While these are both antibiotics, there is no risk of cross-resistance. With the potential of a long bloom, there may be an extended infection risk period. Incorporating Kasumin (maximum 5 applications per year) between Streptomycin (maximum 3 applications per year) sprays will provide a significantly larger management window.

Powdery Mildew

There have been no reports of powdery mildew which is not surprising given how wet it has been this 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. See Table 2-3. Activity of Fungicides on Apple Diseases in the 2018-2019 Publication 360, Fruit Crop Protection Guide for efficacy ratings for most registered powdery mildew products.

Storage Diseases

There has been a number of reports from across the province from growers receiving letters from their packers indicating issues with 2018 fruit coming out of storage with fly speck (Figure 3a), sooty blotch (Figure 3b), black rot (Figure 3c) and/or bitter rot (Figure 3d). This is suggestive that infection occurred late season once scab residues were reduced. Infection by these diseases can occur shortly after petal fall – with the exception of black rot which can occur prebloom – and continues until harvest if temperatures remain warm with rainfall. If these diseases were an issue in your orchard last year, assume inoculum pressure will be high this year, especially if overwintering hosts such as brambles (fly speck, sooty blotch) or decaying wood and fruit (black rot, bitter rot) are present. Consider summer disease control when selecting petal fall and first cover sprays and continue to keep fruit protected throughout the summer with 14-21 day intervals after primary scab is over.

Rapid/Sudden Apple Decline (RAD/SAD)

Some orchards that experienced RAD/SAD in previous years are beginning to see similar symptoms again. Seemingly healthy young trees that have broken bud and growing normally, then suddenly have terminal wilt or even tree collapse. In most cases, the leaves have a pale yellow hue, and then rapidly turn purplish or reddish just before the tree collapses. Cause for this collapse has yet to be determined but an Agriculture & Agri-Food Canada project is underway. If you see signs of this collapse in your orchard, please contact me at 519-420-9422 or


In general, the cooler weather has delayed activity for many insect pests. Some growers in regions that don’t deal with European apple sawfly have been debating a prebloom insecticide. A proper integrated pest management approach is to only apply a chemical control when the pest is present and at economically damaging levels. That said, there is a good possibility of a prolonged bloom this year unless conditions warm up over the next week or two. This could present quite a problem in orchards with a history of damage caused by any insects that tend to be active at this time. A few growers in the earlier regions were disappointed to see their prebloom application washed off in the heavy rains.

Spring Feeding Caterpillar

Small caterpillars have been found feeding on new terminal growth and developing fruit buds. Natural enemies such as spiders have been observed predating on these individuals in some orchards (Figure 4). While not a stand alone control tactic, supporting beneficial populations in the orchard will help suppress some pests such as spring feeding caterpillars. Adults of redbanded leafroller and lesser appleworm have also been caught in pheromone traps.

Spider eating spring feeding caterpillar on apple tree
Figure 4. Spider eating a spring feeding caterpillar on apple tree.

Tarnished Plant Bug

There have been reports of tarnished plant bug and associated feeding on buds from a few regions. Management is not often targeted for this pest but this is 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. Damaged buds exude clear liquid ooze, which becomes amber after several hours. Affected buds do not set and subsequently abort. Fruitlets stung after calyx fall off the tree during June drop, but those that hang on until harvest have a dimple or a deeply sunken conical area caused by the injection of a toxin during feeding.

Apple Leafcurling Midge

Adult catch in pheromone traps has begun in all regions monitoring for apple leafcurling midge including Essex, Norfolk and Niagara. I had hoped the rains would have helped drown some of the overwintering individuals in the soil, but that doesn’t seem to be the case. Pressure is extremely high already in some orchards, as can be seen by the pheromone trap in Figure 5 after only 2 days of catch. Shiny, orange eggs can be found in clusters near leaf folds or margins of the youngest unfurled leaves of shoot tips (Figure 6). I anticipate seeing signs of larval activity and leafrolling over the next 1-2 weeks if the cool weather stays.


European red mite egg hatch has begun in all regions, though relatively low. Delayed dormant oil targeting mites has finished in the latest regions.

Other Insects

The cool temperatures may extend bloom and slow development of many insects. For instance, temperatures need to be above 18⁰C for migration of plum curculio into the orchard from adjacent woodlots. This pest tends not to feed or oviposit when night temperatures are below 10⁰C.

Traps for oriental fruit moth, San Jose scale and ambrosia beetle have gone up. No biofix has been set. Dormant oil timing for scale was difficult for some areas this year with freezing temperatures. A number of growers that were unable to apply oil have opted for a tight cluster to pink application of Sivanto Prime plus oil or Closer to target the maturing scale. The high rate of Closer at this timing may also have efficacy on woolly apple aphid.

No reports of mullein bug or rosy/green apple aphid.

Woolly apple aphid colonies have been found in protected areas along or near the trunk such as pruning cuts, old cankers or under bark (Figure 7). Activity can begin at tight cluster; however, this often remains on the trunk until after petal fall when individuals begin to move out into the canopy.

Signs of a long winter, I was recently in an orchard that had extensive rabbit damage about 1/3 of the way up the trunk of young trees where the snow cover was. With new plantings this spring, a reminder to use a wire screen or guard to prevent feeding damage.

1 comment on “Apple Pest Update: May 20, 2019

  1. John Ardiel

    What control is recommended for Pear Midge?

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