Apples Diseases Insects Pest Management

Apple Update: June 14, 2017

In this update:

  • crop stage
  • thinning
  • irrigation
  • diseases: fire blight, scab, powdery mildew, leaf spots
  • insects: codling moth, obliquebanded leafroller, dogwood borer, San Jose scale, leafcurling midge, plum curculio, brown marmorated stink bug
  • Ontario Apple Orchard Tour

Crop Stages

Apples sizes have jumped quite a bit over the last weekend with all of the high temperatures.

  • In Southwestern Ontario, apple fruitlets are 15-25 mm in diameter
  • In Grey County and east of Toronto, apple fruitlets are at 12-20 mm in diameter


Chemical thinning is wrapping up in most areas of Southwestern Ontario.  Growers in Grey County and East of Toronto will be applying their 2nd or 3rd fruitlet thinner. Hot weather has certainly narrowed down the thinning window as fruit growth has sped up. As chemical thinning and “June drop” finish, it is best to hand thin early to get the most benefit on increasing size of the remaining fruit and improve return bloom. Keep in mind that the REI (Restricted Entry Interval) for Sevin (carbaryl) for hand thinning is significantly longer than the REI for other orchard tasks; the REI is 14 days for high density, trellised orchards and 10 days for low density, traditional orchards. It is recommended to have your thinning crew wear chemical resistant gloves.

In Essex, Lambton and Kent Counties, there has been very little rainfall over the last two weeks and some growers are starting to irrigate.  West of Ridgetown over to Toronto received 10-20 mm and East of Toronto over to Ottawa received 30-40 mm.  For the drier areas of the province, it is difficult to “catch up” your soil moisture with drip irrigation so it is best to start early, especially if you have young trees.


Reports of fire blight strikes have begun to trickle in. With the high humidity, actively oozing cankers can be found (Fig 1). Although the risk of blossom blight has passed, infection can still occur through open wounds caused by strong winds, heavy rain and hail, insect feeding, pruning or mechanical damage. According to Dr George Sundin, Michigan State University, each fire blight ooze droplet can contain approximately 1 billion cells while it only takes 100-1,000 cells to initiate shoot infection. That means a single ooze droplet could have the potential to infect every shoot in an orchard if spread by insect, rain, wind or even workers and contaminated equipment.

Ooze_with arrows
Figure 1. Fire blight ooze (red arrows) can appear clear to orange-red in colour and can often be found in early morning or during days of high humidity.

Cueva and Double Nickel are registered for the control of shoot blight following a trauma event. Like all coppers, there is the risk of russeting with Cueva. However, if fire blight is active in your orchard at the time of trauma, better to lose some fruit this season than trees later. The preharvest intervals for Streptomycin and Kasumin are 50 and 90 days, respectively.

Low levels of apple scab infection can be found in some orchards across the province. However, overall many orchards still appear to be clean at this time. While the degree days suggest we have reached 100% ascospore maturity in the province, some areas have not seen much rain in recent weeks to say for certain all spores have been released. As well, lesions from the most recent infection periods are still not expected to appear for at least another week. Before reducing fungicide rates, be sure your orchard is completely free from primary scab.

The hot and humid weather this week has been ideal for powdery mildew. Growers continue to protect susceptible varieties such as Gala, Honeycrisp, Cortland, Idared, Crimson Crisp, Goldrush, Russet, Fuji and Spy. This disease does not invade mature leaf tissue, so spread stops by terminal bud set.

A number of leaf spots are appearing in orchards around the province, including cedar apple rust (Fig 2), and frog-eye leaf spot (Fig 3). Alternaria leaf blotch has also been reported in some areas. This leaf spot can appear similar to frog-eye but occurs primarily on Delicious varieties, causing round lesions with light brown centers surrounded by a dark purple margin. The frequent and long periods of rain during bloom and petal fall were particularly ideal for infection by these pathogens as spores are transferred through free water. Many scab products will provide control of these diseases. See Table 3-14. Activity of Fungicides on Apple Diseases in 2016-2017 Publication 360, Guide to Fruit Production for efficacy ratings of registered fungicides for rust and summer disease management.

Along with black rot, now is the time to start thinking about bitter rot. The spores of this pathogen can infect fruit at any time during the growing season from as early as bloom right up until harvest. However, symptoms often show up after a period of hot weather accompanied by rain or a thunderstorm. Pristine, Allegro and Granuflo T are registered for control of bitter rot though other fungicides registered for scab may provide some protection as well.



Pheromone trap activity continues. Based on degree-days, codling moth egg hatch has started and control products are being applied. Obliquebanded leafroller flight has picked up this week. Biofix has been set in many areas and degree-days are quickly accumulating with the warm weather. Spray timings will be coming up over the next few weeks. Dogwood borer trap numbers are steadily increasing. If pheromone traps have not been placed in a problem orchard yet, these need to be installed as soon as possible since flight has begun.

San Jose scale adult flight has declined this week. First generation crawler emergence is predicted to begin over the next few weeks. Based on degree-day models, earlier regions should start to see activity June 14-16 while later regions should see activity picking up June 20-25. Growers should continue to visually monitor for the yellow-orange crawlers (Fig. 4) on infested trees to determine activity. Cover sprays such as Sivanto Prime, Closer or TwinGuard will be needed in issue blocks if Movento has not been applied.

Figure 4. Apple limb speckled with numerous small, yellow San Jose scale crawlers.

Apple leafcurling midge adult flight has also declined this week. Larvae inside leaf rolls are turning orange and dropping to soil to pupate, signalling the end of this generation.

Plum curculio damage has been found on developing fruitlets (Fig. 5), particularly around orchard borders. Activity is a little later than previous years in some regions. Often movement into the orchard follows a period of sustained warm weather and rain. Some research suggests weather patterns (sun/cloud) affect whether curculio migrate into the orchard by crawling or flying. If weather is cool at bloom and petal fall as we experienced in many regions this year, adults may not migrate into the orchard until after petal fall.

Figure 5. Fresh plum curculio damage with sap oozing from fruitlet.

The hot and dry weather is ideal for mite development. Overall, pressure has been relatively low this year. Likely helping to keep the mite populations low, a large number of beneficial insects can be found in orchards including adults and larvae of lady beetles, lacewings, mullein bug and minute pirate bug.

Brown marmorated stink bug has begun to be caught in monitoring traps in established areas. These are the overwintering adults becoming active. However, they are not attracted to apples at this time of the year as a food source. Damage is likely to not occur in apples until later in the season. Refer to for updates throughout the year.


The Ontario Apple Orchard Tour will be held in the Niagara area on July 5th. For more information and to register, click here.


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