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Mother Nature has has been throwing everything but the kitchen sink these last few weeks from frosty nights to hot and dry spells to now cooler temperatures and soggy days. The cool weather has certainly slowed bloom in many areas, leaving straggling or variable bloom in some cultivars; late bloom in others, especially cider cultivars.

In the later areas of the province, trees are late bloom to early petal fall while earlier regions are now chemically thinning and applying petal fall insecticide sprays.

Disease

Fire blight

The forecast is indicating the province could be in for another stretch of hot temperatures in the coming days. This will be ideal conditions for rapid multiplication of the fire blight bacteria, Erwinia amlyovora. Windy days, stormy weather, pollinator activity and increasing insect emergence over the last week will have moved bacteria to open blossoms, particularly if fire blight has been an issue in the orchard or neighbourhood.

Fire Blight Prediction Maps

Sudden, sporadic or isolated showers, heavy dew, fog or spraying can all provide enough moisture to wash bacteria into an open bloom. How much water is actually needed? If you look at the requirements for blossom blight infection based on the Maryblyt model, a wetting event is >0.01” (0.25mm) rain, heavy dew or fog sufficient to wet foliage and provide thin film on tissue surface or rain of >0.1” (2.5mm) the previous day.

While bloom has passed in many regions of the province, growers should be aware of blossoms still open, particularly young trees or rat-tail bloom. Protecting these open blossoms from infection will be critical during times of high risk. Antibiotics will break down quickly in sunlight, likely only providing 24-72 hours of protection before a re-application would be required. This means any antibiotics applied last week during the main bloom period will have already degraded away.

If time and labour are available, it may be best to go through the orchard every couple of days and remove the rat-tail blossoms by hand. Be sure to remove blossoms in dry weather to prevent the spread of infection. For many operations, this practice is just not viable. Instead, continue to monitor infection risk and apply protectants if needed.

Warm, humid weather can also bring unsettled or extreme weather including heavy rain, hail and high winds as we saw last weekend. These events can cause trauma blight. If there is concern, Streptomycin or Cueva and/or Double Nickel can protect against spread of infection if used immediately following a trauma event (ideally less than 12 hours after).

Many growers have been applying Apogee starting at king bloom petal fall (typically when growth is 2.2 – 5.5 cm long) to help with shoot blight management. For those with young trees, studies have found Apogee applied at a high rate on young trees provides good control of shoot blight without significantly impacting growth.

Scab

The spring continues to be challenging for apple scab management in areas of the province that have been seeing rainy periods and several extended primary infection periods. While the cooler temperatures slowed new growth some, many growers have been concerned about maintaining protectant fungicide coverage with the heavy or persistent rains. Systemic fungicides have been a go-to for some looking for good kickback activity particularly in cooler weather.

Most areas are still within – though nearing the end of – the peak scab ascospore maturation period. Orchards may have 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.

Some reports of scab lesions are starting to come in.

Powdery mildew

With the high overwintering pressure going into this year for many, it has been no surprise to see early signs of powdery mildew infection following the stretch of warm, dry weather. Unfortunately, when conditions are not necessarily conducive to scab infection, they can often be ideal for powdery mildew. Mildew thrives in dry weather and high relative humidity. In fact, rain deters powdery mildew development by washing off spores. So, protectant sprays may still be required during dry periods when there is little risk from apple scab. Be careful how long the interval between sprays become, especially in mildew susceptible blocks.

Some key points for effective powdery mildew control to consider:

• Maintain a tight spray schedule with high rates during primary scab infection period. Powdery mildew does not invade mature leaf tissue, so spread of mildew ceases when trees stop producing new terminal leaves.

• Getting good mildew control following an outbreak will take several seasons. Mildew infected white shoots from last year’s failure will persist through the season, but does not indicate current fungicides are failing. The current season mildew program is designed to prevent spread that would lead to primary infection for next year.

• Where they are working, include sterol inhibitors, strobilurins or SDHIs during the critical infection period, generally at pink through petal fall. Tank mix these products with Group M fungicide for resistance management.

Black Rot

Frog-eye leaf spot has been found in varieties that tend to be more susceptible to black rot such as Ambrosia and Gala, which holds on to fruit mummies that act as a source of inoculum. These early lesions appear as small purple to reddish flecks with light centers. As the lesion enlarges, it becomes a circular brown spot with a light tan center.

While the optimum temperature for leaf infection is 26⁰C, infection can occur any time above 10⁰C but will just require a longer leaf wetting period (more than 24 hours at 10⁰C). With the frequent rains and overcast conditions, this hasn’t been out of the question.

If you are seeing frog-eye leaf spot now, you now know inoculum is present in the orchard or coming from a sporulating nearby source such as dead or decaying tree in an adjacent woodlot. Black rot produces 2 types of inoculum – ascospores (sexual spores), which are wind blown and conidia (asexual spores), which require a wet period and rain splash for release. Both begin to release shortly after bud break; however, the heaviest ascospore release occurs for a 4-6 week period following petal fall. In other words, if your protectant program considerations haven’t included black rot yet, now is the time!  

Rust

Sporulating cedar apple rust galls on the winter, or alternate host (juniper species including red cedar) have been observed over recent weeks. Orange, gelatinous spore masses called telia horns extend from the galls for a period of a few days after a wet period. This usually coincides with apple bloom.

Spores are dispersed by wind to nearby apple trees. Golden Delicious, Idared, Mutsu and Russet are fairly susceptible to rust. Most scab fungicides used at this timing will have some efficacy on rust diseases. Refer to the fungicide efficacy view for apples on the Ontario Crop Protection Hub for efficacy ratings of registered fungicides.

Insect

Petal fall insecticide sprays are beginning to go on in the earlier regions of the province. The following table, Activity of Petal Fall Insecticides Against Orchard Pests may be of some help to determine the best insecticide for control of pests listed on the product label, while managing resistance and avoiding unnecessary sprays for non-target pests. Efficacy will be based on rate used.

Activity of Petal Fall Insecticides Against Orchard Pests

Insecticide	Group	Obliquebanded leafroller	Oriental fruit moth	Codling moth (eggs)	Plum curculio	Mullein bug	Tentiform leafminer	Japanese beetle	European apple sawfly
Aceta 70 WP	4A	—	✓ *	—	✓ *	✓ *	✓ *	✓	✓ *
Assail 70 WP	4A	—	✓ *	—	✓ *	✓ *	✓ *	✓	✓ *
Calypso 240 SC	4A	—	✓ *	—	✓ *	✓ *	✓ *	✓ *	✓ *
Theme 480 SC	4A	—	✓ *	—	✓ *	✓ *	✓ *	✓ *	✓
Cormoran	4A + 15	—	✓ *	✓ *	✓ *	✓ *	✓ *	✓ *	✓ *
Closer	4C	—	—	—	—	✓ *	—	—	—
TwinGuard	4C + 5	✓ *	✓ *	—	✓s *	✓ *	✓ *	—	—
Delegate	5	✓ *	✓ *	—	✓s *	—	✓ *	—	✓ *
Entrust	5	✓ *	—	—	—	—	✓	—	—
Success	5	✓ *	—	—	—	—	✓	—	—
Minecto Pro	6 + 28	✓ *	✓ *	—	—	—	✓ *	—	✓ *
Bioprotec PLUS	11	✓ *	—	—	—	—	—	—	—
Dipel 2X DF	11	✓ *	—	—	—	—	—	—	—
Xentari WG	11	✓ *	—	—	—	—	—	—	—
Rimon 10 EC	15	✓	✓ *	✓ *	—	—	—	—	—
Intrepid	18	✓ *	✓ *	—	—	—	✓ *	—	—
Altacor	28	✓ *	✓ *	—	—	—	✓ *	✓s *	✓ *
Exirel	28	✓ *	✓ *	—	✓ *	—	✓ *	✓ *	✓ *
Harvanta 50 SL	28	✓ *	✓ *	—	✓s *	—	—	—	—
Vayego 200 SC	28	✓ *	✓ *	—	✓s *	✓s *	—	—	✓ *

Caterpillars

The spray window for oriental fruit moth is quickly approaching in most early regions. Some growers are opting for insecticides with good efficacy on mullein bug, spring feeding caterpillars, European apple sawfly and/or apple leafcurling midge if monitoring had indicated they had reached threshold or have high pest pressure.

Codling moth biofix has been set in some areas but the spray window is still a ways out. Obliquebanded leafroller traps are up. A significant number of redbanded leafroller have been caught at sites monitoring for these moths as well.

European apple sawfly have been caught in traps where monitoring for this pest is on-going. However, catch has been relatively low likely due to the cooler weather during bloom. Nonetheless, early petal fall spray timing will be critical for areas with sawfly pressure. Delaying this spray may result in extensive damage. This is a pest east of Toronto, into Georgian Bay and throughout the GTA. However, damage has been suspected in regions further southwest.

Spring-feeding caterpillar activity can still be found. Of the caterpillar complex, spongy moth can be found causing significant damage in some parts of the province, especially those orchards that saw high pressure last year.

As I mentioned in a previous pest update, the Ontario Ministry of Natural Resources and Forestry (MNRF) are still estimating defoliation to be moderate to severe in certain hotspot regions this year. For more information on these particular high pressure regions, survey information can be found on the MNRF website.

Larva will feed on leaves for several weeks before pupating. Most damaging activity is only present in orchards until July. While you may see the adults later summer, they do not feed and there is only one generation per year. Unfortunately, that early season larvae can feed pretty voraciously until then.

Products applied for leafrollers in apples such as Bacillus thuringienesis (Bt), e.g., Dipel, Bioprotec and Xentari should also provide control of spongy moth. Other products listed under spring-feeding caterpillars in the Ontario Crop Protection Hub may be registered for this pest, including some pyrethroids (Group 3) and Cormoran (Group 4A+15) or have efficacy against spongy moth when applied for a labelled pest.

For best efficacy, apply within the first few weeks after larval emergence. Once the majority of the populations reaches 2.5 cm or larger and develop yellow markings on the head capsule, few products will offer sufficient control. Also try to target application close to the time larvae are actively feeding to ensure exposure. During the day, most of the larva can be found in sheltered areas or on the underside of leaves.

Aphids

Leaf curling and puckering typical of rosy and green apple aphid has been observed around bloom clusters and terminals, respectively. Aphids thrive in cool, wet springs so recent conditions may have encouraged development. While healthy trees can typically withstand fairly large colonies of green apple aphid, rosy apple aphid infestations can cause more significant damage. The most serious rosy apple aphid damage occurs when a toxin found in its saliva translocates from the leaves to fruit, causing apples to remain small, deformed (pigmy fruit) and unmarketable. Control is recommended if more than 5% of fruit clusters are infested with 20 or more aphids.

Mullein bug

Increasing numbers of mullein bugs are being tapped out in various orchards across the province, exceeding the threshold of 7-9 nymphs per 25 taps. No damage has been reported yet in those areas where fruit are sizing. Growers seeing numbers above threshold are applying (or planning to apply) insecticides with activity against this pest particularly on susceptible varieties such as Red Delicious, Spartan, Spy, Ambrosia, Empire, Cortland, Gala, Jonagold and Golden Delicious.

It’s recommended to continue monitoring after the insecticide has been applied to determine if a follow-up spray is required. Delegate, Minecto Pro, Altacor and Exirel do not have efficacy on this pest.

The critical time for damage from mullein bug is generally dime to quarter-sized fruit. Depending on available food sources, fruitlet damage caused by mullein bug populations exceeding threshold can be rather variable. The cooler temperatures in early May seem to have resulted in delayed emergence of some insects mullein bug may feed on such as mites and aphids. However, I have been seeing a significant number of mullein bugs in leafcurling midge rolls. These have seemed to be quite the voracious predators of midge in previous years. So, for those sites with leafcurling midge issues, it is possible if there is enough other food available in the orchard, it can keep the mullein bug from feeding on developing fruitlets. Without such food sources though, those fruitlets may be prime targets for mullein bug attention.

Apple leafcurling midge

Apple leafcurling midge egg hatch continues. Infested terminals are becoming quite apparent already in orchards with a history of this pest. While Movento is registered for suppression, the timing for leafcurling midge is often earlier than for scale and woolly apple aphid so timing would have to be chosen based on which pest is of greater concern in your orchard. However, there are other petal fall insecticides such as Delegate, Closer and Exirel (and for that matter Minecto Pro) that may provide some efficacy as well, which could help delay the Movento application. Summer oils will also help to smother eggs so long as good coverage is achieved to reach within the newest terminal leaves.

San Jose Scale

Orchards in the Norfolk regions that are monitoring for San Jose scale observed the start of adult flight this week. Typically this aligns with 1st generation codling moth biofix which seems to be the case this year. Degree days for San Jose scale have begun to be accumulated to predict crawler emergence. Current estimates are still a couple weeks away but with the warmer nighttime temperatures forecast, this could quicken development.

With petal fall, sticky tape is also currently being placed on infested branches in blocks to monitor crawler activity. Typically, 1^st generation crawlers emerge mid- to late June in Ontario and are active for 4-6 weeks. In following apple pest updates, I will continue to provide alerts as this timing approaches for regions across the province.

Control is rather targeted as most petal fall insecticides will be too early for scale. One option is back-to-back sprays of Movento starting 10-14 days after petal fall, or early June. This is a slower-acting product and requires a couple of weeks to see control so timing is best before crawler activity begins. Another option is a back-to-back spray of Sivanto Prime, Closer or TwinGuard (pre-mix of Delegate and Closer), if available in your area, starting at crawler emergence.

Mites

European red mite activity has begun though overall populations remain fairly low likely due to the cooler weather. Those orchards that did not apply an early season oil should be monitoring with the warmer weather predicted and be aware of the appropriate timing for whichever product you use as some can only be applied within weeks of petal fall (eg., AgriMek, Minecto Pro), require earlier application as populations build (eg. Nealta) or have a slow knock-down (eg., Envidor).