Apple Update: June 30, 2017

In this update:

• Fruit size
• Hedging
• Nutrient analysis
• Weather issues
• Disease: fire blight, apple scab, powdery mildew, fly speck/sooty blotch, sudden apple decline
• Insect: oriental fruit moth, obliquebanded leafroller, rosy apple aphid, San Jose scale, Japanese beetle, brown marmorated stink bug

Crop Stages

• Apple fruitlets are increasing in size and are greater than 30 mm in diameter
• June drop is finishing
• Terminal buds are starting to set in Southwestern Ontario

Production

Now is good time to hedge high density orchards to see the greatest benefit in an increase of flower bud induction and minimize regrowth. Minimizing regrowth will allow apples to be exposed to more sun to improve colour later in the season. Stefano Musacchi and Karen Lewis from Washington State University have found that the best timing for hedging is when new growth has reached 12 leaves.

As June drop is finishing, hand thinning will start. Hand thinning will have more impact on fruit size and improving return bloom when done earlier in the season.

When terminal buds have set, it is the best time to take tissue samples for plant nutrient analysis. Nutrient levels are very variable throughout the growing season but at terminal bud set, the nutrient concentrations are the most stable and it is the standard time for sampling.

• Sample 10 leaves from 10 representative trees.
• Sample from the same trees each year.
• Sample trees separately from areas that you would sample your soil or manage your fertilizer separately.
• Avoid collecting damaged or abnormal leaves.
• Collect tissue samples into labelled paper bags and deliver right away.
• If you can’t deliver right away, dry the leaves in the sun or in an oven at 65°C or less.

Remember, plant tissue sampling does not replace soil testing, the results are the most useful with a visual assessment of the crop and soil conditions and a recent soil test.

Weather has been very erratic this past week.  20-40 mm of heavy rainfall hit most of the province on Friday with the area East of Toronto getting most of the rain receiving up to 90 mm.  Hail (Figure 1) also hit quite a few orchards in localized areas of Lambton, Elgin, Brantford, Niagara, Grey, York and Durham.

Photo: M. Appleby

Figure 1. Variety of hail damage was found in orchards this week from cuts, dents to bruises.

Diseases

Trauma events like hail can increase the risk of infection to diseases such as fire blight or black rot. Although the season has been relatively cool overall, some growers that had active fire blight in their orchard last year erred on the side of caution and applied a streptomycin within 24 hours of hail. Copper can also be used but does not have the systemic characteristics that streptomycin does. It will only kill the bacteria on the surface of the plant tissue and not what has already moved into the tree . There is also a high risk of fruit russetting depending on the type of copper used so this treatment is best for non-bearing trees.

With the humid or wet weather this week, active fire blight cankers could be found oozing (Fig 2). It is best to do any hand labour activities such as thinning, pruning or removing suckers in dry conditions in these blocks to avoid the potential spread of bacteria by workers or contaminated equipment.

More apple scab lesions continue to develop on fruit. In some blocks where there was a lag in fungicide cover, significant leaf scab can be found. However, many growers are surprised to find lesions on the fruit as leaves have remained clean from scab until now. Deformed fruit around lesions indicate infection occurred in initial fruitlet development, likely with the prolonged rains during early petal fall.

As effects of chemical thinners are starting to show, some growers are choosing to hand thin out the scabby fruit.  However, there is a very good chance some lesions will be overlooked. If you see any sign of scab in your orchard, maintain a good summer fungicide program to prevent secondary infection and pinpoint scab at harvest.

Areas that haven’t seen much rain in recent weeks continue to also find powdery mildew strikes in susceptible varieties. In many areas, terminal bud set has begun. Mildew requires actively growing tissue to complete its cycle so fungicides should continue until terminal growth stops. Infected shoots will not cause damage to fruit at this point in the season. Fruit damage is a result of early infection that occurred in the flower buds. However, infected shoots will be the inoculum source for next year. Prune out any strikes that are present.

Figure 3. Apple scab on developing fruitlets.

Figure 4. Powdery mildew infected terminal.

Although symptoms of fly speck and sooty blotch often don’t appear until midsummer to harvest, infection actually starts occurring around petal fall. Fungicides applied during primary scab infection period would have had efficacy on fly speck and sooty blotch. However, now that many growers are beginning to relax back on scab management for the summer, it is important to still consider fungicide sprays for summer diseases, especially since fly speck and sooty blotch appear more frequently during years with cool, wet springs.

Reports of tree collapse have started to increase this week. Seemingly healthy young trees are suddenly dying not only in blocks that had collapse issues last year but also in a number of new orchards. Pennsylvania State University has named this Sudden Apple Decline (SAD) due to the sudden collapse of trees from the time first symptoms appear to tree death.  Unfortunately, very little is known about the cause for tree collapse. The best thing to do is minimize tree stress and remove potential sources of inoculum such as fire blight and black rot where possible.

Typically what is being seen in Ontario this year includes:

• Young (3-8 year old) dwarf trees, typically Gala/M9 rootstock
  • Some collapse on M26 rootstock was observed later in the season in 2016
  • Other varieties affected including Honeycrisp, Ambrosia, Empire, Golden Delicious and Mutsu
• Dead or declining trees intermixed in a block with healthy trees (Fig 5)
• Leaves develop a pale yellow hue, and then rapidly turn purple or reddish just before the tree collapses (Fig 6)
• Purple canker present with necrosis, or dying tissue at the graft union (Fig 7)
• Below the graft union, rootstock and root system appear healthy and often send out a large number of root suckers
• Collapse occurs in blocks as early as bud break to trees with good fruit set and normal crop load

Figure 5. Sudden Apple Decline sees dead or declining trees intermixed in a block with healthy trees.

Figure 6. Leaves turn pale yellow just before collapsing with Sudden Apple Decline.

Figure 7. Purple cankers at graft union are often associated with Sudden Apple Decline.

Insects

Flagging caused by oriental fruit moth was found this week (Fig 8). Young larvae of the first generation can tunnel into new shoots and cause terminals to droop and turn brown. This can sometimes be mistaken for fire blight. Examination of affected shoots often reveal a small entry hole with frass (insect poop) present.

The spray window for obliquebanded leafroller has opened for the early regions and will continue across the province over the next week or so. It has been difficult for growers in the southwest to apply a control with the frequent and strong winds.

Heavy rains have many growers concerned about residue wash-off, particularly with codling moth cover sprays. In general, 2 inches of rain will removed residues for all products and require immediate re-application. With less rain, there are some insecticide classes such as the spinosyns (Delegate, TwinGuard) and diamides (Altacor, Exirel) that have a high rainfast rating.

It’s important to note, if re-application is required, use the same product within a generation for resistance management. Codling moth is still in the first generation window. Second generation activity will not begin for another few weeks – typically early August. Rotate to another insecticide class at that time to ensure each generation is only exposed to one active ingredient.

This seems to have been the year for rosy apple aphid. The cool, wet spring was ideal for aphids and many orchards across the province could easily find rosy apple aphid colonies. At this point, the aphids are no longer active. However, evidence of their feeding is still present with backward curled or crinkled leaves forming tight clusters. There is also a significant amount of small, deformed fruit (Fig 9). Rosy apple aphid have a toxin in their saliva which serves as a “stop drop”, preventing the fruit’s abscission (natural separation from the tree). This is why aphid-infested clusters are so difficult to chemically thin and why fruitlet growth becomes irregular.

Damage from other petal fall insect pests continues to be reported though pressure is relatively low overall. Fruitlet damage from mullein bug (Fig 10), tarnished plant bug (Fig 11) and plum curculio (Fig 12) can be found. The next generation of apple leafcurling midge has started with new leaf curls.

Figure 10. Mullein bug injury on apple.

Figure 11. Tarnished plant bug injury on apple.

Figure 12. Plum curculio injury on apple.

San Jose scale crawler activity is on-going. Very little fruit damage has been reported yet. However, there is typically a lag between initial crawler emergence and signs of damage as the crawlers have to move to developing fruitlets or new shoots, attach and begin feeding. San Jose scale is the only scale pest of apples that contains a toxin in their saliva, causing red halos to develop around feeding site. At this point, these halos can be relatively faint (Fig 13) but will darken over time.

Japanese beetle activity has started. Adults are metallic green with coppery red wings (Fig 14) and feed on leaves, creating a skeletonized appearance. Damage typically starts at the top of the canopy and moves down.

The interesting (but difficult) thing about Japanese beetles are that they produce aggregation pheromones. As well, feeding induces damaged leaf tissue to release volatiles. These two things attract other beetles to the area to feed and mate. Fortunately, Japanese beetle is relatively easy to manage. Most products used for codling moth and apple maggot covers will have efficacy on this pest, too.

Monitoring for brown marmorated stink bug is being carried out across the province again this year. Adults have already started to be trapped in the hot spot areas of Niagara and Hamilton. Relative to previous years, catch is rather large  for this time of the season. At this point, we are not sure if those numbers are the result of a larger than normal overwintering population due to the mild winter, or a function of the new lure combination we are using in 2017. Regardless, those adults are sexually mature and are laying eggs.

For apples, this is generally not a pest of concern until closer to harvest but it is never too late to add stink bug to your monitoring program. For more information and regular updates, go to ontario.ca/stinkbug.