Apples Diseases Insects Pest Management

Apple Pest Update: June 2, 2022 (with audio)

Warm, humid weather increases risk and development of key apple pests including fire blight and San Jose scale

Listen to the audio article here:

Since last week’s update, we’ve yo-yoed back to hot, humid conditions across the province as most regions enter into fruit set and sizing.

Developing Gala fruitlets

These are ideal conditions for disease and insect development. Continue management for scab, rust, black rot and bitter rot during rainy periods; powdery mildew when its hot and dry. Warmer days and nights will also encourage insect development and emergence so be prepared for increased activity of late petal fall to first cover pests such as mites, leafhopper or plum curculio.

Today, however, I wanted to focus specifically on two key pests on the radar this week: fire blight and San Jose scale.

Fire blight

Blossom Blight

The Ontario fire blight prediction maps continue to be updated regularly. With the warmer weather we’re experiencing, conditions have been optimal for fire blight infection.

Regions that are at fruit set should be diligent and monitor their orchards closely for new plantings that may still be blooming or for secondary, or “rat tail” blossoms. There have been a number of reports of fairly substantial straggling bloom or rattail bloom in many varieties including Honeycrisp, Ambrosia, Gala, Empire, Cortland and Crimson Crisp. Take necessary action to remove or protect open blossoms from infection. Generally, continuing a protective program for a couple of weeks following petal fall will help ensure any late blooms are covered. Keep in mind, the antibiotics (Streptomycin, Kasumin) provide control for only 2-3 days prior to a rain event. So for extended infection periods, a subsequent spray may be needed. Also be mindful of preharvest intervals when using these products. For instance, Kasumin has a preharvest interval of 90 days.

Secondary, or rat tail bloom in Ambrosia

However if there are a few days of dry weather and labour allows, it may be best to go through the orchard and pinch off the rat tail blooms altogether since the fruit produced by the late blossoms are more of a risk than they are an asset.

According to Dr. George Sundin from Michigan State University, temperatures conducive for growth can result in fire blight populations multiplying to one million cells per flower within 1-2 days. Imagine how quickly bees or flies could spread infection around an orchard with bacterial populations like that, even if there are just a few blooms!

Shoot Blight

Getting through bloom with no sign of infection, unfortunately, does not mean you are out of the woods. Symptoms of shoot blight can develop without blossom infection occurring if the pathogen is present in or around the orchard (ie. neighbouring farms). With the warmth and rains, trees will be going through extensive growth with rapidly expanding leaves. During this time of rapid growth, microscopic tears in the leaf tissue can be enough of an entry point for fire blight bacteria to get into the shoot and cause an infection.

Shoot blight on Idared

In terms of shoot blight management, prohexadione-calcium, or Apogee/Kudos is an important management tool to help slow the growth of these vigorous shoots and reduce the spread of fire blight. The use of LifeGuard, a Bacillus product, can also be picked up again now that most orchards are out of bloom or hives have been removed. This product works by activating the tree’s defense mechanism and is most effective when it is applied before infection occurs. Some level of response can occur shortly after application but ideally 3-5 days are needed for maximum level of protection. It’s important to note that since this is a biological product, it should not be tank-mixed with antibiotics such as Streptomycin and Kasumin.

Damage caused by leaf-feeding insects can also create more susceptible shoots to bacteria invading. As well, it is possible these little pests may also contribute to bacterial spread from one developing shoot to the next. For instance, I have seen situations where shoot blight development in an orchard has correlated with potato leafhopper movement and damage. Carving out a small piece of time each week – even with the pressures of thinning this time of year – to regularly monitor insect activity is critical to preventing or managing the spread of fire blight.

Fire blight infected terminal with ‘hopperburn’ damage caused by potato leafhopper.

The unsettled weather may also bring hail, strong winds or heavy rains that could trigger trauma blight. These could be very isolated events, occurring in one area but not another nearby. Assess for signs of damage in all blocks following any storm. Winds and rain can spread inoculum while injuries caused by these events provide an entrance for the pathogen and shock or traumatize trees, reducing their natural defense mechanisms. Shoot or fruit blight can develop as a result.

Be sure to apply Streptomycin or copper as soon as possible after the trauma event, especially if foliage damage is extensive or fire blight is known to be active in the orchard (ie., cankers, etc). Ideal timing is 4 to 12 hours after a storm. Anything applied later runs the risk that the bacteria will have already established and begun to grow, reducing the efficacy of the product.

Soon you will see if the fruits (pun intended) of your labour paid off or if fire blight symptoms develop. Scout your orchard for wilting flower clusters or the characteristic “shepherd’s crook” every week until terminal bud set.

San Jose scale

Crawler emergence for 1st generation San Jose scale can be predicted using a degree day model with a March 1st biofix and base temperature 10C. Based on current degree day accumulations, crawler emergence is predicted to begin at the end of next week in the earliest regions of the province.

Small yellow San Jose scale crawlers on trunk.

The table below highlights the predicted timings for various locations using the 14 day forecast for the respective regions. These dates will continue to be updated as timings approach. We are also actively monitoring for crawlers in a number of Simcoe orchards to validate model predictions.

Region
Current Degree Day Celsius (DDC)
Predicted Crawler Emergence (278 DDC)
Harrow
228 DDC
June 8, 2022
London
179 DDC
June 16, 2022
Simcoe
199 DDC
June 13, 2022
Grimsby
192 DDC
June 14, 2022
Clarksburg
166 DDC
14+ days out
Newcastle
173 DDC
14+ days out
Degree day accumulations based on March 1st biofix and base temperature 10C

Once crawler activity (ie., movement to new shoots and developing fruit) begins, this will continue for 4-6 weeks. Registered products including Closer and Sivanto Prime should be applied at the beginning of crawler activity and reapplied 10-14 days later due to the extended generation. Movento is a slow-acting product and should be applied at least 1 week prior to predicted crawler emergence, again followed up 10-14 days later. Depending on the product, this timing may also have efficacy on woolly apple aphid and/or leafcurling midge.

For orchards with historically high scale pressure, even with dormant oil applied, in-season crawler management may be needed for a number of years until pressure can be reduced. Of the summer management, 1st generation is certainly the most critical timing. Management of the second generation, which generally emerges in early August, would be warranted if signs of scale damage on developing fruitlets is observed in July.

San Jose scale damage on developing apple.

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