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Spring is not taking it’s time this year and bud break is already beginning in early regions of the province. Thankfully, things have slowed down this week compared to last which buys some time for preparation. Now is the time to get dormant pest management strategies in place to start the season out right!
The first line of defense for pest management in an orchard is prevention. There is no silver bullet to eradicate something like disease once it becomes well established. This means management is a year-long process to reduce inoculum in the orchard and prevent the spread to healthy trees or fruit.
- Prune out dead or diseased branches and rotten or mummified fruit that often habour overwintering pests, such as fire blight, black rot or bitter rot.
- Get rid of wood or cull piles and stumps that may have provided overwintering sites for insects, such as codling moth and plum curculio.
- Clean up the orchard floor of leaves, branches and fruit.
If you haven’t already, early spring is a good opportunity to clean up any remaining infected limbs, cankers or – in the case of rootstock blight – trees that will act as sources of inoculum and get as much fire blight out of the orchard as possible. These overwintering cankers will be the source of disease in the coming year as the bacteria is sitting in vascular tissue in the canker margins. As the tree begins to push in the spring, these cankers become active and the bacterial cells start to multiply. Generally, as temperatures increase above 18C, the enlarging cankers start oozing bacterial-laden sap that may or may not be visible. The bacteria are then carried by insects or rain splashed to developing tissue – whether that’s blossoms or shoot tips.
Dormant pruning is effective for many reasons. Firstly, the fire blight pathogen, Erwinia amylovora is not active in the cold temperatures and therefore will not spread on pruning shears, tools or infected tissue. This means there’s no need to disinfect tools between cuts like with summer pruning. Secondly, the cankers are easier to see in the orchard without foliage blocking or shading your view.
What to Look for When Pruning
Spend some time to thoroughly scout for cankers while pruning. Try to do this more than once during the early spring and at different times during the day to have different light direction. While this does take additional time, that extra work may save you trees down the road by removing inoculum sources.
Shoot blight can be easily identified by the characteristic shepherd’s crook, or J-shaped limb. Since the limb is dead, blighted leaves often remain on the limb throughout winter.
Cankers may not be as obvious. They can vary in size on twigs, limbs or trunks of trees. Typical characteristics of fire blight canker include:
- Sunken or wrinkled area with a narrow ridge along the margin
- Rough or darkened bark often located around a spur, wound or pruning stub
- May develop cracks within or around the margins
- Inner bark of canker may appear reddish brown and ooze a sticky amber substance during the spring and early summer, especially in humid conditions
- Black sooty mould can grow on the sugary substances, giving the canker a black appearance
- May become colonized by other fungi such as the black rot fungus, Botryosphaeria obtusa
Making a Cut
Pruning cuts should be made at least 30 cm (12 inches) beyond the canker or visible infected tissue into 2nd year wood. When the canker extends near or up to the main leader, you will need to decide whether to:
- Risk making just the limb cut and hope the bacteria hasn’t spread further.
- Cut the trunk below the canker and encourage a new leader.
- Depending on various factors including tree age, size, vigour, etc, leave as is and see what happens during the season.
However, for young trees (less than 3 years old), trees that appear to be infected perennially or trees that have the canker established in the trunk should be removed altogether.
Mulch limbs in the row middles in the early spring if you don’t want to remove limbs entirely to burn them. An application of urea applied at this time for apple scab may help with degradation as well.
Trees with rootstock blight should be removed as soon as possible to reduce any chance of further spread. Cut off and remove the scion from the orchard, making sure to leave enough to allow the roots to be pulled out. Leave the remaining trunk to let the roots dry out prior to removal before the start of the season.
It’s time to also start thinking about in-season fire blight management as bloom can come quickly. Having products ready and at your fingertips will allow you to act fast should conditions for infection occur. Don’t get caught unprepared!
- Have enough control product to cover all rows of susceptible blocks every 3 days during bloom.
- Become acquainted (or refresh your memory) with forecasting models such as Cougar Blight or MaryBlyt if you will be running these for your farm. Otherwise, bookmark the Ontario Fire Blight Prediction Maps in your browser for quick reference during bloom. These maps are updated 3 times per week with a 7-day predicted risk forecast.
- Make a plan for management
- What products you will use? Consider your options: antibiotics (Streptomycin, Kasumin), coppers (Cueva), biologicals (Blossom Protect, Serenade, Double Nickel, Lifegard), Apogee, etc.
- How will you use them? Keep in mind, the newly registered Lifegard should be applied prior to bloom and again at petal fall. Biologicals often need to be applied preventatively ahead of an infection to give time to multiply, invade the area, initiate a plant response, etc. A good resistance management practice to maintain the life of antibiotics is chemistry rotation.
- Don’t forget to consider your action plan in case of trauma blight.
But first and foremost, be prepared to apply a dormant copper spray at silver tip to ½” green to help protect the spread of bacteria from any oozing cankers that were missed during dormant pruning.
The effectiveness of a dormant copper spray really comes down to how it is applied and post-application weather. Copper provides an unfriendly environment over the bark and bud surfaces of the tree, preventing bacteria from getting established or spreading. Thus, it must be applied as a high-volume spray to ensure sufficient coverage.
Dormant copper such as Copper Spray, Copper 53W, Cueva and Parasol can safely be applied up to ¼” green (possibly ½” green) without risk of phytotoxicity. However, the use of a softer copper registered for season-long control such as Cueva could be extended in those early spray timings to ½” green or tight cluster in blocks with low scab inoculum (ie., free of scab last year) to provide some scab protection.
Residual activity typically last about 7-10 days under ideal spring conditions. However, once rainfall exceeds 2” from last copper application, it should be assumed all residue has been washed off.
Using 1-2% dormant oil, unless the label states otherwise, will act as a sticker/spreader to help with dormant copper as well as provide efficacy on scale, European red mite and suppression of powdery mildew. However, Cueva is formulated with a fatty acid so there is little benefit to adding oil as a sticker for this product. Keep in mind if using dormant oil, do not apply captan or sulphur products within 14 days of application since oil can enhance penetration of these products into sensitive tissue, resulting in phytotoxicity issues. There are other cautions around the use of oil, which will be discussed below.
Practical and inexpensive, inoculum-reducing strategies such urea and mowing can be an important component to an effective scab management program and contribute to the reduction of overwintering ascospores that will infect green tissue at the start of the season. This is especially important given the reduced arsenal of available fungicides effective against scab beginning this year. Table 1 summarizes research from New Hampshire that looked at the impact of inoculum pressure on primary scab infection the following spring. An orchard with 20% overwintering leaf scab has the potential to produce 7,000 times more ascospores than an orchard with less than 1% overwintering leaf scab (Gadoury and MacHardy, 1986).
Table 1. Primary scab infection activity based on overwintering inoculum levels, New Hampshire1
Leaf scab in previous fall (%)
Total ascospore produced/acre in spring (‘000)
Ascospores/acre released at green tip (‘000)2
Potential lesions/acre from green tip infection period3
You can save time and money usually invested in fungicide applications by reducing the initial scab pressure going into the season:
- In a typical year, only a small proportion of ascospores are actually mature early season. Therefore, reducing (or eliminating) the risk of infection, and potential need for chemical control, at green tip.
- Scab spores do not travel far and most infections start from within the orchard. Again, getting the inoculum out greatly reduces the risk of infection.
- Most fungicides tend to work better in low-inoculum orchards, particularly when dealing with our Ontario spring weather.
- The trend in new product registrations is towards single-site fungicides which have high resistance potential. Reducing scab inoculum means less selection pressure placed on these products (ie., longer life of these products).
Urea works in a number of ways:
- It directly inhibits the development of ascospores.
- It stimulates the growth of naturally occurring organisms that are antagonistic against the scab fungus.
- It facilitates the breakdown of the leaves.
Since the early 2000’s, research has shown this practice to be effective in reducing over-wintering spores. Research from the University of New Hampshire showed a 97% reduction in ascospore productivity in leaves sprayed with 5% urea just before leaf-fall, 50% reduction when urea was applied to the leaf litter when approximately 95% of the leaves had fallen, and 70% reduction when urea was applied to the leaf litter in spring (Sutton et al., 2000).
If you haven’t already done so this spring, apply 45 kg of agricultural urea per 1,000 L of water/ha to the orchard floor before bud break.
In addition to a urea application, scab inoculum can be reduced 80-90% by shredding overwintering leaves (Sutton et al., 2000; Vincent et al., 2004). Rake or blow leaves from under trees and shred them using a flail mower. This helps encourage leaf decay and may re-orient the leaves to prevent spores from discharging up into the trees.
Spring-applied dormant oil can seem at times a risky game to play, hoping the right growth stage will align with the right weather conditions. However, by preparing early for this spray and following the forecasted weather, dormant oil can be an effective tool for managing some rather difficult-to-control pests.
Over the years, there has been some question whether oil may reduce the overall health of the trees. To date, there is no scientific evidence to support these claims. Oil has been used for many years across apple growing regions of the world to control various overwintering pests with no ill effects on the health of the tree when properly applied.
How Does It Work?
Oil sprays work mainly by suffocation. Coating the insect – which means in a high-volume spray to reach all the cracks in the tree bark – prevents normal respiration from the air holes (spiracles) where they breathe. This works best on the immobile and immature stages where:
- The insect can’t move away to avoid the spray,
- The scale coverings have still not hardened and oil can penetrate,
- Respiration rate is the highest.
However, oil can also interfere with egg development, prevent settling of scale crawlers and deter feeding by pests such as aphids which is why summer oil programs are also worthwhile.
What Pests Does It Target?
There are several species of scale insects affecting apples; San Jose scale (SJS) is the most common in Ontario orchards. This insect overwinters as an immature scale under bark and emerges just prior to bud break. As the immature scales feed, they exude a waxy substance that forms a protective layer. Dormant oil sprays are the best timing for this pest before they develop that waxy covering.
Without the foliage to block the spray, dormant oil applications can get reasonable coverage of limbs and trunk where the overwintering SJS population is located. Targeting individuals at this stage will help reduce the population that will produce the summer generation crawlers. Postbloom management targets these crawlers which move from the infested area to maturing fruit. These sprays can be very effective at reducing the amount of fruit damage; however, they do not always provide good control of the crawlers that move elsewhere such as to new branches, a different spot on the trunk or to an adjacent tree. In other words, you could find yourself in a continuous cycle of managing fruit damage if the SJS population is not suppressed. While it may be hard to find time and good weather early season, an oil application is well worth it.
European red mites overwinter as eggs on roughened bark around the bases of buds and spurs, or in the inner parts of the tree close to the main trunk and branches. Oil sprays should be applied before egg hatch, between half-inch green and tight cluster.
While delayed dormant oil applications primarily target scale and mites, you may see some additional efficacy against other pests at this timing including impeding egg hatch and movement of aphids and some spring feeding caterpillars, interfering with egg laying and development of apple leafcurling midge and preventing release of overwintering powdery mildew spores as infected buds open.
When Is It Too Late For Dormant Oil?
Depending on the target pest, the term “dormant” oil can be rather misleading as sprays can be applied from the true dormant state prior to bud break up until pink. Unfortunately, optimal dormant timing for scale is not necessarily the same for mites.
If monitoring indicates scale is a bigger issue in the orchard, oils need to be applied before or shortly after bud break. This efficacy against scale is significantly reduced with later oil applications for European red mite as they develop a waxy protective layer that impedes the oil from effectively penetrating and preventing respiration.
However, if European red mite populations are the problem, sprays can be delayed. Ideal timing is half-inch green to tight cluster but can be delayed to pink; however, blossoms can be quite sensitive to oil under adverse conditions so consider using a lower rate at this timing.
Original precautions around the use of dormant oil were developed prior to the refinement processes that are carried out now with the commonly registered products. Most impurities that were associated with phytotoxic effects with some of the older “heavy” horticultural oils are removed through extra filtration and distillation. If you have concerns with using oil, especially with sensitive varieties like Red Delicious, Empire, Mutsu and Ambrosia, consider a product such as Purespray Green Spray Oil 13E as it is registered for both dormant and summer use.
However, even highly refined “summer” oils can cause crop injury when they are applied:
- when temperatures are consistently below 4C
- within 48 hours before or after a freezing event
- in slow drying or prolonged wet conditions
- with or too close to products containing sulphur or captan
- do not apply oil within 14 days before or after these products
- above label rate
- 2% solution (20L/1,000L) for dormant sprays
- 1% solution (10L/1,000L) for summer sprays
- High water volumes are essential for good coverage
- to plants are under moisture stress
- when temperatures are very high (above 25°C)
Always read the product label for additional instructions and precautions.
Mild Winter & Pest Pressure
In a typical winter, insect populations often experience winter kill to some degree, depending on many factors within the micro-climate of the orchard, such as air drainage, snow cover and extreme temperature. However, in a mild winter like we just had with very few cold snaps, overwintering survival for most insects is very likely. Even with an extremely cold winter, favourable weather early in the spring like we’re experiencing can easily compensate for those pests that survived and may lead to early emergence. With a warm, dry spring, we could see a quick emergence of European red mite, rosy apple aphid, tarnished plant bug, apple leafcurling midge and mullein bug.
Mother Nature typically doesn’t give us much reprive from most overwintering disease in a typical year, as inoculum is often well protected from cold weather in infected branches or cankers (e.g., fireblight), in leaves on the orchard floor (e.g., apple scab), in dead wood or mummified fruit in the tree or on the ground (e.g., black rot, bitter rot), or in alternate hosts to be carried in from other areas on wind currents (e.g., rust).
The exception to this would be powdery mildew, which overwinters as mycelium in dormant fruit and shoot buds that were infected the previous season. Conidia grow out of buds in the spring as infected tissue and spread to other leaves causing secondary infection. Overwintering powdery mildew infected buds and shoots lack winter hardiness. In a cold winter with temperatures below -24°C, the survival rate is generally less than 5%. Unfortunately, with the mild winter, growers who had problems with powdery mildew last year should anticipate survival of those infected buds and be prepared for early prevention this season.