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The 2022 growing season is upon us. The warm days and nights has moved development along quickly in some areas. Trees in the earliest regions of the province are at green tip to half inch green, with the first primary apple scab event happening earlier this week. Later regions are beginning to see bud swell and break.
So, with the season on our doorstep, are you prepared for early spring pest management? Let’s go through a checklist of things to consider:
Orchard Sanitation: First Line of Defense
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. Instead, management is about reducing inoculum and preventing spread to healthy trees or fruit.
- Prune out dead or diseased branches and rotten or mummified fruit that are still on the tree. These habour overwintering pests, such as fire blight, black rot or bitter rot.
- Get rid of wood or cull piles and stumps that may have been in or near the orchard over the winter. These often provide overwintering sites for insects, such as codling moth and plum curculio.
- If the ground is dry enough to get the tractor through, mow the orchard floor of leaves, branches and fruit.
While you’re finishing up pruning, early spring is a good opportunity to clean up any remaining fire blight infected tissue like limbs, cankers or – in the case of rootstock blight – trees. 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 this spring, these cankers become active and the bacterial cells start to multiply. Generally, as temperatures increase above 18C, the enlarging cankers start oozing this bacterial-laden sap that may or may not be visible and 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 equipment 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.
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.
For more information on what to look for when pruning fire blight infected trees, see the February 2021 Orchard Network Newsletter article, Put Out the Fire – Part 1: Managing Rootstock Fire Blight.
Another this to consider this time of year for fire blight management is 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 tree surfaces, preventing bacteria from getting established or spreading. So, it must be applied as a high-volume spray to ensure sufficient coverage.
Dormant copper such as Copper Spray, Copper 53W, Cueva, Parasol and the newly registered Kocide 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. For some copper formulations, using a dormant oil will act as a sticker/spreader as well as provide efficacy on scale, European red mite and suppression of powdery mildew. This is not the case for all registered coppers. Always refer to precautions listed on the product label prior to use.
Reducing overwintering inoculum
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 not a new concept. It has been around for decades but for those growers not currently doing it, it may be something to really consider given the reduced arsenal of available fungicides effective against scab Ontario now faces. A study in New Hampshire in the 1980s 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).
You can save time and money usually invested in fungicide applications by reducing the initial scab pressure going into the season. This about this:
- In a typical year, only a small proportion of ascospores are actually mature early season. Therefore, reducing (or eliminating) the risk of infection, the need for chemical control at green tip is significantly less.
- 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, and
- It facilitates the breakdown of the leaves.
For the later areas of the province, 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 with a flail mower (Sutton et al., 2000; Vincent et al., 2004). This helps encourage leaf decay and may re-orient the leaves to prevent spores from releasing up into the trees.
Early season management
For those regions further along in development, consider the following for the first cover sprays for primary scab infection:
- Leaves are susceptible to infection as soon as green tissue is present, especially if you had scab in your orchard last year.
- Green tip to tight cluster is a period of extensive new growth. Keep covered with a good protectant fungicide program and re-apply every 5-7 days during periods conducive to disease development or following heavy (greater than 1”) rain.
- Protectant fungicides such as captan, folpet or mancozeb do not provide effective post-infection or anti-sporulant activity.
- That means if sprays are applied in less than ideal conditions, ie., windy, alternate rows, or washed off in rain, the risk of scab infection is increased.
- Ascospores mature slowly early season and infection takes longer, peaking over bloom period so plan to save your systemic scab products for when infection risk is greatest.
- For example, more than 20 hours of leaf wetness are required for primary ascospore infection at an average temperature of 5⁰C. This goes down to only 6 hours during average temperatures of 15-25⁰C.
- During cool, wet springs, protectant fungicides may not be enough. Consider tank-mixing with post-infection products such as Syllit, Scala, Inspire Super, Luna Tranquility or Buran.
This is another disease to look out for while pruning. Dormant buds infected with powdery mildew are typically feathered, pointed or shriveled and usually break dormancy later than healthy buds. This means susceptible, green tissue may already be present when the first conidia are produced. With the high powdery mildew pressure across the province last year and the relatively mild winter, be prepared for early season management of this pest.
Protectant fungicides used for early season scab management do not have activity on powdery mildew. Tank-mixing a low rate of sulphur (3-5 kg/ha) with a scab protectant, beginning at ½” green, will provide good activity on scab while also suppressing powdery mildew. This early season program will help free up other chemistries, such as the sterol inhibitors (Nova, Fullback, Cevya, Inspire Super), SDHIs (Aprovia, Fontelis, Sercadis, Excalia, Luna Tranquility) and strobilurins (Pristine, Merivon, Flint) for later use when plenty of new growth is present.
Some key points for effective powdery mildew control this year include:
- 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.
- Rain deters powdery mildew development by washing off spores. Instead, mildew thrives in dry weather and high relative humidity. So, protectant sprays may still be required during dry periods when there is little risk from apple scab.
- 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.
- Include a mildewcide, such as sulphur in all sprays beginning at ½” green until temperatures are greater than 25°C or when applying oil. Sulphur lacks post-infection activity, so must be applied early season. A tank-mix that includes captan, EBDC and sulphur provides excellent protection against scab, rust and mildew.
- If pressure was low last year, oil applied for mites may provide suppression of powdery mildew. Do not use captan- or sulphur-containing products within 14 days of an oil application.
- Where they are working, include fungicides from other groups, including sterol inhibitors, strobilurins or SDHIs during the critical infection period, generally at pink and petal fall.
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. Insect development and activity is driven by temperature; the milder it is, the faster the insect matures. This improves the efficacy of how oil works as well as ensures a lower risk of phytotoxicity concerns.
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, and
- 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, often referred to as a “delayed dormant” application.
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.
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 newer generation oil products such as Purespray Green, Suffoil-X or Vegol as these are 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
- restrictions also apply to Vegol Crop Oil with copper compounds
- above label rate
- higher labeled concentrations can be used for dormant applications vs summer applications (e.g., 2% solution vs 1% solution of Purespray for dormant vs summer use, respectively)
- 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.