Put Out the Fire – Part 2: Managing Blossom and Shoot Blight

In the February issue of Orchard Network Newsletter, Put Out the Fire – Part 1 looked at rootstock fire blight, overwintering management and preparing for dormant copper applications in early spring.

Ideal timing for dormant copper is between silver tip and ½” inch green but residues need to last until at least pink when cankers are most active. David Rosenberger, Cornell University (retired) suggests fixed copper products such as copper oxychloride (Copper Spray), basic copper sulphate (Copper 53 W) and copper hydroxide (Parasol) are less soluble or prone to wash-off so would provide the longest residual activity.  At this point, most regions in the province are beyond this stage and phytotoxicity can occur if these products are applied. However, the use of a softer copper registered for season-long control such as Cueva could be extended into those early spray timings of tight cluster or pink in blocks where fire blight cankers have been a problem or there are concerns of wash-off.

When it comes to in-season fire blight management, there are two critical factors for success: 1) prevent blossom blight, and 2) prevent shoot blight. Control of blossom blight does not mean you are out of the woods for shoot blight. In this article, we will take a deeper dive into summer management including risk prediction, control products and pruning.

What’s the Risk?

Figure 1. Overwintering fire blight canker with amber-coloured ooze on trunk of apple tree in April 2021.

Fire blight bacteria overwinter at the edges of cankers. In the spring, as temperatures increase above 18ºC, cankers become active and droplets containing high numbers of bacteria ooze out of infected bark tissue (Figure 1). The ooze can be rain-splashed or carried by insects to open blossoms and developing shoot tips within the orchard. The bacteria can also move into the orchard from nearby infected ornamental and wild hosts or neighbouring orchards, making fire blight a significant regional issue.

In order for infection to occur, three key events must happen simultaneously:

  1. The bacteria is present.
  2. A wound or natural opening is created on a susceptible host.
  3. Weather conditions are favourable to disease development.

Open blossoms are the most susceptible tissue since they provide a natural opening for the bacteria to enter. Once the bacteria are introduced to an open blossom by wind, rain and insects, they multiply very rapidly on stigmas. Free moisture (e.g. rain, heavy dew, sprayer, overhead irrigation) washes the bacteria into the nectaries.

Fire blight bacteria are capable of existing in the orchard without causing disease symptoms. Once favourable conditions occur, bacteria multiply rapidly and enter susceptible tissue. The bacteria grow over a range of temperatures from 4 – 32ºC, with rapid multiplication leading to infection occurring most frequently when temperatures are between 24 – 28ºC. Infection risk is highest during extended periods of hot, wet weather as this favours the multiplication of the pathogen and encourages the succulent growth of susceptible plant tissue. That said, cooler temperatures when preceded by warmer temperatures can still be conducive to infection. If the cells are in the flowers from that warmer period, they can grow even if temperatures then cool down so risk remains high.

Interesting new research from the Sundin lab at Michigan State University found most new growth happens overnight between 10:00 pm and 2:00 am. This is likely because the drop in temperature at night results in the formation of dew, which encourages the growth of the fire blight bacteria.

Once infection occurs, the disease moves quickly into succulent new growth (Figure 2). In young vigorous plantings, movement of the pathogen from infected blossom to the root can occur in one month under favourable weather conditions (20 – 28ºC). With most infections, ooze is often one of the earliest and more obvious signs of infection (Figure 3 & 4). This can be seen within 3 days of infection, often in the early morning, during humid conditions or following a rain.

Shoot blight (Figure 5) can develop throughout the growing season either by spread from infected blossoms or oozing cankers. Growers can sometimes be caught off guard by shoot blight after having good blossom blight control. This is often an indication there are overwintering cankers that were missed during dormant pruning.

The fire blight bacteria enter the developing shoots through wounds made by foliar feeding insects, strong wind, hail or spread from contaminated equipment. It has also been determined that during period of rapid growth, there can be microscopic tears in the susceptible tissue that can act as entry points. As the season progresses, infection slows and overwintering cankers develop in the bark.

Predicting Infection

Infection can occur any time temperatures exceed 18˚C with high humidity, heavy dew or rainfall. According to George Sundin from Michigan State University, favourable conditions for growth can result in fire blight populations multiplying to one million cells per flower within 1-2 days. Imagine how quickly flies or bees could spread infection around an orchard with bacterial populations like that!

Prediction models such as Maryblyt and Cougar Blight are extremely valuable decision-making tools to determine conditions in the orchard that could lead to fire blight infection. This allows for precise application timing and strategy selection (i.e.., what product to use and when).

The Ontario fire blight prediction maps (based on the Cougar Blight model) for apple and pear are now available for the 2021 season. Bookmark these maps and predicted forecast for your area to prepare for potential infection periods. The risk is based on the 7-day weather forecasts from over 70 locations throughout the province and updated three times per week (Monday, Wednesday and Friday) until bloom period is over.

As with any model, the fire blight risk maps are a general guide. Environmental conditions may be more conducive for fire blight infection in a particular orchard than what is indicated by the maps. All apple and pear growers are encouraged to run either the Cougar Blight or Maryblyt model with data generated from their orchards for a more accurate prediction.

Control During Bloom

As mentioned earlier, the first factor to a successful fire blight management program is control of blossom blight. Depending on the level of infection risk and percent bloom, control options include antibiotics, copper, biologicals and systemic acquired resistance (SAR) inducers.

Antibiotics

The antibiotics, Streptomycin and Kasumin provide the most effective control for blossom blight during periods of high infection risk. Frequent protective sprays may be needed during extended risk as new blossoms are opening daily and will not be protected by sprays made while these blooms were closed. As well, antibiotics degrade rapidly in sunlight so are only active for 2-3 days MAXIMUM. Apply these products as close to a wetting event as possible. For resistance management with antibiotics, rotate between Streptomycin and Kasumin.

During the OFVC Educational Series fire blight webinar on February 3, 2021, Dr. Sundin recommended timing antibiotic applications for the evening before high risk to account for the rapid bacterial growth that seems to occur overnight. He also suggested limiting use of antibiotics during early bloom when risk is not as high. Instead, try targeting for the last spray coming out of bloom to be Streptomycin.

Biologicals

The use of antibiotic-alternatives in early bloom has become more widely discussed for fire blight management in recent years. The early timing for products like biopesticides is important for several reasons:

  • Most of these products work by competitive exclusion, inhibiting growth or triggering a defense mechanism in the plant. Time is needed for colonization and establishment.
  • Many of these products provide only limited control under high pressure or during certain environmental conditions. These are a good substitute during low to moderate infection risk.
  • Using these suppression products when there are few blooms and risk is lower, allows you to save your uses of antibiotics like Streptomycin and Kasumin for peak bloom and high infection risk events.

Research trials by Quan Zeng (Connecticut Agricultural Experiment Station [CAES]), Dan Cooley (UMass) and Neil Schultes (CAES) looked at non-antibiotic control methods such as Double Nickel, Blossom Protect, Bloomtime, Cueva and Oxidate over 4 years on Red Delicious. Blossom Protect consistently out-performed the other products as a stand alone program and was not significantly different from Streptomycin control in some years. This efficacy was only improved with the addition of the surface sterilant, Oxidate. The recipe for the most effective non-antibiotic method they found were:

  • Early to full bloom – 2 applications of Blossom Protect
  • Full bloom – 1 application of Oxidate
  • 24 hours after bloom – 1 application of Oxidate, if pressure is high or if there is a history of fire blight in the orchard

Trials out of Oregon State University (Ken Johnson) for organic apple production recommend the use of Blossom Protect early to full bloom similar to above, followed by Serenade OPTI from full bloom through petal fall every 2-4 days. Keep in mind, we have a much wetter climate in the northeast so may see differences in consistency or efficacy.

Copper and some fungicides are not compatible with certain biologicals. Check the label before use.

Systemic Acquired Resistance Inducers

Products that do not directly affect the pathogen but rather elicit a response from the host plant are known as systemic acquired resistance (SAR) inducers. These inducers activate plant defense mechanisms such as the production of thicker cell walls or anti-fungal proteins.

Lifegard WG was recently registered in Canada for suppression of fire blight. Initial plant defense response occurs soon after application, but 3-5 days are required to attain maximum level of protection. This product is toxic to bees to applications should be made at pink and again beginning at petal fall.

Figure 6. Secondary, or rattail bloom in apple can pose a significant fire blight risk if left unmanaged.

Secondary Bloom

Secondary blossoms, referred to as “rattail” bloom (Figure 6) often open when environmental conditions for infection are high or extreme. Since they open much later than the majority of blossoms, they are often unprotected and therefore extremely vulnerable to fire blight infection. If possible, remove secondary blossoms immediately since they rarely produce marketable fruit and are more of a liability. Otherwise, continue with a bloom control program when a forecasting model predicts that there is a fire blight infection risk. Be mindful of preharvest intervals when using these products.

Managing Shoot Blight

Shoot blight management strategies include:

  • Plant less susceptible varieties where possible
    • For more information, see Table 3-8. Disease Susceptibility Ratings of Common Apple Cultivars in the 2021 Publication 360A, Crop Protection Guide for Apples.
  • Plant blocks such that more resistant varieties surround susceptible ones
  • Control blossom blight
  • Prune out cankers and infected tissue (see ‘Summer Pruning’ below)
    • Limit any hand labour activities such as thinning, pruning or removing suckers to dry conditions to avoid the potential spread of bacteria.
  • Apply prohexadione calcium (Apogee, Kudos) to manage vigour, except on Empire, Stayman or Winesap (see ‘Prohexadione Calcium’ below)
  • Avoid excessive succulent growth caused by excess nitrogen
    • Apply nitrogen fertilizer only where warranted by annual leaf analysis.
    • Consider a split application of nutrients, half in the spring before growth starts and half again after petal fall. If severe blossom blight occurs, withhold the later application.
    • Similarly, avoid late cultivation that can make nitrogen available to the tree resulting in late succulent growth.
  • Maintain good insect control, especially plant-sucking insects such as leafhoppers, aphids and plant bugs.

Prohexadione Calcium

Prohexadione calcium, or Pro-Ca (Apogee, Kudos) use for fire blight management is becoming more common in the province. Most growers try to aim for king bloom petal fall for management of fire blight shoot infection with subsequent applications at 2 week intervals. However, some growers are applying Pro-Ca earlier between pink to bloom if there is terminal growth.

This product works by:

  1. Reducing vegetative growth of susceptible shoots and limiting spread of fire blight infection
  2. Thickening the cell wall to provide a physical barrier from bacterial infection
  3. Stimulating plant defense system and production of antimicrobial compounds to reduce infections.

During the OFVC fire blight webinar, George Sundin described Pro-Ca as the best tool for fire blight as it provides physical barrier and no chance for resistance development.

Following the Apogee or Kudos label, Pro-Ca can be applied between 2.5-5.5 cm of new shoot growth. That early range can easily occur prebloom depending on the spring.

Since the use of Pro-Ca has been known to reduce shoot blight largely due to the thickening of the cell walls within the shoot, recently published research by Anna Wallis and Kerik Cox from Cornell University proposed Pro-Ca could do the same to reduce blossom blight by thickening the cell walls of blossom pedicels. With their trials on mature Gala, they saw over 70% control of blossom and shoot blight with a pink application of Pro-Ca and minimal effect on tree growth.

Trauma Blight

Unsettled weather common in Ontario summers can bring hail, strong winds or heavy rains that could trigger trauma blight. These can 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, Cueva or Cueva + Double Nickel 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). New research is now suggesting 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.

Like all coppers, there is the risk of russeting with Cueva. However, if fire blight is active in your orchard at the time of trauma, better to lose some fruit this season than trees later.

Because Pro-Ca will take 7-10 days to activate, Apogee or Kudos should not be used as a trauma blight treatment. However, if the trees are on an existing program, this can slow the spread of infection in a tree.

Summer Pruning

When trying to decide when the best time to prune out fire blight infection, Figure 7 summarizes some key points to consider based on tree age and vigour.

Figure 7. Fire blight pruning decision tree.

Keep in mind that once the terminal buds have set, fire blight stops spreading within infected trees. Summer pruning in vigorous trees can encourage new shoot growth and extend the susceptibility period for shoot blight. However, leaving the disease in the orchard may serve as a source of inoculum, and cause increased infections in the result of hail, severe storm, or high wind damage.

If you decide to prune out fire blight strikes, remember:

  • Cut at least 30 cm (12 in) beyond the visible infected tissue, preferably into 2nd or 3rd year wood. However, recent trials by Kari Peter, Penn State University (PSU) and Ken Johnson, Oregon State University (OSU) found significantly more aggressive cuts greater than 75 cm (30 in) yielded best results.
  • Only prune out strikes when there are 2-3 consecutive days of low humidity and temperatures below 25°C to allow pruning wound time to heal without conditions conducive to disease spread.
  • Pruning flush to another healthy limb or spur may result in more cankers based on PSU and OSU trials. Where possible, leaving a pruning stub.
  • When pruning in-season, disinfect pruning tools between each cut by soaking them in a solution of 1 part bleach to 5 parts water or in a 65-70% alcohol solution.
  • In sections where trees are severely affected or trees are continuously infected year after year, it may be more cost-effective to simply remove the whole tree.

Remove Pruning Cuts

If time and labour allows, remove any small pruning cuts from the orchard as soon as possible and burn. Take extra caution during removal to not touch branches to healthy trees along the row. To prevent potential spread of bacteria, some growers immediately bag cuts, while others place cuts directly in a covered wagon.

For large cuts or if limited on time, prunings can be tossed in the row middles and dried thoroughly before flail mowing. The general research community suggests dry cankers do not present a danger to spreading the disease. However, trials at OSU sprayed cankers on pruning cuts with red paint, flailed and returned 30 days later. Fire blight bacteria and ooze could still be found on the painted wood.

References

Cox, K., DuPont, T., Johnson, K. and Peter, K. 2021. The Latest Fire Blight Research. Washington State University, Tree Fruit Days (virtual).

Sundin, G. 2021. Fire Blight Management in High Density Orchards. Ontario Fruit & Vegetable Convention Educational Series: https://youtu.be/MOSdGtd0R0w

Wallis & Cox. 2020. Management of Fire Blight Using Pre-bloom Application of Prohexadione-Calcium. Plant Disease: 104. DOI https://doi.org/10.1094/PDIS-09-19-1948-RE

Zeng, Q., Cooley, D. and Schultes, N. 2019. Fire Blight IPM Using Non-Antibiotic Control Methods. Northeastern IPM Center: The IPM Toolbox Webinars: https://www.northeastipm.org/ipm-in-action/the-ipm-toolbox/fire-blight-ipm-using-non-antibiotic-control-methods/

Kristy Grigg-McGuffin
Kristy Grigg-McGuffin

Horticulture IPM Specialist, OMAFRA