With bloom across the province, fire blight is top of mind for many. But with the cooler weather, risk may not be high despite the rains in the forecast. To help in decision-making, the Ontario Fire Blight Prediction Maps are available and continue to predict 7-day risk across the province.

Fire Blight Prediction Maps

Here are a few quick reminders to deal with this disease:

What is the risk?

The risk of fire blight infection is possible if the following conditions are met:

1. Flowers are opened and/or are opening – There is no risk of infection if bloom is not present in the orchard despite the fire blight maps stating caution to extreme; however, keep an eye on any early signs of bloom as it can happen quickly.
2. Enough heat units have accumulate based on forecasting models – The fire blight maps use Cougarblight but this applies to MaryBlyt as well. Similar to degree days, daily temperature is accumulated and must reach a certain point before risk increases.
3. A wetting event is predicted – Any type of wetting event will wash the bacteria into the open bloom including rain, dew or even the artificial rain shower you create with the sprayer during critical infection periods.

So far this season, fire blight infection risk has remained low to moderate with the cooler average daily temperatures. Ideal conditions for rapid multiplication of the fire blight bacteria is above 18C. Research suggests bacteria are capable of surviving on open flowers for several days so it is possible for infection should a wetting event occur, even with potentially cooler conditions, though likelihood is much less. If there is a history of fire blight in your orchard or neighbourhood, keep a close eye should conditions change.

What causes infection?

Fire blight bacteria multiply rapidly in a very short period of time when temperatures are above 18⁰C and relative humidity above 90%. Bacteria can be transferred to new blooms by rain, wind or insect. Bees and other pollinators are very active in the orchard right now and can easily spread the bacteria around.

Sudden, sporadic or isolated showers, heavy dew, fog or spraying can all provide enough moisture to wash bacteria into an open bloom. How much water is actually needed? If you look at the requirements for blossom blight infection based on the Maryblyt model, a wetting event is >0.01” (0.25mm) rain, heavy dew or fog sufficient to wet foliage and provide thin film on tissue surface or rain of >0.1” (2.5mm) the previous day.

When does blossom blight risk end?

Susceptibility to fire blight decreases as the blossom ages and calyx begins to close so following petal fall blossom blight risk ends. The staggered bloom that some growers are seeing this year within blocks (and even within an individual tree) will extend the risk until the last blossom falls.

However, in areas of the province that are seeing petal fall, growers should be aware of delayed bloom, particularly young trees or rat-tail bloom. Protecting these open blossoms from infection will be critical during times of high risk. We have seen situations in previous years where blocks have been severely affected by fire blight as a result of rat-tail bloom infection.

If time and labour are available, it may be best to go through the orchard every couple of days and remove the rat-tail blossoms by hand. Be sure to remove blossoms in dry weather to prevent the spread of infection. For many operations, this practice is just not viable. Instead, continue to monitor infection risk and apply protectants if needed.

Blossom blight management

Antibiotics

Frequent protective sprays may be needed if risk increases and remains high 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.

If there are any concerns about missing a potential infection event, Streptomycin provides kickback activity but should be applied within 12 hours after the event.

Antibiotic alternatives

Consider alternatives to antibiotics if risk is low to moderate such as a year we’re experiencing but fire blight has been an issue in your orchard before. These include:

• Biologicals – eg., Blossom Protect, Buran, Serenade, Regalia or Double Nickel
• Surface sterilant – eg., Oxidate
• Copper labelled for in-season use – eg., Cueva – note that copper and some fungicides are not compatible with certain biologicals. Check the label before use.

The early timing for products like biopesticides is important for a number of reasons.

• Firstly, because most of these products work by competitive exclusion, inhibiting growth or triggering a defense mechanism in the plant, they need time to colonize and become established. A good approach to timing for biologicals is when risk is coming in the next 3-4 days.
• Many of these products provide only limited control under high pressure or during certain environmental conditions.
• 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 (50-80% bloom) and high infection risk events.

Postbloom management

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.

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. King bloom petal fall is the ideal timing for applying these products (typically when growth is 2.2 – 5.5 cm long). Studies have found Apogee applied at a high rate on young trees provides good control of shoot blight without significantly impacting growth.

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.