Why is fire blight showing up in apple orchards during 2016

Fire blight symptoms have been present in many orchards this year even though fire blight protection products were applied during bloom time (Figure 1). There are many reasons why fire blight continues to show up in orchards this year. The Cougar Blight models predicted a high to extreme risk of blossom infection from May 24 through to May 30 across most of Ontario's apple growing regions. Growers should have applied fire blight protection products preventatively several times during this period to protect open blossoms from becoming infected. It should be noted that fire blight protection products such as Streptomycin are only effective for about 2 days and should have been reapplied several times during extreme infection risk periods. Although the southwestern regions of the province may have been finished bloom during the last week of May when conditions were favourable for blossom infection, it is very likely that late blossoms or 'rattail' blossoms were present on a few trees in many orchards. 'Rattail' blossoms were reported in most apple growing regions of the province earlier this year and are very susceptible to fire blight infection. During this same period, trees in other apple growing regions of the province were still in various stages of bloom which made them very susceptible to infection.

Figure 1. Fire blight causes shoots to turn brown and curl down in a 'shepherds crook'.

Figure 1. Fire blight causes shoots to turn brown and curl down in a 'shepherds crook'.

Growers who did not spray a fire blight protection product preventatively during blossom time, or did not spray frequently enough or at the proper time during bloom or did not remove the 'rattail' blossoms, would have seen fire blight symptoms in their orchards in early to late June. Unfortunately, since the end of May, temperatures have been favourable for the multiplication of the fire blight bacterial cells in infected shoots or spurs that may have gone unnoticed. Temperatures between 21 to 27°C are optimal for the bacterial cells to grow and multiply that can spread to actively growing shoots through rain splashing and insects.

Growers should continue to prune out and remove the infected shoots promptly in orchards 10 years old or younger and particularly in very young orchards between 1 to 3 years old if there are only a few strikes per tree. When removing infected shoots, it is important to cut at least 30 to 60 cm (12 to 24 inches) beyond the water soaked margin of the infected shoot, preferably into healthy 2nd or 3rd year old wood. Wood 2 years old or older is more resistant to infection. Only prune out the fire blight strikes when there is a stretch of 2 to 3 consecutive days of dry weather forecasted with low humidity and preferably when temperatures are below 25°C. During periods of hot humid weather, it may be best to wait. Now that terminal buds have set, the trees are not producing succulent shoots and are much less susceptible to infection. Pruning later this fall is another option. Regardless of when you prune, disinfect pruning tools between each cut by soaking them in a solution of 1 part bleach to 5 parts water or in a 65 to 70% alcohol solution. The infected shoots can be thrown in the grassy pathways where they will dry up within a few days and not be infectious and then mowed to further break down the infected shoots.

For more information:
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E-mail: ag.info.omafra@ontario.ca