HORTICULTURE
Fighting Fireblight
in Southwest Michigan
Mark Longstroth
If you are reading this page you probably have a serious
problem with fire blight. Fire blight is a serious disease of apples and pears caused by a
bacterium, which is harmless to humans. Fire blight attacks all the tissues of the trees;
blossoms, leaves, shoots, branches, fruits, and roots. Once established in the tree fire
blight quickly moves through the current season's growth into older growth. Death of
infected branches is so rapid that the leaves do not have time to fall off the tree. Young
trees, both bearing and non-bearing, can be killed but mature bearing trees usually
survive even if all the new growth seems to be killed. For more information on the disease
see Fire Blight in Southwest Michigan. To figure out what to do
with your trees now that you have it read on. Many of the links on this page go to
my posting of fire blight symptoms.
The bacteria overwinters in cankers caused by the disease.
Cankers should be cut out during dormant pruning in the winter and spring to reduce the
amount of bacteria in the Spring during bloom. Some apple varieties are more susceptible to fire blight than others.
The first symptoms of fire blight are blossom blight
strikes in young fruit. The shoot blight phase which follows is usually the one growers
notice and is the phase where growers ask if they should prune out the strikes.
When to Prune Out Fire Blight: To Prune or Not to Prune
The question of pruning out Fire blight strikes during the growing season,
leaving them until the winter or using the "ugly stub method" has long been
debated. The answer to this question depends upon the number of strikes and the weather
conditions suitable for infection. While growers usually consider pruning out blight when
it is severe, I think that great benefit comes from pruning out blight in years when there
is only a little blight.
Experience in 2000
During the 2000 season, a warm, humid spring and a long bloom period resulted in
scattered outbreaks of Fire Blight in Southwest Michigan apples. Then a severe storm
brought 2 ½ inches of rain and hail to the region in mid May resulting in severe trauma
blight several weeks later. After two years, Fire Blight is still a concern for many
Michigan apple growers. During the 2000 Fire Blight epidemic in Michigan, weather
conditions at bloom allowed bacteria populations to rise to very high levels. When rains
came late in bloom, some growers did not apply timely streptomycin applications in all
their orchards, others only sprayed the most susceptible varieties. Consequently, blossom
blight developed on the unsprayed rows in many orchards and the trauma event in May spread
the disease throughout the orchards in the region. Some apple growers immediately pruned
out fire blight infections and continued to prune out strikes as they appeared. Other
growers, especially those with severe widespread infections left the fire blight strikes
until winter before pruning. Regardless of when they pruned, both found some fire blight
in their orchards the following season.
Under dry conditions when only a few strikes occur, immediately pruning down to
non-infected 2-year-old wood would reduce the potential of the disease spreading. This
strategy also works when infections are light or are located in only an isolated area of
the orchard. More about aggressive pruning later.
We had a lot of orchards where the grower walked away from older trees and the blight
killed all the young wood less than 3 or 4 years old. By late July the trees had grown new
shoots and you could hardly tell they were infected from the road. Some growers pruned out
all the fire blight affected wood that winter and others waited a year and did their
pruning after the tree's growth had settled down. Both strategies seem to work well and I
think waiting a year was cheaper but you do have a much-increased risk the following
spring if you leave it in for a year. This strategy works best with old trees.
Unfortunately, leaving the disease in the orchard may serve, as a source of inoculum
should a severe storm roll through that could result in a "trauma blight" event.
The "ugly stub" method would work better than leaving the diseased strikes in
the orchard until winter. Dave Rosenberger, in the Hudson River Valley, advocates the
"ugly stub method".
Pruning Out Blight:
Pruning out strikes during years when fireblight is not a major problem allows
growers to reduce the small amounts of bacteria that will be present in their orchards in
the years that the bloom conditions are right for the spread of the disease. "I tell
my growers to scout for and prune out strikes when the weather in dry and don't go out
until the dew has dried."
If you decide to prune out strikes in a young orchard, the strikes should
be pruned out as soon as they appear. Failure to do so increases the likelihood that
blight will continue to spread both to adjacent trees and into the rootstocks of affected
trees. Pruning out infections in mature trees may not be practical, but mature trees with
a full crop will set terminal shoot buds earlier than young trees. When trees set terminal
buds, blight stops spreading both between trees and within the affected trees.
Remove strikes before the cankers extend into the tree. Trees must be
examined at least two or three times weekly until the epidemic slows as tree growth slows.
In sections where trees are severely affected, it may be more cost-effective to
immediately remove entire trees, especially if trees are a susceptible cultivar like Gala.
Pulling out badly affected trees will allow blight removal crews to focus their efforts on
trees that can be salvaged.
Blight removal crews should be trained to recognize the early symptoms of blight on terminal shoots. On
terminals just beginning to show symptoms, the first or second fully expanded leaf will
droop and closer examination will show blackening along the mid-vein at the base of the
leaf blade. The entire shoot tip may appear to be slightly yellowed. Remove such shoots by
cutting back into two-year-old wood at least 8-12 inches below the last visible symptoms.
If a spur or shoot on the central leader shows signs of blight, immediately remove the
central leader down to 8-12 inches below the last visible symptom. Immediate and
aggressive removals reduce the need for repeated pruning in the same tree and may result
in fewer trees lost to root stock blight.
Making cuts into at least 2-year-old wood where bacteria will be less able
to multiply. Also, leave "ugly stubs" by cutting branches between nodes and at
least several inches away from the central leader. Small cankers that form on these
stubs can then be removed during winter pruning whereas a canker that forms at a flush cut
on the central leader will be missed during winter pruning.
Prune during dry weather. An extension specialist in California reported
that he failed to transmit fire blight with pruning tools when he purposely made cuts
through active cankers in dry weather. However, he succeeded in transmitting blight on
pruning tools when pruning was done in wet weather. Blight removal operations should
usually be suspended in wet weather, but that is not always possible. As a precaution,
perhaps pruning tools should still be disinfected if blight removal must be done in wet
weather.
In the ideal world, blight removal would only be done in dry weather.
However, when a week of rain is predicted just as the first symptoms of blight appear, one
must weigh the risks of spreading blight by pruning in wet weather versus the risks of
giving the epidemic a full-week, or even a two or three day head start. With highly
susceptible cultivars like Gala, I would remove blight as quickly as possible, even if
that meant that some removal would be done in less than ideal weather.
Should prunings be removed from the orchard? My recommendation is to toss
prunings in the row middles and allow them to thoroughly dry before mowing them. Dry means
that the bark no longer slips on the cut branches, and the cambium is brown. With
today's tightly spaced orchards, I am concerned that carrying prunings out of the orchard
may spread more blight than occurs when prunings are left to dry in the row middles.
Avoid hand thinning, bud pinching and other manipulation activities until
after terminal bud set. Delaying hand thinning may result in some loss of fruit size, but
risks of spreading blight out-weigh the benefits of early hand-thinning. You can spread
blight on your fingers while pinching buds (or hand-thinning). Pinching is done to
succulent shoot tips that are highly susceptible to blight whereas cuts made to remove
blight are made in wood that is at least two years old.
Streptomycin or other antibiotic sprays should NOT be applied during
summer because summer applications will result in rapid development streptomycin-resistant
strains of the blight pathogen. The only exception is that streptomycin should be applied
immediately after any hailstorm if there is active blight in the orchard (i.e., orchards
where blight was present this year and terminal shoots are still growing).
Copper
sprays applied in summer should reduce or inactivate blight bacteria on plant
surfaces and slow the epidemic. However, attempts to document the benefits of summer
copper sprays have been inconsistent. Proponents of using copper during summer admit that
benefits of copper are limited because copper is not systemic and therefore will not
affect bacteria inside plant tissue. Furthermore, actively growing terminal shoots
"outgrow" the copper residue, thereby leaving the blight-susceptible shoot tips
unprotected within several days after an application. Copper applied in summer is also
phytotoxic to fruit, with injury appearing as necrotic black spots at fruit lenticels.
Thus, copper sprays are not acceptable where the crop is destined for fresh market. In
young orchards, salvaging the crop may be less important than salvaging the trees. The
bottom line: If I was managing a young block with fire blight, I would be applying a low
rate (about 4 oz/100 gallons dilute spray) of a fixed copper on a 7-10 day schedule until
terminal buds are set. Copper sprays should be applied under good drying conditions. The
alkaline nature of copper sprays means that they probably cannot be combined with other
pesticides that are subject to alkaline hydrolysis.
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First posted: October 3, 2002
Last Modified: February 24, 2005