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Lorraine P. Berkett
June 10, 2013
Quick Update -Fire Blight Update -- Excellent article on scouting for blossom blight symptoms on web from MSU.
Mark Longstroth of Michigan State University Extension has written an excellent article entitled:
“Scouting for Blossom Blight Symptoms of Fire Blight in Apples.” It includes explicit pictures of the
symptoms for which one should look.
The article is below. It was posted on the web at:
http://msue.anr.msu.edu/news/scouting_for_blossom_blight_symptoms_of_fire_blight_in_apples?utm_source=Fruit+Prod
uction+-+MSU+Extension+News+-+6-7-13&utm_campaign=Fruit+6-7-13&utm_medium=email
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“Scouting for blossom blight symptoms of fire blight in apples”
Scouting for blossom blight symptoms allows you to gauge how effective your protection was during
bloom. This article describes the blossom blight or spur blight phase of fire blight with pictures taken
in southwest Michigan.
Posted on June 5, 2013 by Mark Longstroth, Michigan State University Extension
Fire blight is a serious bacterial disease of apples and pears. Fire blight attacks all tissues of apple trees:
blossoms, leaves, shoots, branches, fruits and roots. The bacterial pathogen Erwinia amylovora that causes fire
blight is harmless to humans. The bacteria usually enter the tree through flowers during bloom. Under rainy
conditions, the bacteria then move into the tissues of the flower and the rest of the flower cluster. This infection
causes the death of the flowering spur, which is often the first visible symptom of the disease.
Once established in the tree, fire blight quickly spreads from the current season’s growth into older growth.
Death of infected branches is rapid and the leaves do not have time to fall off the tree. Young, non-bearing and
newly bearing trees can easily be killed by the infection while mature-bearing trees usually survive even if all the
new growth is killed.
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Photo 1. Wet apple blossoms. Rain during bloom can be a disaster.
Photo credit: Mark Longstroth, MSU Extension
Fire blight blossom infection
Blossom infections are initiated by bacteria carried to open blossoms during bloom by bees and other insects.
The bacteria thrive on the stigma of open blossoms where pollen lands to pollinate the flower. With warm
temperatures, bacterial numbers can quickly rise to incredibly high numbers on the nutrient rich exudates of the
stigma, but do no harm there.
Rain during warm weather during bloom can cause infection (Photo 1). Rain washes the bacteria down off the
stigma to the base of the flower. At the base of the flower are nectar pores where the bacteria can enter the
plant. Once in the plant, if the conditions are warm, the bacteria begin growing and kill the plant tissues. Disease
symptoms are not readily apparent since the disease needs warm conditions and time before symptoms
develop. Often the first flowers to bloom out are not infected even when the conditions are hot and wet
because the flowers were not inoculated with the bacteria. The bacteria are spread from flower to flower by
bees and other insects.
Blossom blight symptoms
Often the initial fire blight symptom seen in an orchard, blossom blight usually indicates where the pathogen
first gains entry into the tree. The bacteria kill the flower (blossom blight) and often the spur (spur blight). The
damage may resemble frost injury to fruit spurs.
Photo 2. One flower in this cluster was infected with the fire
blight bacteria. Photo credit: Mark Longstroth, MSU Extension
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Blossom blight symptoms are hard to see unless you are actively inspecting the bloom for fire blight symptoms
or assessing fruit drop to make thinning decisions. Affected tissues first have a water-soaked appearance that
quickly turns black or brown. The bacteria then moves out of the infected blossom cluster and symptoms of
necrotic (dead, brown or black) tissue can be seen in nearby leaves and fruit. These symptoms appear where the
petiole joins the leaf or fruit.
The time of appearance of the symptoms depends on the temperature after infection. The warmer the
temperature, the sooner symptoms appear. In Photo 2, you can see not only the infected fruitlet, but leaf
symptoms on some of the spur leaves as the bacteria have begun to move out of the fruit. The first leaf
symptom is dying tissue where the petiole or leaf stem meets the leaf blade.
Photo 3. This fire blight infection has spread from one cluster to
other shoots and leaves. Photo credit: Mark Longstroth, MSU Extension
Spur blight symptoms
Spur blight is the collapse of entire spurs after the initial fire blight blossom blight symptoms on single flowers,
the bacteria begins to move inside the plant, killing nearby tissues. Photo 3 shows spreading fire blight where
nearby tissues are collapsing as the bacteria moves into healthy spurs and shoots.
Photo 4 shows several symptoms of fire blight. The flowers on the right were infected and you can see the black
stems of the flowers. The bacteria have spread into the tree and infected the shoot on the opposite side of the
branch. The systemically infected shoot has orange-colored leaves at the tip and is starting to wilt. You can also
see wilting leaves around the infection and the woody stem is also starting to ooze, both above and below the
initial infection.
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Photos 4 and 5. Left, Flowers on the cluster on the right were the first infected and the infection has
spread to the shoot on the right. Right, Older spur blight symptoms. Photo credits: Mark Longstroth, MSU
Extension
The accompanying picture (Photo 5) was taken several weeks after the onset of symptoms. On this tree the
infection has killed several spurs and caused oozing at the base of the spurs. This canker has not moved far, but
has the potential to girdle the limb and kill the shoot. If this canker remains active it will spread fire blight in this
orchard next year.
Photos 6 and 7. Left, Fire blight leaf symptoms. Right, Initial fire blight symptoms in shoot tip.
Photo credits: Mark Longstroth, MSU Extension
Photo 6 shows the dead tissue in the midrib of the leaves on and near an active infection indicates that the
bacteria are moving through the stem and have now moved into this leaf. Photo 7 shows the orange or yellow
shoot tips are often the first sign that a shoot is infected systemically and will die. The color indicates that the
bacteria have arrived at the shot tip. The entire shoot will quickly wilt and die.
For more information from Michigan State University Extension, see these articles:
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Watch video on fire blight shoot blight development from George Sundin’s lab.
When to prune out fire blight: To prune or not to prune
New video resources in tree fruit pathology from MSU
Prune out fire blight in the winter
Summer pruning apples
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This article was published by Michigan State University Extension. “
Thank you Mark for writing this very informative article !
Also see Mark’s article listed above on “When to prune out fire blight: To prune or not to prune”.
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Update on Potential Fungicide Phytotoxicity - Leaf Spotting - On June 4, a “Heads-Up” was sent out
when Dr. Dave Rosenberger of Cornell’s Hudson Valley Lab noted that some Fontelis fungicide
combinations may cause leaf spotting problems on some cultivars under some conditions. In today’s
issue of Scaffolds, an article written by Dr. Rosenberger entitled “THE CAPTAN CONUNDRUM: SCAB
CONTROL VS. PHYTOTOXICITY” expands on the topic of potential captan phytoxicity and also includes a
summary of observations on injury associated with Fontelis-captan mixtures. Dr. Rosenberger also
writes: “It is important to note that some pathogens cause leaf spotting that is very similar to leaf
spotting caused by captan injury. Rust-induced leaf spotting occurs when cedar apple rust spores
germinate on apple cultivars that are resistant to rust. The invading rust fungus soon dies due to the
host incompatibility reaction, but the cells killed or damaged by the germinating rust spores provide
entry points for leaf spotting fungi. Rust-induced leaf spotting can be differentiated from leaf spotting
due to phytotoxicity by the fact that rust-affected leaves usually show some bright yellow-orange
pinpoint spots either at the center of lesions or at other locations on the leaves where the rust spots
were not followed by secondary pathogens. Frog-eye leaf spot caused by Botryosphaeria obtusa can
also cause severe leaf spotting, but distribution of this disease is very uneven within trees, with most
infections occurring below over-wintering fruitlet mummies that supplied the inoculum.” Below is a
picture (Fig. 4) which is included in the article that shows rust-induced leaf spots on Liberty.
(Source: Dr. Rosenberger, Scaffolds, Vo. 22, No. 12, June 10, 2013)
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FYI -- ”New” Insect Pest found in UVM Hort. Research Orchard - Ann Hazelrigg who is conducting
organic apple research at the UVM Hort. Research Center found the larva pictured below boring in
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new shoot growth. She sent a specimen off to Dr. Alan Eaton at UNH who identified the larva as that
of the Pith Moth.
Pith moth larva from organic orchard at the UVM HRC.
Dr. Eaton has written a fact sheet on “Borers in New Hampshire Apple Trees” which contained the
following information about the Pith Moth and which can be found at:
http://extension.unh.edu/resources/files/Resource001830_Rep2590.pdf
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_____________________________________________________________
Where trade names or commercial products are used for identification, no discrimination is intended and no endorsement
is implied. Always read the label before using any pesticide. The label is the legal document for the product use. Disregard
any information in this newsletter if it is in conflict with the label.
The UVM Apple Program is supported by the University of Vermont Agriculture Experiment Station, a USDA NIFA E-IPM
Grant, and USDA Risk Management Agency Funds.
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