Fire Blight
Infections Occur During Bloom
Risk for fire blight infections are high this weekend. Apple are in bloom throughout most of the
commonwealth, and Friday’s rain created ideal conditions for infection.
Initial infections from the fire blight bacterium occur
during bloom. The pathogen is carried to
blossoms through rain or insects. With
sufficient moisture, the bacterium moves down into blossoms and infects natural
openings of flower parts. Thus, it is
critical to protect apple blossoms from infections throughout bloom, especially
when weather is warm and rainy.
Predictive weather models are available (http://wwwagwx.ca.uky.edu/plant_disease.html)
for evaluating risk for infection.
Applications of bactericides (streptomycin or
oxytetracycline) begin as the first blossoms open and continue until petal
fall. When weather conditions are
conducive for infection, sprays should be repeated every 4 to 5 days. Even when risk is low, a minimum of 2
applications is necessary to protect blossoms.
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Fig 1 – Blighting of shoots, also called shepherd’s crook,
is the most recognized symptom of fire blight on apple.
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Fire blight can be a devastating bacterial disease of apple,
crabapple, pear, and flowering pear, but disease epidemics are often sporadic. In fact, optimal conditions must be met for
severe disease to occur. Our current
conditions are an indication that fire blight may be severe this spring.
Erwinia amylovora
infects trees through flowers. However,
large numbers of bacterial cells must be present during flowering in order for the
disease to develop into an epidemic. The
fire blight pathogen favors rain and temperatures above 60˚F. Under these conditions, bacterial cells
multiply quickly. Thus, if conditions
are favorable during flowering, infection can be severe.
Predictive systems
are available for growers.
University of Kentucky’s Cougarblight model evaluates the potential for
infection by analyzing temperature and leaf wetness data from the previous four
days in order to estimate potential risk for infection. Trees must be in bloom for this predictor to
be effective. Cougarblight is an
excellent decision-making tool for growers and can be accessed at http://wwwagwx.ca.uky.edu/plant_disease.html
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Most growers are familiar with shoot blight, the most
obvious fire blight symptom in which infected shoots die quickly, causing
branch tips to form a distinct crook (photo 1).
Shoot blight, however, does not result from infection of blossoms. Direct penetration of bacteria into green
shoots or the upper leaves of young shoots after bloom typically causes shoot
blight symptoms.
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Fig 2 – Initial infection by the fire blight pathogen occurs
through blossoms. Notice bacterial ooze
coming from the pedicel.
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Initial fire blight
infections occur through flowers. We
call this symptom blossom blight. Petal
browning is the first sign of petal blight, but many growers do not notice
it. Browning of pedicels (stems that
attach flowers to stems) follows. Often,
droplets of bacterial ooze can be seen coming from pedicels (photo 2). Bacteria quickly travel down the spur and
into the twig. Cankers that form around
the spur-attachment site girdle branches, and then branch parts above the
canker also die (photo 3).
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Fig 3 – Bacteria can spread through flowers and spurs into
twigs. Resulting cankers can girdle
entire limbs and branches.
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Fire blight control
measures include anti-bacterial pesticides applied during bloom. Applications made after bloom are
ineffective. When fire blight risk is
high (warm temperatures combined with rain) during bloom or if fire blight was
a problem last year, the following spray schedule should be followed:
1.
Apply fixed copper at silver tip. Homeowners should not skip this step, as it
is their only tool available to combat fire blight. Do not use copper fungicides after bud break.
2.
Apply streptomycin beginning at pink stage,
repeating every 4-5 days, through petal fall.
At least 2 applications are required, but up to 4 sprays may be applied,
depending on rain and temperature conditions.
Ideally, bactericides should be applied just before rains. Pay extra attention to susceptible varieties
(i.e. Gala, Jonathan, and Rome). Utilize
Cougarblight or MARYBLIGHT predictive systems for assistance. Mycoshield (oxytetracycline) is also
available for management of fire blight but is not as effective as
streptomycin. Neither product is
recommended for homeowner use.
Various cultural practices may be implemented to aid in
disease management. Combine these
practices with bactericide sprays above for best control.
1.
Select disease resistant or disease tolerant
varieties. Liberty, Pricilla, SirPrize,
Enterprise, Gold Rush, and Sundance are recommendations from ID-21.
2.
Prune last year’s cankers and dead wood before
bud break. Burn, bury, or completely
remove prunings from the orchard to eliminate the possibility of bacterial
cells being carried back to healthy tissue.
Monitor predictive systems. Know
your risk. Cool temperatures or no
rainfall will result in low disease incidence.
3.
Remove flower/fruit spurs immediately after
symptoms develop so bacteria cannot continue infection into branches. Dip tools in 10% bleach, 10% Lysol
concentrate, or pure rubbing alcohol after each cut to keep from spreading
bacteria.
4.
Do not prune limbs or branches during the
growing season. Trees natural defenses
wall off infection sites and stop disease spread. Remove these branches during the dormant season,
instead, when threat of disease spread is lowest. Removal of all infected wood is critical to
prevent spread of inoculum.
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