The Biology and Epidemiology of Fire
Blight
Paul W. Steiner, Professor &
Extension Fruit Pathologist
Department of Natural Resource Sciences, University of Maryland, College Park,
MD 20742
(Presented at the Illinois
Horticultural Society Meeting, January 2000)
INTRODUCTION
Fire blight has been a mystery for scientists and fruit
growers for most of the 200 years since the first symptoms were described in
the New York Hudson Valley. Epidemics can develop rapidly in orchards with no
history of the disease, destroying much of the current crop and killing many
large limbs or whole trees within a few months. Epidemics also can be minor
affairs, causing no significant economic damage, even in orchards that suffered
severe blight the previous season. Between these extremes, variation in the
incidence and severity of fire blight that seems to follow no particular
pattern from season to season and orchard to orchard is characteristic.
The cause of fire blight remained elusive for another
century after symptoms were first seen when it was proven to be a bacterial
pathogen we now know as Erwinia amylovora. Despite the importance of this
discovery, the mystery would continue for yet another century before we could
predict certain infection events.. Use of the Maryblyt™ predictive programs in
managing fire blight will be discussed later. The purpose of this discussion is
to present an overview on the biology and epidemiology of fire blight as a
basis for understanding its effective management in the orchard.
DISEASE CYCLE
Plant disease is a dynamic process, not a static condition.
It develops as the result of a three-way interaction over time involving the
host plant, the pathogen and the environment. The parallel development of the
host plant as it begins growth and matures and the pathogen as it becomes
available, multiplies and incites infections during any given season tends to
be cyclic in nature. Infection events occur only when the host is in a
susceptible condition, pathogen inoculum is adequate and environmental
conditions (e.g., temperature, moisture, wind, etc.) are suitable. Disrupting
these basic interactions at any point (host x pathogen, host x environment,
pathogen x environment) prevents disease from occurring and is the essence of
any good control program.
2.1 Overwintering Sources of Inoculum. The pathogen overwinters in living bark tissues surrounding
some cankers formed at the base of spurs or shoots killed the previous season.
They can also form in the bark surrounding cuts made to remove infected shoots
during the growing season. There are two types of cankers: determinate and
indeterminate. Determinate cankers have strongly delimited margins, often
marked by a distinct crack or separation of the bark caused by an effective,
early season resistance mechanism in which a barrier of suberized, corky tissue
isolates the pathogen from the surrounding healthy bark tissue. Determinate
cankers seldom serve as sources of inoculum the following season. Indeterminate
cankers lack this physical barrier zone so that their margins usually appear
smooth and continuous with the surrounding healthy bark surface. Here, damage
caused by the bacteria in the intercellular spaces withdrawing water from
healthy cells appears to be halted only by the high carbohydrate reserves that
develop in the bark during the mid- to late- season (e.g., after mid-June). The
bacteria do not overwinter in the dead tissue of indeterminate cankers but in
the living bark tissue that surrounds them.
Data obtained in developing the Maryblyt™ predictive model
for fire blight suggests that bacterial multiplication and the renewal of
infectious activity at indeterminate canker margins probably begins in the
early spring at about 93 cumulative degree days (CCD) >550F after green tip
which is about the time of the tight cluster to early pink stage on apples.
This is an important event in the development of fire blight epidemics because
it marks the time when the pathogen is probably first available for dispersal
in the orchard.
2.2 Inoculum Dispersal and Colonization. Once the bacteria become available as droplets of sweet,
sappy ooze on the surface of bark, many different species insects (mostly
flies) are attracted and to the oozing canker sites and begin the casual
dispersal of inoculum from tree to tree, leaving colonies of bacteria wherever
they walk. E. amylovora is a competent epiphyte in that it is capable of
surviving and multiplying on plant surfaces several weeks before flowering
begins. The bacteria can also be dispersed by rain either by direct splashing
or as aerosols carried on even modest winds. Once the first early opening
flowers are colonized by bacteria, further dispersal is not only rapid but
specifically directed at open flowers through the activities of honey bees and
other pollinators. The five stigmas in the center and top of each blossom have
a moist, nutrient-rich surface that supports their selective colonization by
the bacteria to high levels even though infection has not yet occurred. As honey
bees arrive to collect pollen, the bacteria are picked up on their body hairs
and are then subsequently moved to other flowers in the orchard.
2.3 Multiple Infection Types. One of the most significant findings in our work to develop
the Maryblyt™ program was the identification of five distinct types of
infections that can occur over the course of a fire blight epidemic. These are
identified as blossom, canker, shoot, trauma and rootstock blight. Of these,
only canker blight occurs every year where fire blight occurred the previous
year and the incidence and severity of the others will vary depending upon the
prevailing weather conditions. These various types of infections differ in the
sources of inoculum, the type of tissues invaded, and the weather conditions
governing the infection process. While each type can be readily distinguished
at the early stage of symptom development, once an epidemic gets under way, it
is more difficult to separate all the various types. This is important because
the types of control efforts we select and use against one type may not be
effective against another, leading to costly investments in efforts that don't
work.
2.3.1 Blossom blight.
If rain or dew does not occur during bloom, the pollinated flowers will go
ahead and set healthy fruit despite the presence of the bacteria. If rain or
dew does occur during flowering, a gradient of several simple organic acids
seems to form in water films between the stigmas and the nectarthodes in the
base of the flower. Because the bacteria have flagella and, attracted to this
strong nutrient gradient, will follow it directly into the nectarthodes where
over 90% of blossom infections occur. Below 600F, the bacteria, while still
motile, lose the ability to move in a directed fashion following the chemical
gradient. Above 600F, however, colonization and infection of the nectarthodes
appears to occur within minutes. Once a blossom infection event does occur,
symptoms of that specific event can be predicted accurately (usually + 1-2
days) using an interval of 103 CCD >550F from the date of the event. Since
this threshold is temperature dependent the real time in calendar days for
symptom development can range from 5-6 days under warm conditions to 30 days or
more under cool conditions, a fact that explains the puzzling appearance of
blossom blight as much as a month after petal fall.
The importance of blossom blight cannot be overstated, even
when the number of overwintering cankers might be small. This is because
blossom blight has the potential to develop explosively due to several key
factors. First, primary inoculum is often dispersed from holdover cankers to
surrounding trees and then throughout the orchards up to several weeks before
bloom. Second, once the stigmas of the first open flower are colonized, the
bacteria are spread quickly by pollinating insects to new flowers as they open
and this can occur over a period of days before an infection event might occur.
Third, under suitable temperature conditions, E. amylovora populations can
double within 20-30 minutes so that with just 31 doublings, one bacterium gives
rise to over a trillion, each of which is capable of initiating an infection.
And, finally, when all conditions for blossom infection exist, infections probably
occur within minutes. This 'quiet colonization' of the orchard and blossoms is
perhaps best imagined by playing the sound track of the movie 'Jaws' from tight
cluster until the first wetting event during bloom when the average temperature
is 600F or above. When such an infection event occurs the number of new sources
of secondary inoculum in the orchard can suddenly reach several hundred
thousand or more, making efficient control over the rest of the season
difficult.
2.3.2 Canker blight.
Although the infectious activity of E. amylovora begins at the margins of
overwintering cankers much earlier than bloom, the actual symptoms associated
with this infection event usually do not become apparent until some time after
bloom. Unlike the other infection types, canker blight can be expected to
develop every year in orchards where overwintering indeterminate cankers remain
from the previous year. The first symptom that can be detected can only be
found by cutting into the bark at the canker margin and then appears as only a
narrow zone (1-2mm) of water-soaked green or diffuse brown tissue at the margin
between the necrotic tissue of the canker and the surrounding healthy bark.
This occurs routinely with the accumulation of 196 CDD >550F after green
tip. A second, more easily recognized and striking symptom of canker blight
occurs 103 CCD >550F later (=299 CDD >550F after green tip) when the tips
of vegetative shoots, especially water sprouts, close to active canker sites
develop a distinct yellow to orange color and begin to wilt. At the same time
the bottom few leaves on these shoots often show darkened mid-veins indicating
that these shoots have been invaded from the base as the result of systemic
infections arising at canker margins. Even without these tell-tale tip and
basal leaf symptoms, close examination of new shoots arising near the margins
of cankers will often show droplets of bacterial ooze on their stems.
Canker blight symptoms are often overlooked in the light of
much more numerous and dramatic blossom infections or because of their
similarity to the more familiar shoot tip (=shoot blight) infections that occur
later. Because of the limited number of overwintering cankers in a well managed
orchard the significance of canker blight is often underestimated. Indeed,
their importance is probably insignificant in terms of overall damage when
blossom blight occurs. However, in years when blossom infection events do not
occur or have been well controlled, active canker sites serve as the primary
source of inoculum for a continuing epidemic of secondary shoot blight
infections that can lead to major limb, fruit and tree losses. Such sources of
inoculum can also be important for new orchards with no history of fire blight
when they occur in older, surrounding orchards from which the bacteria can be
moved into young orchards by wind, blowing rain and certain insect species.
2.3.3 Shoot blight. Shoot
tip infections are common in areas where fire blight occurs and, as the name
implies, are incited on the youngest 2-3 tender, un-expanded leaves at the tips
of vegetative shoots. The symptoms of these shoot tip infections differ from
those of the systemically invaded shoots associated with canker blight in that
the shoot tips are usually still green (not yellow to orange) when they wilt
and there are no symptoms on the basal leaf mid-veins. The significance of
shoot infections is two fold. First, these infections tend to progress downward
rapidly, often invading and destroying larger supporting limbs resulting in the
loss of large portions of trees and a proportion of the total fruit bearing
surface. Secondly, Al Biggs in West Virginia has evidence that the leading edge
of up to 80% of shoot infections that occur after mid-June ends as an
indeterminate canker suitable for the overwintering of the bacteria for the
next season.
It now appears that as inoculum becomes abundant in the
orchard, leaf surfaces are colonized by the bacteria (arriving from earlier
blossom infections, active cankers or young shoots systemically invaded by
bacteria from nearby cankers), but cause no harm so long as they remain on the
surface. In the past, while we have often associated shoot blight outbreaks
with insect activity, which insect species might be involved, if at all, is
still being investigated. There is good evidence that green apple aphids and,
probably, white apple leafhoppers are not important because of their peculiar
feeding habits. Potato leafhoppers, however, which feed on shoot tips and often
occur in high numbers may still play a role.
A more likely factor is wind and then not necessarily the
high winds associated with storms. There is evidence from work published in the
1960s of a direct relationship between the incidence of shoot blight in apple
nursery trees and their distance from Lombardy poplar wind breaks. More recent
work in Germany provides clear evidence that the simple damage to leaf hairs
along the midrib of pear leaves (only a few cells away from the xylem
parenchyma and vascular tissues) provides suitable wounds for the bacteria to
enter and to incite infections leading to typical shoot blight symptoms. Thus,
the greatest number of shoot tip infections may well occur during days with
gusty winds that might cause a whipping type injury to shoot tips leaves, an
event that is, unfortunately, all too common in the temperate climates where
apples and pears are grown.
2..3.4 Trauma blight. The
incidence of severe fire blight associated with damage caused hail and high
wind is well known by experience and has been well documented in the
literature. Much like shoot blight, it appears that leaf surfaces already
colonized by the bacteria are severely injured during hail and wind storms so
that the bacteria have ready access to internal leaf tissues and the vascular
system. A similar observation has been made following a late season frost
<280F, where the bacteria may be drawn from surfaces into internal leaf
tissues during the thawing process. It is important to note that trauma blight
is not limited to highly susceptible cultivars, but can also occur in more
resistant Red Delicious orchards as well since the injuries seem to breech the
normal defense mechanisms active in these trees
When such trauma-inducing events occur, the amount of fire
blight that follows appears to be directly related to the amount of foliar
colonization by the bacteria in the orchard, being heaviest near good sources
of inoculum such as active blossom, canker or shoot blight symptoms or active
cankers not previously removed. In one recent case in Maryland, a grower who
had followed a rigorous fire management program in his apple and pear orchards
for several years experienced a severe hail storm that struck his entire
planting. The trauma blight symptoms that subsequently developed, however, were
limited to less than 20 trees scattered in small clusters around the orchard
where, in nearly all cases close examination revealed the presence of an
overlooked active canker. Because of his previous good efforts at limiting the
number and distribution of these primary sources of inoculum, this grower was
able to cut out and remove all of the blighted wood in his orchards within a
few hours after symptoms were discovered.
2.3.5 Rootstock blight. Rootstock
blight can be especially damaging where M.26 and M.9 apple rootstocks are used
for high density plantings. Here, we have shown that the bacteria from a single
shoot infection can move rapidly down through the otherwise healthy
superstructure of branches, limbs and trunk into the rootstock (or C-6
inter-stem) where the bacteria initiate a canker that quickly expands to girdle
the tree causing the death of the whole tree. In the Midwest, Mid-Atlantic and
Southeast part of the U.S., whole trees may collapse and die suddenly in June
or July following an earlier bout with blossom, shoot or trauma blight.
Additional trees show early fall red coloration in late summer to early autumn
and still, more trees may not show symptoms until they begin to decline and die
in the early spring following the infection. While not all trees showing
infections in the scion ultimately succumb to rootstock blight, 5-10% tree loss
per year for the first 5-6 years is not uncommon and can be as high as 20-40%
or more in some orchards. In New York and New England, where summers are
generally cooler, the mid-summer death of trees seems rare, but the losses from
fall and spring deaths are about the same. Note, too, that such losses can
occur even with relatively resistant apple varieties such as Red Delicious
where scion infections can occur in trauma blight situations.
3. SUMMARY
As fire blight epidemics unfold, the symptoms that appear
can be associated with up to five different types of infections, each
determined by the source of inoculum and the conditions in the host and
environment. Not all of these infection types occur every year in all orchards
nor with the same intensity. Canker blight, while often limited to a few
locations were overwintering cankers were not removed, occurs regularly and
predictably every year, can serve to fuel a major epidemic of shoot blight or
set the stage for serious damage following summer storms. Blossom blight may or
may not occur in any given season and varies in its incidence and severity with
the number of open flowers colonized by the bacteria, temperatures above 600F,
and the thoroughness of wetting by rain or dew. Shoot blight outbreaks have
been associated independently with both insect activity and modest wind damage
where young shoot tips are already colonized by the bacteria. Trauma blight
also requires a degree of foliar colonization by the pathogen and is triggered
by the wounding that occurs with hail, high winds and late season frosts.
Rootstock blight is limited to apple cultivars planted on M.26 and M.9
rootstocks and can cause severe damage in young orchards where blossom, shoot
or trauma blight events occur. At present, there is sufficient information
regarding blossom, canker, shoot and trauma blight to allow for their accurate
prediction in advance so that effective control measures can be taken. The conditions
governing rootstock blight, however, have yet to be defined well enough to make
prediction reliable.
January, 2000