©2006 clipart.com |
Abstract
This introduction to commercial organic pear production covers
pear diseases, disease-resistant cultivars, insect and mite pests
and their treatment, Asian pears, and marketing. Electronic and
print resources are provided for further research.
Table of Contents
Introduction
In most of the U.S., pears may be the easiest of the tree fruits
to produce organically or with minimal spraying. Pears' fertility
requirements are not high, they are adapted to a wide range of climates
and soils, and they have fewer pest problems than other tree fruits.
Standard cultural considerations—such as pruning, choice
of rootstock, planting, spacing, and thinning—are generally
the same for organic and conventional growers. For this type of
cultural information consult your county or state Cooperative Extension
Service and/or find the information in any number of orcharding
texts, articles, and Web sites (see the Additional
Resources section at the end of this document). For cultural
information more specific to organic production (organic fertilization,
organic weed control, etc.) see ATTRA's Tree
Fruits: Organic Production Overview.
Pears have most of the same pest and disease problems that apples
have, but usually to a considerably lesser degree. ATTRA's Organic
and Low-Spray Apple Production identifies pests and suggests
organic remedies that are just as appropriate to pears; therefore,
most of these problems will not be discussed further in this publication.
However, because of its importance and prevalence on pears, fireblight
will be considered in more depth. Other pests and diseases peculiar
or especially troublesome to pears will also be discussed.
Comparing
a Pair of Pears |
|
European pears |
Asian pears |
Scientific names |
Pyrus communis |
Pyrus ussuriensis
P. serotina
P. bretschneideri |
Common cultivars |
Bartlett, Bosc, D'Anjou, Seckel, Magness,
Maxine, Moonglow, Comice |
20th Century, Shinseiki, Korean Giant, Shinko,
Chojuro, Niitaka |
Texture and flavor |
Buttery smooth and sweet. |
Crisp. Flavors vary widely, including bland,
sweet, sweet-tart, pineapple-like, and more. |
Color |
Green to yellow, though russetted types
are dull bronze, and there are a few red types. |
Green, yellow to golden, russetted bronze,
and many types with white lenticels (spots). |
Shape |
Pyriform (i.e., round & bulb-like on
the calyx end, narrowing to a slender neck at stem end). |
Most cultivars are round. Some are misshapenly
round. A few are somewhat pyriform. |
Harvest and postharvest |
Harvest before fully ripe. Allow to ripen
(cure) at room temp. Buyer usually does curing. |
Allow to tree-ripen. Ready to consume or
sell. |
Pests and diseases |
Same as for Asian (see text). |
Same as for European (see text). |
Back to top
Diseases
Fireblight
Fireblight, caused by the bacterium Erwinia amylovora,
is one of the most serious and economically damaging diseases of
pears. Occurring throughout the pear's range, it is particularly
troublesome in the humid eastern United States. Infection is triggered
by heat and moisture, and can spread rapidly—even within a
matter of hours. It can be transmitted by bees, aphids, psylla,
or other insects, and can also be spread by blowing wind and rain.
Pruning can be another source of infection. Affected branches wither
and turn black or brownish black, as if scorched. Most branch tips,
once infected, wilt rapidly, taking on the characteristic shape
of a "shepherd's crook." The bacteria gain entry to the
tree through blossoms or lush new growth and, once inside, begin
to work toward the roots. If the disease spreads unchecked to the
trunk and roots, it can kill the tree; however, in resistant varieties
the bacteria rarely invade beyond young wood. Under the bark, the
bacteria form a canker where they will survive the winter, only
to infect more trees the next year. If on a warm day you see pustules
on a tree, oozing an orange-brown liquid, you are looking at fireblight.
(1)
Fireblight development is greatly favored by the presence of young,
succulent tissues. Cultural practices that favor moderate
growth of trees are recommended. These include using only half as
much compost as for apples, never using manure, and avoiding heavy
populations of clovers and other legumes around the base of the
tree. (2) Pruning should also be limited, and
only minimum fertilization applied.
Choosing fireblight-resistant pear cultivars is a good start to
managing fireblight, but it is by no means a panacea (see the Appendix
"Fireblight, Disease Resistance, and the Disease Triangle").
There are several European-type pears with a comparatively high
level of fireblight resistance (Ayres, Magness, Potomac, Warren,
Maxine, Moon Glow, Tyson, Honeysweet, Kieffer, etc.) that are adapted
to most of the contiguous United States. Clapps, Bartlett, Comice,
Anjou, Bosc, and most other cultivars that are not expressly
described as "fireblight resistant" in a nursery catalog
should be considered susceptible.
Asian-pear cultivars also vary in their resistance to fireblight.
Shinko has a high level of resistance, while Ya Li, Chojuro, Shinsei,
and Seuri are among the moderately resistant varieties. (3)
Shinseiki has been rated as "resistant" by Kentucky researchers
and "susceptible" by Alabama A&M researchers. Korean
Giant and Shin Li are listed as resistant in some literature, but
grower experience with heavy fireblight pressure in Arkansas indicates
that they are at best only slightly resistant. (4)
Blight resistance for some cultivars appears to vary with growing
conditions. It may be some time before all Asian pear cultivars,
being relatively new in the U.S., are well classified in terms of
blight resistance.
Preventing infection is the next key to fireblight control. Sprays
of agricultural-grade streptomycin, applied at early bloom to prevent
infection, have been the standard commercial control since the 1950s.
In some regions, particularly Western apple-growing areas, E.
amylovora has developed resistance to streptomycin. In order
to preserve the effectiveness of this antibiotic, it is important
not to spray it excessively. In areas where resistance has already
developed, terramycin (Myco-Shield®) can be used as a substitute,
though in general it is less effective. Both streptomycin and terramycin
are listed as approved substances for fireblight control in the
National Standards on Organic Agricultural Production and Handling.
(5)
Bordeaux mix (copper sulfate and lime) and other copper formulations
sprayed at green-tip stage are organic options that provide some
protection from fireblight infection. For best results, these formulations
should be applied to all the trees in a block, not only the blight-susceptible
cultivars. (6) Copper treatments can cause fruit
scarring, and are phytotoxic to some cultivars, so careful application
is important.
In 1996 a biocontrol product called BlightBan® A506 came on
the market, labeled primarily for reducing frost damage, but also
for fireblight suppression. BlightBan is a formulation of the bacterium
Pseudomonas fluorescens strain A506. P. fluorescens
is a non-pathogenic competitor with E. amylovora, and as
such does not directly kill propagules of E. amylovora;
rather, it occupies the same sites that E. amylovora would,
provided it gets there first. Therefore, in order to be effective,
BlightBan should be applied to newly-opening flowers (multiple applications
will probably be necessary) or applied in combination with streptomycin
(P. fluorescens, strain A506 is resistant to streptomycin).
In fact, research indicates that fire blight suppression is best
when streptomycin and BlightBan are combined.
Using these two substances together can reduce the amount of streptomycin
sprayed each year, which may help to protect the antibiotic's effectiveness.
By itself, BlightBan can provide up to 50% suppression, but not
control. It cannot be used in combination with copper sprays. The
biocontrol bacteria live only about three weeks in the orchard,
and there is no carry-over from year to year. BlightBan is marketed
by J. R. Simplot Company/Plant Health Technologies. (7)
Note that it is not registered for use in all states.
A computer software program called Maryblyt™ is available
to help in timing antibiotic sprays for their most efficient application
against fireblight. The grower enters daily minimum and maximum
temperatures, rainfall, and stage of blossom development, and the
program predicts infection events and symptom development for most
phases of fireblight. Further information on the program is available
at the USDA's
Kearneysville, West Virginia, web site. In the Pacific Northwest
the similar but freely-distributed Cougarblite
Model is more often used to help time spraying, and elsewhere
other degree-hour models from local sources may be more appropriate.
Once fireblight infection has occurred, there is no spray or other
treatment, beyond quickly cutting out newly infected limbs, that
will minimize damage. However, infection has almost certainly extended
beyond what the grower sees; therefore, it is all too easy to spread
the disease by trying to prune it out during the growing season.
If you do cut during the growing season, remove all blighted twigs,
branches, and cankers at least 8 inches—some sources recommend
12—below the last point of visible infection, and burn them.
After each cut, the shears can be sterilized in a strong bleach
or Lysol® solution (1 part household bleach or Lysol to 4 parts
water) to help avoid transmitting the disease from one branch to
another, although there is some disagreement about the effectiveness
of disinfection. Lysol is less corrosive than bleach to the metal
parts of the pruners. Some have found it more convenient to use
a spray can of Lysol disinfectant carried in an apron rather than
a plastic holster or glass jar with a liquid solution.
During the winter, when the temperature renders the bacteria inactive,
pruning out fireblight-infected wood can proceed without sterilization
of pruning tools, and need not extend as far below the visible canker.
Blossom blast
Another bacterial disease, blossom blast (causal organism: Pseudomonas
syringae, not to be confused with Psedomonas fluorescens
discussed above), may afflict pears, usually as a blossom blight
resulting in reduced fruit set. It can also cause twig dieback and
bark cankers, and may lead to severe wood damage of Asian-pear cultivars
in particular . Because the presence of blossom-blast bacteria allows
ice crystals to form at higher-than-normal temperatures, the disease
increases the incidence of freeze damage during cold wet weather.
Asian pears are especially affected because their early bloom makes
them more susceptible to frost injury. (8) Of
the Asian-pear cultivars mentioned above, Shinko and Ya Li are moderately
resistant to P. syringae.
Controlling this disease is difficult because its occurrence is
widespread on many plant species and not easily predicted; once
symptoms appear, control efforts are too late. Protecting orchards
from frost damage can limit injury. An early application of BlightBan®
A506 can help reduce frost damage potential by excluding the ice-nucleating
bacteria. In California, the application of fixed copper at the
green-tip stage followed by streptomycin at early bloom has provided
reasonable control. This treatment has also been used in Oregon,
where cool wet weather makes blossom blast a particular problem
in pear production. Streptomycin or terramycin applied at early
bloom to control fire blight also help to control blossom blast,
although the most effective timing of application for the two diseases
is somewhat different.
Pear scab
Pear scab (Venturia pirina), a fungal disease closely
related to apple scab, is neither as common nor as economically
important on pears as apple scab is on apples. Nevertheless, it
can cause economic damage by marring the appearance of the fruit.
Pear scab causes lesions on leaves, shoots, and fruit, and—unlike
apple scab—infects twigs, where it can overwinter. Pear cultivars
differ in their susceptibility to scab. Organic control is identical
to that for apple scab (9), usually consisting
of treatment with sulfur, lime-sulfur, or Bordeaux mixture. See
ATTRA's Organic and
Low-Spray Apple Production for details, including the use
of the Mills Table to predict infection periods.
Other diseases
Two foliar diseases, fabraea leaf spot (Fabraea maculata)
and mycosphaerella leaf spot (Mycosphaerella pyri), are
usually no problem in sprayed orchards but can reach damaging levels
in unsprayed ones. (9) Susceptible cultivars,
which include nearly all European pears, can be defoliated, resulting
in reduced buds and dwarfed or—if the fruit itself is infected—unmarketable
fruit. Sprays for pear scab, such as Bordeaux mixture, will generally
control these two maladies as well. Also, Surround™, a kaolin-clay-based
insect repellant, is registered and labeled for suppression of fabraea
leaf spot. More information on Surround is available in ATTRA's
Insect
IPM in Apples -- Kaolin Clay.
Back to top
Insect and Mite Pests
Many of the same pests that affect apples also prey on pears, although
often to a lesser degree. More information on dealing with these
pests can be found in ATTRA's Organic
and Low-Spray Apple Production. Some of the pests that
particularly affect pears are discussed below. Proper pest identification
and orchard scouting are key to implementing successful IPM programs.
ATTRA's Biointensive
Integrated Pest Management (IPM) publication provides additional
information.
Pear psylla
The pear psylla (Cacopsylla pyricola), an aphid-like insect
whose only host is the pear, is the crop's most significant insect
pest. In conjunction with fireblight, pear psylla is largely responsible
for the decline in Eastern pear production. The honeydew left by
the psylla damages the fruit by supporting growth of sooty mold
and causing a black russeting; these two effects account for most
of the economic damage caused by the psylla. It is also an important
vector of fireblight and "pear decline disease," and can
weaken trees in areas of heavy infection.
Although the psylla develops resistance to insecticides, it can
usually be controlled either with dormant oil sprays or with sulfur
sprays (but not a combination of the two, which damages plants.)
Insecticidal soap can reduce active populations during the growing
season. Fortunately, the recently-introduced Surround™ (discussed
above under Other Diseases) is effective against
pear psylla. In fact, use of Surround for pear psylla control has
made organic pear production viable for the first time in states
like Michigan. (10) The psylla also has a number
of natural enemies such as predatory flies, minute pirate bugs,
and lacewing larvae. In organic orchards parasites and predators
can help control psylla below economically damaging levels, especially
when combined with a program of oil treatments. (11)
True bugs
Stink bugs, tarnished plant bug, and other true bugs (insects in
the order Hemiptera) will readily feed on pears throughout the growing
season. Early feeding damage may result in a pucker or dimple in
the fruit. Mid- and late-season feeding often results in the development
of so-called "stone cells" immediately beneath the feeding
site. These stone cells are very hard and can seriously compromise
the marketability of affected fruit.
Some fruit and nut growers reduce true-bug damage by maintaining
unmown leguminous trap crops (clovers, vetches, peas, etc.) near
the orchard. (12) Unmown areas and cover crops
can also serve as habitat for predator insects-for more information;
see the ATTRA publication Farmscaping
to Enhance Biological Control. Surround™ is labeled
for suppression of stinkbugs and lygus bugs in pears. Also, some
of the relatively new biological insecticides derived from the seeds
of the neem tree are effective against true bugs on fruit crops.
For example, Aza-Direct™, which is listed by the Organic Materials
Review Institute (OMRI) for use in organic production, is labeled
for use on all pome fruits for the control of weevils, thrips, true
bugs, leafhoppers, aphids, leafrollers, cutworms, flies, and mites.
Aza-Direct is produced by Gowan Company. (13)
Mites
In high enough numbers, pearleaf blister mites (Phytoptus pyri)
and pear rust mites (Epitrimerus pyri) can reduce the photosynthetic
efficiency of leaves and cause russeting on fruit. Both species
will likely be more troublesome in dusty conditions, as the dust
interferes with their natural predators. These predators—including
green lacewings, predatory mites, and various species of ladybird
beetles, as well as bigeyed bugs, minute pirate bugs, and predatory
thrips—will usually control mite outbreaks if they are not
disrupted by dust or pesticides. Orchard monitoring can help establish
whether adequate numbers of beneficial insects are present. If not,
horticultural oils sprayed during the dormant season can suppress
mite eggs through suffocation. Neither horticultural oils nor other
materials approved for organic production provide economic control
of damaging populations.
Another pest that contributes to poor yields and smaller fruit
is the spider mite. Dormant horticultural oils and insecticidal
soaps can both help control spider mite outbreaks. Neem-based biological
insecticides are also labeled for controlling mites on pears. European-pear
cultivars are more sensitive to mite feeding than Asian pears, and
any pear that is drought-stressed will be more susceptible to mite
damage. (14)
Other insects
Other insect pests that may affect pears include codling moth,
apple maggot, scale, and plum curculio. These insects are covered
in ATTRA's Organic
and Low-Spray Apple Production publication. The pear slug
(actually a sawfly larva) causes leaf damage; it can be controlled
with a forceful stream of water, soap, or wood ashes. The leafroller
complex—including tufted apple bud moth, variegated leafroller,
and redbanded leafrollers—represents another potential, but
comparatively minor, pear pest problem. Mating disruption tools
that are effective for some leafrollers are being developed. In
addition, there are biological insecticides labeled for control
of leafhoppers, aphids, and leafrollers, among other pests.
Peshastin
Creek Growers Areawide Organic Project
In a Washington valley, twelve family-owned orchards with
approximately 300 acres of pears have joined together to establish
an areawide insect pest management program based on the use
of organic insect-control tactics.
Dennis Nicholson and Rich Miller, Co-Chairpersons
P.O. Box 55
Peshastin, WA 98847
(509) 548-4207
nichorch@rightathome.com
|
Back to top
Asian Pears
Because they are relatively new to much of the United States, Asian
pears deserve some additional comment. A crisp, juicy fruit, Asian
pears are also known as Oriental pears, nashi, sand pears, apple
pears, or salad pears. The more than 1,000 varieties of Asian pear
range widely in shape, color, and taste. Only about a dozen varieties
are commonly grown in U.S. commercial orchards.
Asian pears are slightly less cold-hardy than European types; they
may suffer tissue damage at temperatures below -10° F but are
generally hardy to -20°, making them best adapted to USDA climatic
zones 5-9. Most Asian pears bloom slightly earlier than their European
counterparts and may lose some blooms or buds to freezing in areas
with a highly variable spring climate.
Culture of Asian pears is similar to that of European types, but
not identical. One significant difference is the common tendency
for many of the Asian types to set too heavy a fruit crop, which
requires hand thinning of young fruit soon after bloom. Shinko,
a popular cultivar because of its high fruit quality and fireblight
resistance, is especially prone to this problem. If not properly
thinned, Shinko and any other heavy-bearing Asian pear cultivar
will not be able to properly mature such a load. Fruit size and
quality will suffer. On the other hand, thinning to promote fruit
size may result in reduced yield per acre compared to European pear
varieties.
Another difference between Asian and European pears is that Asian
pears ripen on the tree—they do not have to be picked and
then cured like European pears. This is an advantage for most growers,
especially growers who are retailing direct to consumers. The fruit
can be tree ripened and is ready to eat when picked, or can be held
in cold storage. You do not have to educate the buyer about curing,
as you might with European pears.
It is important to note that the delicate fruit of many Asian-pear
varieties must be carefully handled during and after harvest to
minimize bruising, punctures, and roller marks. The skin of Asian
pears discolors quickly following rough handling, making careful
picking and packaging a necessity, and mechanical handling risky.
To prevent damage to Asian pears, it may be best to pack them into
padded boxes or trays in the field where they are picked.
More information on Asian pears can be found in the sources provided
in the References and Additional
Resources sections.
Back to top
Economics and Marketing
Worldwide, China is by far the world's largest pear-producing country,
followed by Italy. The United States is the third-largest pear producer
in the world. Export markets are important for U.S. producers—more
than 30% of the U.S. fresh pear crop is shipped to foreign markets.
(15) At one time pears were grown commercially
on a large scale throughout the United States. However, because
of the prevalence of fireblight in the humid eastern and southern
states, most pear production has been relocated to the drier areas
of the Pacific Northwest. More than 95% of the pears produced in
the U.S. are grown in Washington, Oregon, and Northern California.
(16) Bartletts remain the most popular pear
variety grown commercially in the U.S., outnumbering all other species
combined.
Bartlett pear production has been declining slightly since 2000,
resulting in higher prices for producers who continue to grow them.
(15) In general, pear prices tend to fluctuate
based on crop size—which can be highly variable on account
of weather and fruit set rates—and on the number of growers
producing pears in a given year. Fluctuation in the organic market
is also based on these factors, and the premium that organic growers
can expect to receive for their product is unpredictable at best.
The market for pears is weaker than it could be, largely because
of consumers' unfamiliarity with the different types of pears and
how to use them. One survey showed that Bartletts and winter pears
were consumed by no more than half of America's households, and
then only occasionally. A market study funded by the Pear Bureau
showed that only 3 out of 10 people have tried d'Anjou pears, and
2 out of 10 have tried Boscs. One of the obstacles limiting pear
consumption is that many consumers do not know how to ripen them.
Providing information on ripening may help increase sales (though
the Pear Bureau study also found that most people preferred to buy
pears already ripened). A substantial percentage of purchases are
impulse buys, so appearance is critical. Many appearance problems
come from fruit being handled by retail employees and consumers.
Pears should be handled gently, no matter what stage of ripeness
they have reached. Rough handling causes brown marks on the skin
that may not appear until the fruit has begun to ripen, and will
decrease customer appeal. Sources of consumer, food-service, and
other trade-education materials related to European pears are available
from the Pear Bureau Northwest. (17)
The market for Asian pears is mixed. Until 1995 or so, the wholesale
market for Asian pears was quite open and prices were high, particularly
on the West Coast and in cities with large Asian-American populations.
But Asian pears have been heavily planted in California for more
than 20 years, and those orchards are now mature and bearing, more
than meeting local demand and causing price drops in some areas.
On the other hand, outside of California, Asian pear growers that
serve a retail clientele—especially a local Asian-American
population—report brisk sales and good prices from locations
across the country. (3, 4)
Offering taste sampling of fruit, where practical and allowed by
local health regulations, is probably a good idea for building markets,
since so many Americans are still unfamiliar with Asian pears.
Asian pears are breaking out of their niche to join the mainstream
market, and may represent an opportunity for producers as market
awareness and demand continue to grow.
Back to top
Back to top
References
- Agrios, G. 1978. Plant Pathology.
Academic Press, New York, NY. p. 457–463.
- Hall-Beyer, Bart, and Jean Richard.
1983. Ecological Fruit Production in the North. Jean Richard Publishing
Co., Trois-Rivieres, Quebec, Canada. p. 33–34.
- Jones, Terry, and Ed Fackler. 1995.
Asian pears—A past & future in Kentucky. Pomona. Fall.
p. 31–35.
- Ames, Guy. 2000. Personal communication.
- Anon. 2000. The National List of
Allowed and Prohibited Substances Description of Regulations General
Requirements. Accessed August 2002. www.ams.usda.gov/nop/NationalList/FinalRule.html.
- Steiner, Paul W. 1995. Maryblyt
beats fire blight. American Fruit Grower. February. p. 30–31.
- Plant Health Technologies
POB 15057
Boise, ID 83715
800-535-9101
- Elkins, R.B., and W. D. Gubler.
2001. Plant Pathology, UC IPM Pest Management Guidelines: Pear.
UC DANR Publication 3339. Accessed August 2002.
www.ipm.ucdavis.edu/PMG/r603100111.html.
- Jones, Alan, and Turner B. Sutton.
1996. Diseases of Tree Fruits in the East. NCR 45. Michigan State
University, East Lansing, MI. p. 22–23.
- Elkins, R.B., R. A. Van Steenwyk,
L. G. Varela, and C. Pickel. 2001. UC
IPM Pest Management Guidelines: Pear Psylla. UC DANR Publication
3339. Accessed August 2002. www.ipm.ucdavis.edu/PMG/r603301111.html.
- Anon. 2002. Organic pears are
a viable opportunity for Michigan growers. IPM Report. Spring.
p. 1. Accessed August 2002 at www.msue.msu.edu/ipm/ipmrptv8n1.htm.
- Anon. 1999. Pea patch diverts
stinkbugs. Common Ground. Autumn. p. 4.
- Gowan Company
P.O. Box 5569
Yuma, AZ 85366-5569
800-883-1844 ext. 2
www.aza-direct.com
- University of California Statewide
Integrated Pest Management Project. 1991. Integrated Pest Management
for Apples & Pears. University of California Division of Agriculture
and Natural Resources. Publication 3340. Oakland, CA. p. 116.
- Perez, Agnes. 2001. Smaller
2001 U.S. Pear Crop to Boost Prices. Agricultural Outlook. November
2001. p.4.
- Foreign Agricultural Service.
1999. World Pear Situation. FAS Online. Accessed July 2002. www.fas.usda.gov/htp2/circular/1999/99-04/pear.htm.
- Pear
Bureau Northwest
4382 SE International Way Ste. A
Milwaukie, OR 97222-4635
503-652-9720
Back to top
Additional Resources
Electronic
University
of California Crop Information Asian Pear Index
Horticultural information and links to factsheets on Asian Pear
production and disorders.
University
of California Web site for European pears
Extensive links to sources of information on European pear varieties,
disorders, and economics.
Washington State
University Tree Fruit Research Center-Pear Entomology
A new Web site designed for farmers and field workers, and devoted
to information and current research on pear entomology.
Washington
State University Cooperative Extension-Fire Blight: 7 Key Steps
to Control
A direct, basic fact sheet recommending 7 key steps to fireblight
control.
Pennsylvania
Tree Fruit Production Guide: Asian Pears Recommended cultivars
Discussion of Asian pears in general, and descriptions of common
cultivars in the United States, and less-common cultivars.
Print
Jones, A. L. and H. S. Aldwinckle. 1990. Compendium of Apple and
Pear Diseases. APS Press, St. Paul, MN. 100 p.
Excellent descriptions and photos. $37 from:
APS Press
3340 Pilot Knob Rd.
St. Paul, MN 55121-2097
1-800-328-7560
www.scisoc.org/
University of California Statewide Integrated Pest Management Project.
1999 (2nd Edition). Integrated Pest Management for Apples and Pears
(No. 3340). University of California, Oakland. 216 p.
Available for $30 (plus $5 shipping), made payable to U.C.
Regents, from:
University of California
Division of Agriculture & Natural Resources
Communication Services-Publications
6701 San Pablo Ave.
Oakland, CA 94608
1-800-994-8849
Van der Zwet, Tom and Norman F. Childers (eds.) 1982. The Pear.
Horticultural Publications, Gainesville, FL. 503 p.
Out of print, but obtainable through the Interlibrary Loan
System at your local library.
Back to top
Appendix: Fireblight, Disease Resistance,
and the Disease Triangle
The occurrence of disease—any disease on any plant or animal—is
dependent on three factors: 1) a susceptible host, 2) the presence
of the disease-causing pathogen, and 3) a suitable environment for
infection and disease development (the "disease triangle;"
see Figure 1). This fact is very important
to understanding fireblight incidence in pear orchards, especially
in terms of managing this potentially devastating disease in different
parts of the country.
Let's start at the top of the triangle with a corollary to point 1:
heritable disease resistance is rarely absolute. When a plant breeder
or plant pathologist refers to a pear cultivar as "fireblight
resistant," he or she is speaking in relative terms. Usually
we read or hear something like "somewhat resistant," "very
resistant," "moderately resistant," "slightly
susceptible," etc. To further complicate matters, different researchers
use different rating scales to describe resistance/susceptibility—there
is no agreed-upon standard. For example, some published studies rely
on a "1 to 10" scale, which is usually based on a visual
estimate of damage, while others may use more absolute measurements,
such as the centimeters of shoot tissue affected by fireblight.
Regardless of the failings of the various rating systems, the phenomenon
of differential resistance/susceptibility is real—pear cultivars
vary greatly in their reaction to the presence of Erwinia amylovora.
Some cultivars, such as Magness and Shinko, will only rarely suffer
any fireblight strikes, and, if they do, the blight will rarely
intrude into any wood older than one year. In contrast, when all
three factors of the disease triangle conspire, fireblight can kill
even some older trees of susceptible European cultivars in a single
year.
The second face of the triangle—presence of the pathogen—is
the one that seems to prompt most of our efforts as growers. When
we spray copper, Blight Ban™, or one of the antibiotics, we
are trying to reduce or exclude (in the case of Blight Ban) the
pathogen from potential infection sites. There is also a geographic
component to this part of the triangle, as E. amylovora
is much less prevalent in some parts of the world than in others.
The third face of the triangle—an environment conducive to
the disease—is also related to geography, mostly by climate.
Where the climate is warm and wet, especially in the spring, fireblight
infection and development are favored. The more arid parts of the
western U.S. are not nearly as prone to fireblight problems as most
of the East. But these large climatic and geographic features are
not the only components of a disease-promoting or disease-suppressive
environment—the orchardist can manipulate parts of the orchard
micro-environment to help suppress disease development.
For example, reducing fertilization can limit fast-growing succulent
tissue, which is especially prone to infection and seems to provide
the disease its fastest entry into the wood.
Another practice that can be adjusted to alter the orchard environment
is pruning, which can "open up" a tree to allow more rapid
drying of plant tissues and thus prevent disease. However, a grower
in a different situation might need to reduce pruning in
order to avoid fostering the lush shoot growth (which is more susceptible
to fireblight) that usually follows heavy pruning.
There is considerable interplay, of course, between the faces of
the disease triangle. It is especially important to understand that
given high levels of inoculum and the proper environmental conditions,
medium levels of varietal resistance can be overcome. For example,
Shin-Li—released by the University of California as "fireblight
resistant"—is relatively resistant in the climate of
California, but can suffer severe fireblight infection in the Southeast
if not adequately protected by spraying. If you plant Bartlett in
South Carolina, for another example, and don't spray, you can watch
the disease triangle imitate the Bermuda Triangle, as your trees
disappear one-by-one and year-by-year.
Organic Pear Production
By Martin Guerena and Holly Born
NCAT Agriculture Specialist
Richard Earles, Editor
Cole Loeffler, HTML Production
IP167
Slot 36
Back to top |