Sooty blotch
and flyspeck (sbfs) affect apples in nearly all orchards in the
Midwest. SBFS is caused by fungi that make
the fruit appear smudged with soot (sooty blotch) or speckled with clusters of
tiny black dots (flyspeck). Several IPM
techniques can be effective in reducing the risk of these diseases on apples.
Causal Organisms
Sooty blotch
and flyspeck (SBFS) are caused by several fungi, including Gloeodes pomigena, Zygophiala
jamaicensis, Leptodontium elatius, Peltaster fructicola, and Geastrumia
polystigmatis. Black smudges (sooty
blotch) on the fruit are caused by growth of fungal strands. Clusters of tiny black spots (flyspeck) on
the fruit are fungal reproductive structures.
Since SB and FS fungi do not penetrate the skin of the fruit, they can
sometimes be brushed or washed off.
Ecology
More than 50 species of plants act as
reservoir hosts of SBFS, including brambles, weeds, and apple fruit and twigs. Spores of the fungi are spread from
reservoir hosts by wind-driven rain beginning in the early summer. Although little is known about SBFS ecology,
the disease is often prevalent in years of high humidity and frequent rainfall.
Cultural Methods for SBFS Management
Most
growers apply fungicide sprays on a protectant schedule, every 10 to 14 days,
from late spring through summer to control SBFS in Iowa orchards. However, a wide range of Integrated Pest
Management (IPM) options are available to growers seeking to reduce pesticide
use and save money.
1. Pruning. A properly pruned tree will slow the development
of SBFS. Annual pruning will allow
better and faster drying of fruit
after rain or dew. SBFS fungi require
periods of wetness in order to grow, so keeping the surface of the fruit and
foliage as dry as possible can reduce disease. In addition, a well-pruned tree allows better
penetration of fungicides. Fruit in
the inner canopy of an inadequately pruned tree often show SBFS damage, even
in orchards that receive frequent fungicide sprays.
2. Bramble
Management. Various species of Rubus,
including blackberries and raspberries, are major reservoir hosts of SBFS. If you produce these berries on your farm,
it is advisable to plant them far away from the orchard or on the other side
of a windbreak or hedgerow. Wild brambles
located near the edges of orchard blocks should be mowed during the spring
and summer to reduce the amount of SB and FS spores.
3. Thinning. When SBFS fungi coat the surface of apples
early in the season, the apples are too small to touch each other. When the apples grow larger and form clusters,
tight clusters can provide SBFS fungi with a humid environment and a refuge
from fungicide. Harvested apples may
then have SBFS blotches on the surfaces that were protected by the cluster. Proper thinning will reduce clustering, thus
reducing the ability of these fungi to “hide” from fungicides.
4. Mowing. Mowing is another strategy that can promote better airflow and
faster drying in an orchard. Apples on
low-hanging branches, especially in tall grass, can remain wet with dew until
late morning. Regular mowing allows
low-hanging apples to dry quickly and discourages multiplication of SBFS.
5. Cultivar
Selection. Although no apple cultivars
are resistant to SBFS, some cultivars have physiological characteristics that
may reduce the risk of SBFS damage. For
example, early-maturing cultivars may show less damage because SBFS fungi have
less time to multiply on the apple surface.
Symptoms are also less visible on dark-skinned fruit than on lighter or
yellow-skinned fruit. Although dark red
apples have as many SBFS blemishes as yellow apples, the red apples can be more
appealing to consumers because the spots are not as obvious. Planting early-maturing, dark red apple
cultivars can therefore reduce the damage caused by SBFS.
Post-harvest dip treatment, followed by
brushing on an apple grading line, can remove some of the signs of SBFS.
Various
chlorine- and soap-based, food-grade commercial dipping solutions are
commercially available. Dip treatments
require floating the apples in the treatment solutions in poly tanks for about
5 to 7 minutes, followed by brushing for 30 to 60 seconds and spray-rinsing in
water. ISU research has shown that post-harvest chlorine dip treatment can
remove up to 80% of SBFS signs. This
treatment can pay off by significantly increasing the marketability of a
lightly to moderately diseased crop.
Chemical Methods for SBFS Management
Fungicides can be used effectively and
safely to manage SBFS. Various IPM
methods for timing sprays and choosing fungicides have been successful in
orchards in the Midwest.
1. Weather-based
Spray Timing. In recent years,
a warning system for SBFS, developed in North Carolina and Kentucky, has been
tested in the Midwest. In this system,
a total of 175 hours of wetness due to rain must be measured after the first
cover spray before the second cover spray is applied. Once the second cover spray is applied, fungicides are applied
preventatively (approximately every 10 to 14 days) until close to harvest. Experience with the warning system in North
Carolina indicates that a tank mix of a benzimidazole fungicide (Topsin M or
Benlate) and a contact fungicide (such as captan) should be used in the first-
and second-cover fungicide sprays when applying the warning system.
At the Iowa State University Horticulture Farm and the University of Illinois
Pomology Research Farm, the SBFS disease warning system has consistently saved
one to three summer fungicide sprays without increasing SBFS on apples.
Saving an average of two fungicide sprays per season can reduce input
costs by about $2,000/year for a 50-acre orchard.
Growers can obtain wetness data from
either of two sources:
an
electronic weather monitors in the orchard, or a commercial weather provider of
estimated wetness data.
Electronic weather monitors, such as the Wetness/Temperature Logger (Spectrum
Technologies, Plainfield, IL), can be purchased for about $250.
These sensors are about the size of a deck of cards and are mounted
in the lower canopy of a representative tree in the orchard at an angle of
45˚ to horizontal, beginning just before the first-cover spray.
The data gathered by the sensor can be downloaded to a personal computer,
usually about once per week. Easy-to-use software (cost: about $100) helps
you to organize the data and total the number of wetness hours per day.
Commercial services (such as SkyBit, Inc., Bellefonte,
PA) provide estimates of wetness hours in an orchard using the orchard’s geographic
location and data from the National Weather Service.
This data is e-mailed or faxed daily to subscribers for about $50 per
month. Field trials at Iowa State University, University
of Illinois, and University of Wisconsin indicate that SkyBit overestimated
wetness hours compared to a sensor placed in the orchard, but still resulted
in an average savings of about one spray per season.
With either type of wetness
measurement system, data collection begins immediately after the first-cover
fungicide spray is applied. Wetness
hours are added each day until they total 175 hours. Wetness period less than 4 hours long are not included in the
count. When the 175-hour threshold is
reached, the second cover spray is applied, after which a standard protectant
fungicide-spray schedule (every 10 to 14 days) is followed for the rest of the
season.
2. Reduced-risk
fungicides. SBFS control by
two “reduced risk” strobilurin fungicides, Sovran and Flint, was evaluated at the University of Illinois, Iowa State University,
and the University of Wisconsin-Madison. When sprays of kresoxim-methyl (Sovran7) or trifloxystrobin (Flint7) were alternated with tank-mixed
sprays of thiophanate-methyl (Topsin M) and captan, control of SBFS equaled
that of full-season sprays of thiophanate-methyl and captan. These strobilurin fungicides are less dangerous
to human health and the environment than older fungicides, but are considerably
more expensive than thiophanate-methyl plus captan.
3. Organic fungicides. Midwest trials have shown that potassium
bicarbonate (KHCO3), formulated as Kaligreen7 and other product names, is a moderately
effective treatment for SBFS. However,
although many potassium bicarbonate fungicides are approved for use by organic
growers against powdery mildew, they are not labeled for control of SBFS. These products should be applied only for
control of powdery mildew and are far less effective against SBFS than the
strobilurin fungicides (Sovran, Flint) or benzimidazoles (Benlate, Topsin M).
Effectiveness of Fungicides against SBFS
|
Fungicide
|
Activity against SB
|
Activity against FS
|
|
Flint, Sovran
|
Excellent 
|
Excellent 
|
|
Benlate, Topsin M
|
Excellent 
|
Good
|
|
Captan, Ziram
|
Fair to Good
|
Fair to Good
|
|
Ferbam, Thiram
|
Fair
|
Fair
|
|
Mancozeb (Dithane, Manzate, Penncozeb)*
|
Excellent 
|
Excellent 
|
|
Sulfur, Syllit
|
Poor
|
Poor
|
|
Bayleton, Nova, Procure,
Rubigan
|
None
|
None
|
*Cannot be sprayed within 77 days
of harvest.
As in any
disease management situation, cultural methods are very important to successful
management of SBFS. Cultural strategies
can be combined with post-harvest treatment and the use of fungicides in an
IPM-based program for environmentally safe and economical SBFS control.
Plant Disease
Clinic
323 Bessey
Hall
Dept. Plant
Pathology
Iowa State
University
Ames, IA
50011
Plant Clinic
University of Illinois
1401 W. St. Mary's Rd.
Urbana, IL 61802
Plant Disease Diagnostics
Clinic
Dept. Plant
Pathology
Univ.
Wisconsin-Madison
1630 Linden Drive
Madison, WI
53706-1598
Plant Diagnostic Clinic
42 Agriculture Building
University of Missouri
Columbia, Missouri 65211
• For more
information on SBFS, see the
Midwest Apple IPM web page
at:
www.public.iastate.edu/~appleipm
Prepared by
·
Mark Gleason, Extension Plant Pathologist and Professor,
Iowa State University
·
Sara Helland, Assistant Scientist, Department of Plant
Pathology, Iowa State University
·
Mohammad Babadoost, Extension Plant Pathologist and
Assistant Professor, University of Illinois
·
Patricia McManus, Extension Plant Pathologist and
Associate Professor, University of Wisconsin
·
Bruce Barrett, Extension Entomologist and Associate Professor,
University of Missouri
·
Richard Weinzierl, Extension Entomologist and
Professor, University of Illinois
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Issued
in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914,
in cooperation with the U.S. Department of Agriculture. Stanley R. Johnson, director, Cooperative
Extension Service, Iowa State University of Science and Technology, Ames,
IA.
sity
of Science and Technology, Ames, IA.
