Apple Scab Presentation

Apple Scab
Disease
a
Plant
Pathology
Presentation
by
Lisa K. Schlag
30 November 2006

Apple Scab: Introduction
For centuries, apple scab has been a disease problem plaguing
susceptible species and cultivars in the Malus genus.

A painting by Italian artist Michelangelo Merisi Caravaggio,
Basket of Fruit, 1596, depicts scab-infected apples.


A painting by American artist James Peale, Fruit Still Life with
Chinese Export Basket, 1824, depicts scab-infected apples.

Apple scab, caused by the fungus Venturia inaequalis, is still
one of the most serious and economically important diseases
of susceptible species and cultivars in the Malus genus that
exists worldwide.
In the United States, it is especially severe in the
northeastern and north central states where cool,
moist spring and summer weather favors the causal
fungal pathogen.
significant defoliation for 2-3 years causes a general
weakening or loss of vigor that could result in:
♦ increased winter injury; and
♦ susceptibility to secondary diseases and insects.

Disease losses attributable to apple scab if scab control
measures are not taken
reduction in amount and quality of marketable apple
harvest.
poor fruit bud development for next year.

http://ohioline.osu.edu/hyg-fact/3000/3003.html

Est. Production Value losses if control measures are not taken
www.ers.usda.gov/publications/fts/sep04/FT

25

20
70% crop loss: (Volk, 2002)

15
OH Total Production:

10

5
S312.pdf

0
1999 2000 2001 2002 2003 2004(est.)
OH Ttl PV of Apple Crop (in 14.858 no data 20.296 18.76 24.66 27.4
millions)
Ttl PV less 70% disease loss (in 4.457 no data 6.088 5.628 7.398 7.289
millions)

Disease losses attributable to apple scab if scab control
measures are not taken
reduction in crabapples aesthetic appeal and wildlife value
caused by premature defoliation and fruit drop.

www.apsnet..org

Apple Scab: Disease Cycle
Pathogen
V. inaequalis is an Ascomycetes sac fungus producing both
sexual and asexual spores
ascospores are sexual spores that
initiate one primary cycle of apple
www.apsnet.org

scab disease annually
♦inaequalis named for unequal-
sized brown “footprint”shaped
two-celled spores
conidia are asexual spores that
initiate one or more secondary
cycle(s) of apple scab disease
annually
♦ cycle name Spilocaea pomi
www.caf.wvu.edu/kearneysville/disease_descriptions
/disease_images/F7a.jpg

Pathogen
V. inaequalis belongs to the group of pathogenic fungi
known as hemitrophes
parasitic phase is spent as mycelium and spores on
living tissue penetrating only the cuticle and staying
between cuticle and epidermal cells.

www.agf.gov.bc.ca/cropprot/tflpm/applescb.htm

Pathogen
V. inaequalis belongs to the group of pathogenic fungi
known as hemitrophes
saprophytic phase overwinters as psuedothecia initial
penetrating dead leaf tissue of fallen debris.

(Wilcox, 2001)

Growth Stages of Northern Ohio Malus sp.
♦ Dormant
www.ipmcenters.org/cropprofiles/docs/ohapples.html

◊ before growth starts in spring
♦ Green Tip (Early April)
◊ when buds start to break, showing green color of new
leaves
♦ Tight Cluster - buds short-stem’d & closely pack’d (End of April)
♦ Pink (Early May)
◊ when flower buds start showing color of petals
♦ Bloom (Mid-May)
◊ when flower buds open
♦ Petal Fall (Mid- to Late May)
◊ when flowers drop their petals
♦ First, second, and third cover
◊ 10-14 days after petal fall (End of May)
◊ 10-14 days after first cover (Early June)
◊ 10-14 days after second cover (End of June)

Dissemination
Movement of fungal pathogen V. inaequalis is passive
moisture must be present to induce expulsion of
ascospores from psuedothecia
♦ ascospores disseminated by splashing rain and wind
♦ few spores mature at green tip with maturity
progressing slowly until tight cluster
♦ discharge usually peaks between stages pink
through bloom, and nearly all discharged
within 2-4 weeks after petal fall
conidia disseminated by rain, washed or splashed,
and/or wind

Inoculation
Primary inoculum
♦ ascospores forcibly released by asci into air
when mature pseudothecia become thoroughly
wet during spring rain
♦ discharged over a 5-9 week period
◊ starting at bud break (green tip) and peaking
from pink to petal fall stages
Secondary inoculum
♦ conidia arise from primary and secondary lesions
when humidity levels are at least 60-70%
♦ discharged over growing season
◊ throughout remainder of growing season

Penetration
V. inaequalis is a nonobligate hemitrophe
direct penetration
♦ascospores and conidia develop fine hyphae which
directly pierces cuticle and grows in a subcuticular
position between cuticle and epidermal cells
◊ fungus may degrade and use cuticle as well as
some host cell wall and pectic materials for
nutrients
◊ once leaves fall to ground, mycelium penetrates
tissue below existing lesions
does not always lead to infection

Infection
V. inaequalis spores require a film of water on plant tissue for a
specific period of temperature-dependent time to germinate
and infect susceptible Malus sp. hosts
infections affected by age of leaves and fruit
♦ leaves most susceptible 1-5 days after unfolding
♦ fruit most susceptible until 3-4 weeks after petal fall
♦ leaves and fruit become less susceptible with maturity
Primary scab infection
♦ occurs from bud break until 2-4 weeks past petal fall
Secondary scab infection(s)
♦ can occur several times during growing season when
suitable conditions prevail

Infection -- Revised Mills Table
Average Temperature1
www.nysaes.cornell.edu/pp/extension/tfabp/components

Average temperature during the wetting period in Fahrenheit or

Incubation
Fº Cº Wetting Period (hr)2 Period (days)3
79 26 11.3 --
77 25 8 --
2Minimum wetting period needed to achieve infection

75 24 6.1 --
73-63 23-17 6 9-10
61 16 6.1 9-10
3Time in days until visible symptoms appear

59-57 14-15 7 12-13
55 13 8 14
54 12 8.3 14
52 11 9 15
50 10 11 16
48 9 12.2 17
46 8 13.4 17
44 7 15.4 17
43 6 18 17
/revmills.html

40 5 21.2 --
39 4 27.8 --
37 3 29.6 --
Celsius

35 2 34.7 --
34 1 40.5 --
1

Infection -- Revised Mills Table
Mills Curves www.nysaes.cornell.edu/pp/extension/tfabp/mills.shtml

♦ 1944, first attempt at using forecasting to help growers
time applications of sulphur dusts
♦ relates hours of leaf wetting and temperature during
wetting period to scab infection
Revised Mills Table – reflects most up to date information
♦ shorter times for infection by ascospores and conidia
♦ does not rate severity of infection periods
◊ rating severity on wetness duration misleading
◊ other factors important – suceptibility; inoculum level
♦ day vs night ascospores
◊ over 95% of ascospores not released until after sunrise
◊ conidia not affected by light or darkness

Growth & Reproduction
Primary scab cycle
♦ by late winter or early spring, each
www.caf.wvu.edu/kearneysville/wvufarm8.html

mature, flask-shaped pseudothecia
produces 50-100 asci with each asci
containing 8 ascospores
◊ first ascospores become mature
about bud break
♦ within 9-17 days of primary
infection, ascospore lesions
produce conidiophores which bear
10,000’s+ of potentially infectious
secondary scab spores called conidia
These produce, produce, produce… additional conidiaphores and
conidia in fall, V. inaequalis colonizes tissues beneath former scab
lesions producing pseudothecia during dormant season

Growth & Reproduction
Secondary scab cycle
♦ conidia form olive-green to
brown, velvety textured

www.apsnet.org
scab lesions that push up
through the cuticle
♦ can continue throughout the
spring, summer, and
autumn
♦ in autumn, V. inaequalis
(cited: slide 8)

colonizes tissues beneath
former scab lesions
producing pseudothecia
initial during dormant
season

P Primary scab cycle
o
l
y
c
y
c

Secondary
scab cycle
l
i
c
D
i
s
e
a
s (Agrios, 2005)
e

Apple Scab: Symptoms
Susceptible Plant Parts
In Spring, symptomatic spots (lesions) are most
noticeable on leaves and fruit; however, symptoms
of V. inaequalis infection can also appear on:
♦ petioles;
♦ pedicels;
♦ apple blossoms;
♦ bud scales; and
♦ young twigs/shoots

Lower Leaf Surface
In spring, first spots (lesions) usually located on lower
surface of expanding leaves

(Wilcox, 2001)

Upper Leaf Surface
Lesions progress to olive-green to brown coloration and
velvety texture. As fungal growth stops the lesion margins
change from feathery, indistinct to distinct

www.uni-kl.de/FB-Biologie/AG-Hahn/Research/venturia%20engl.html

Upper Leaf Surface
Scab sheet is a condition where
entire leaf surface is covered
with apple scab disease
www.ppdl.purdue.edu/ppdl/weeklypics/Weekly_Pict
ure6-10-02-1.html, Gail Ruhl

later yellowing stage

Fruit
circular, necrotic olive-
green or brown colored
warty lesions with distinct
margins on fruit infected
early in development

fruit infected late in season
small black spots of “pinpoint”
scab noticeable after harvest
apples in storage
www.caf.wvu.edu/kearneysville/disease_
descriptions/disease_images/photo2-9.jpg

Pedicel just after Petal Fall
Small, dark green lesions on pedicel before, during, or after
bloom

www.caf.wvu.edu/kearneysville/disease_descriptions/disease_images/AppleScabPedicel.jpg

Apple Scab: Disease Control
Treatments
Natural
♦ disease-resistant Malus sp. cultivars
♦ biological
♦ environment create environment
unfavorable for pathogen
Cultural
♦ sanitation
♦ pruning
♦ growing practices
Chemical
♦ protectant (protective) fungicides
♦ post-infection (suppressant) fungicides
Integrated Pest Management

Treatments
orchard.uvm.edu/uvmapple/pest/backtobasics/diseases

www.apsnet.org
Flail mower chopping leaf debris Airblast sprayer applying fungicides

www.apsnet.org
/scab.htm

Leaf wetness monitor Mills Table-based weather monitoring system

Natural
Plant disease-resistant Malus sp.cultivars
♦ best means of managing apple scab on crabapples
♦ fresh market apples not as popular
♦ disease resistancy may be temporary
◊ through sexual reproduction, V. inaequalis can
overcome resistance
biological
♦ New Zealand -- yucca extract prevents germination of
fungus and had curative effect
♦ antagonistic fungi may decrease ascospore
production -- viable option in future
environment
♦ humidity < 60% and drier weather conditions

Natural
top varieties of fresh market apples grown in Ohio1 vary in
their susceptibility to apple scab disease:
Very susceptible Moderately susceptible
♦ Red Delicious ♦ Golden Delicious
All require
♦ Rome Beauty ♦ Jonathan fungicide
♦ McIntosh usage
while some are highly resistant and immune to scab and
well suited to organic commercial & backyard growers:
♦ Enterprise ♦ Liberty ♦Priscilla
♦ Freedom ♦ Macfree ♦ Redfree
♦ Gold Rush ♦ Nova Easy ♦ Sir Prize
♦ Jonafree ♦ Prima ♦ William’s Pride
1www.ipmcenters.org/cropprofiles/docs/ohapples.html
ohioline.osu.edu/hyg-fact/3000/3003.html

Natural
crabapple cultivars with excellent scab resistance and
superior horticultural characteristics for Ohio, including
being small-fruited for wildlife:
♦ Malus baccara ‘Jackii’ ♦ Red Jewel ♦ Silver Moon
♦ Bob White ♦ Royal Splendor ♦ Strawberry Parfait
♦ Ormiston Roy ♦ Malus sargentii ♦ Sugar Tyme
♦ Prairifire ♦ Sentinel ♦ Malus x zumi
‘Calocarpa’
crabapples initiate buds for next season mid-June to early
July

Cultural
Create unfavorable environment for pathogen
reduce occurrence of primary innoculum
♦ collect & destroy all fallen leaves
♦ dolomitic lime or 5% urea applied to ground of fallen
leaves hastens leaf decomposition
♦ flail mowing to chop leaf litter in autumn
decrease drying time of plant tissues during growth
♦ prune to increase sunlight penetration into and
improve air movement through tree canopy
♦ avoid overhead irrigation
◊ irrigate soil around trees
plant right tree in right location
♦ plant new trees in well-drained, sunny locations
♦ pathogens can attack healthy plants

Chemical
no chemical treatments were employed until the
late 1800’s
♦ at that time, cooper- and sulphur- based fungicides
were used as pre-infection protection.
◊ these treatments caused substantial foliar damage
cultural practices influence fungicide usage
♦ fungicides are not an alternative to the use of sound
cultural practices
standard Malus sp. cultivars vary widely in their
susceptibility to scab
♦ influences intensity of control program

Chemical -- Fungicides
Protectant -- 5-7 day spraying schedule in early season
♦ prevent spores from germinating or penetrating leaf tissue
♦ must apply to susceptible tissue before infection occurs
◊ green tip & 1 week past petal fall critical time
Post-infection – every 7-10 days later in season
♦ penetrates tissue to eliminate/inhibit lesion development
♦ effectiveness limited to few hours/days depending on
fungicide
monitor rainfall in orchard
♦ 1/2”-1” rain needed to deplete half fungicide residue on
fruit and foliage

www.ipmcenters.org/ te n il
n na l ta

eb
ta

m
cropprofiles/docs/

lp e
ha hy bu

su lim

z
r
ap

ba

hu

co
p clo
ohapples.html
c io et

r

an
fe
th -m y

m
m
dormant x x
green tip x x x
tight cluster x x x x
pink x x x
bloom x x x x
petal fall x x
1st & 2d cover x x x x
3d cover x x
remaining x x

Fungicide selection should be based on:
♦ other diseases that must be managed
♦ scab fungus resistance
♦ history of disease in orchard or property landscape
♦ final market for fruit
♦ other social and economic factors
Multi-purpose sprays containing insecticides and
fungicides are convenient , but have disadvantages
♦ none controls all insects and diseases
♦ lack flexibility
◊ only use after bloom period to prevent lethal harm
to bees; fungicides alone are not harmful to bees
♦ more expensive than chemicals you prepare

Integrated Pest Management (IPM)
Integration of all available control methods
Effective disease management program should
emphasize integrated use of:
♦ knowledge of pathogen and disease;
♦ disease resistant plants;
♦ specific cultural practices; and
♦ timely application of chemicals when needed.
◊ weather monitoring equipment and disease
forecasting computer programs can predict scab
infection periods
▪ Envirocaster ; Metros

Apple Scab: References
Books
Agrios, G.N. (2005). Plant Pathology (5th ed.) Burlington, MA:Elsevier
Academic Press.
Cloyd, R.A., Nixon, P.L., & Pataky, N.R. (2004). IPM for Gardeners:
A Guide to Integrated Pest Management. Portland, OR: Timber
Press.
Dirr, M.A. (1998). Manual of Woody Landscape Plants (5th ed.)(pp. 630-
660). Champaign, IL: Stipes Publishing.
Sinclair, W.A., & Lyon, H.H. (2005). Diseases of Trees and Shrubs (2nd
ed.)(pp. 86-87). Ithaca, NY: Cornell University Press.

Electronic
Apple Scab (April 2006). Retrieved 2 Nov. 2006, from Cornell University, Plant
Disease Diagnostic Clinic Web site:
http://plantclinic.cornell.edu/FactSheets/apple_scab_factsheets/apple_scab.htm
Apple Scab. Retrieved 17 Nov. 2006, from New York State IPM Program, from
Cornell Cooperative Extension Web site:
www.nysipm.cornell.edu/factsheets/treefruit/diseases/as/as.asp
Apple Scab of Flowering Crabapples (March 2002). Retrieved 20 Nov. 2006,Bull.BP-39-W,
from Purdue University Cooperative Extension Service Web site.
Biggs A.R. & Hickey K.D. Apple Scab. Retreived 20 Nov. 2006, from NYS IPM Tree
Fruit Factsheet, New York State Agricultural Experiment Station Web site:
http://www.nysaes.cornell.edu/pp/extension/tfabp/apscpm.shtml
Chatfield J.A., Draper E.A., Cochran K.C., Bristol P.W., & Tubesing C.F. (1999).
Ornamental Plants – Annual Reports and Research Reviews 1999. Retrieved 7 Nov. 2006,
from Ohio State University, Ohioline Web site:
http://ohioline.osu.edu/sc173/sc173_16.html
Chatfield J.A., Rose M.A., Ellett C.W.,et al.(1996). Disease Control in the Landscape.
Retrieved 10 Nov. 2006,Bull.614, from Ohio State University Ohioline Web site:
http://ohioline.osu.edu/b614/b614.html

Electronic
Ellis M.A., Chatfield J., & Draper E. (1994). Scab of Apple and Crabapple. Retrieved 7
Nov. 2006, from Ohio State University, Ohioline Web site:
http://ohioline.osu.edu/hyg-fact/3000/3003.html
EU researches promise disease-free apples (July 2006). Retrieved 21 Nov. 2006, from
Food navigator Web site:
http://www.foodnavigator.com/news/printNewsBis.asp?id=69114
Grove G., & Xiao, C-L (2005). Apple Scab. Retrieved 2 Nov. 2006, from Washington
State University Extension Web site:
http://cru.cahe.wsu.edu/CEPublications/pnw0582/pnw0582.pdf
Hartman J.R.(June 1996). Apple Scab. Retrieved 2 Nov. 2006, from University of
Kentucky, Cooperative Extension Service Web site:
http://www.ca.uky.edu/agc/pubs/ppa/ppa24/ppa24.pdf
Schroeder D.B. Marrotte E.L. (Ed.), Apple Scab (February 2003). Retrieved 7 Nov.
2006, from University of Connecticut, Integrated Pest Management Web site:
http://www.hort.uconn.edu/ipm/homegrnd/htms/19apscab.htm
Turechek J. & Carroll J., The Mill’s Table. Retrieved 27 Nov. 2006, from New York
State Agricultural Experiment Station Web site:
http://www.nysaes.cornell.edu/pp/extension/tfabp/mills.shtml

Electronic
Volk, T. (2002). Tom Volk’s Fungus of the Month for September 2002. Retrieved 7 Nov.
2006, from University of Wisconsin-La Crosse, Tom Volk Fungi Web site:
http://botit.botany.wisc.edu/toms_fungi/sep2002.html
Wilcox W.F. Apple Scab. Retrieved 17 Nov. 2006, from New York State Integrated Pest
Management Web site:
www.nysipm.cornell.edu/factsheets/treefruit/diseases/as/as.asp
Wilcox W.(2001). Apple Scab. Retrieved 20 Nov. 2006, from NYS IPM Tree Fruit
Factsheet, New York State Agricultural Experiment Station Web site:
http://www.nysaes.cornell.edu/pp/extension/tfabp/apscpm.shtml
The American Phytopathological Society web site presents information on many plant
diseases and their control. Retrieved 31 October 2006.
www.apsnet.org

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