Strawberry Diseases in Michigan

Michigan State University Extension
MSU Extension Fruit Bulletins - 17289401
11/30/99

Strawberry Diseases in Michigan Michigan State University Extension Extension Bulletin E-1728

Major revision October 1994
Sandra Perry and Donald Ramsdell, Department of Botany and
Plant Pathology
AG FACTS

Soil-Borne Root Diseases

Verticillium Wilt
The fungus (Verticillium albo-atrum) that causes
Verticillium wilt can be very damaging to strawberry
plantings. Several common garden crops (tomatoes, potatoes,
peppers, eggplants) and some woody ornamentals are also
susceptible to this disease. The fungus can
survive in the soil for many years. Wounds are not
necessary for infection: it occurs by direct penetration of
roots in infested soil. In new strawberry plantings,
disease symptoms appear about the time runner production
begins (mid-July). In established
plantings, the symptoms appear as the fruit begins to
ripen.

Symptoms
1. Older outer leaves wilt, turn brown around the edge and
between the veins (Fig. 1).

2. Leaf Petioles may turn red.

3. The crown and roots may show a brown discoloration
indicating decay (Fig.2).

Control: The fungus can be introduced into "clean" soil by
infected planting stock or by using implements contaminated
with Verticillium-infested soil.

The disease is most easily controlled by planting resistant
varieties of strawberries (see Table 1 for a list of
varieties and their disease resistance). To reduce the
chances of Verticillium wilt, do not plant strawberries
following tomatoes, potatoes, peppers, or eggplants.

Preplant soil fumigation should be done in the autumn prior
to spring planting, to help control Verticillium wilt.
Proper soil preparation for several months prior to
fumigation is essential. Consult the MSU Fruit Spraying
Calendar (Extension Bulletin E-154) for soil fumigant
recommendations.

Red Stele
Red stele root rot is caused by the fungus, Phytophthora
fragariae. This pathogen overwinters in infected roots and
can persist in the soil for many years. Red stele disease
is worse in heavy soil or poorly drained planting sites.
The fungus is most active in the spring and fall when soils
are cold and wet.

Symptoms: Disease symptoms don't become evident until the
bearing year (second season of growth). The fungus produces
spores (zoospores) that are spread by water and mechanical
means (machinery, tools, shoes, etc.). These spores infect
and destroy the small hair roots giving a "rat tail"
appearance to the remaining root system. The normally cream
colored core (stele) of the roots turns a rusty red to
dark-brown (Figs. 2 & 3). The discoloration may include the
entire stele or only the stele at the root tips. These
symptoms are best seen in the spring by cutting a root
lengthwise.The fungus is restricted to the roots but any
interference with root function is soon seen in leaf
symptoms. New leaves are small and bluish green, while old
leaves turn yellow or red. Plants are stunted, and little
or no fruit is produced. Infected plants die out very
quickly during dry weather.

Winter injury can also cause plants to appear stunted but
there will be discoloration in the plant crown that is not
present with red stele.

Control: Select a well drained planting site with loamy
soil. Purchase resistant plants from reputable growers.
Resistance to red stele disease is a relative term,
however. There are at least five distinct biological races
of the causal fungus and all varieties of strawberries are
not resistant to all races of the fungus. Resistant
varieties are an effective means of control. Also, consult
the MSU Fruit Spraying Calendar (Extension Bulletin E-154)
for chemical control recommendations.

Black Root Rot
The exact cause of this root rot is not known. Any
condition that causes root injury or generally weakens
plants will produce symptoms of black root rot. One or more
of the following factors are associated with its
appearance: soilborne fungi, nematodes, drought, winter
injury, excessive fertilizer application, or excessive soil
moisture. Plants may wilt, appear stunted, or die outright.
The leaves may turn purplish with red petioles.

By the second growing season, the main, or perennial roots,
of a healthy strawberry plant become woody. They have a
dark brown or black outer covering and the inside is
usually lighter colored. There should be masses of fine
white feeder roots branching from the main roots. A plant
affected by black root rot will have brown or black
(inactive) feeder roots or none at all. The outer root
covering will die and pull off easily. The white core of
the main roots may turn black in patches or along the
entire length of the root (Figs.2 & 4).

Control: If the disease is present in an established bed,
abandon that bed and choose a new site. The alternative is
to plow the plants under, work the soil for several months,
fumigate the soil in the autumn, and plant healthy
strawberry plants in the spring. Consult the MSU Fruit
Spraying Calendar (Extension Bulletin E-154) for fumigant
recommendations. Proper cultural practices can help
minimize the occurrence of black root rot. Plant certified
disease-free stock into a fertile, well-drained sandy loam.
Incorporating organic matter at planting time stimulates
beneficial biological activity in the soil. Adequate summer
mulch prevents drought stress, and a layer of mulch over
the plants for winter prevents winter injury to the crown.

Fruit Rot Diseases

Grey Mold
This disease is caused by the fungus, Botrytis cinerea,
which overwinters on plant debris on the ground. Rainy or
humid periods favor disease development. At the beginning
of bloom, the fungus attacks the blossoms and causes a
blossom blight leading to considerable crop loss. Fungus
spores form on the blighted blossoms and infection spreads
to both green developing fruit and ripening fruit. The
disease first appears on fruit as small, water-soaked areas
which are soon covered with grey, fuzzy-spore masses (Fig.
5).

Control: Fungicide sprays beginning at first bloom and
continuing at 7 to 10 day intervals through harvest are
effective in controlling grey mold. See Extension Bulletin
E-154 for fungicide recommendations.

Stem End Rot
This disease causes a rot at the stem end of the strawberry
fruit. The disease is caused by the fungus Dendrophoma
obscurans which overwinters in plant debris on the ground.
In early to mid-April, at about the time buds emerge from
the crown, the fungus attacks the new leaves and causes the
primary infection. Later, when fruit has formed, the calyx
(fruit cap) becomes infected and instead of remaining green
and healthy, turns brown. The fungus enters the fruit
through the infected calyx, causing it to rot (Fig. 6). If
uncontrolled, a considerable number of fruits may be rotted
by harvest time. There is a leaf infection phase connected
with stem end rot. It is called strawberry leaf blight:
described under "leaf diseases" in this
publication.Control: Apply the first fungicide spray when
leaves have expanded in the spring and flower buds are just
emerging from the crown. Subsequent sprays primarily aimed
at control of grey mold will control stem end rot for the
duration of the season. Consult Extension Bulletin E-154,
Fruit Spraying Calendar, for fungicide recommendations

Leather Rot
This disease is caused by the fungus Phytophthora cactorum.
This disease can become very important in some years, given
favorable environmental conditions. Ripening fruit clusters
that are touching the ground in standing water after
prolonged warm rains suddenly (in a day or two) turn
grey-brown and become mushy. The fruit stems often become
rotted. The disease progresses rapidly and a rotten smell
pervades the field. After a few days, infected berries
start to dry out and become leathery. If infected berries
are cut longitudinally, the vascular system of the fruit is
darkened and shows up as dark streaksradiating from the
fruit core outward (Fig. 7).
Control: Straw mulch on the ground to keep berries off the
soil is a very useful cultural control. See Extension
Bulletin E-154 for chemical control recommendations.

Leaf Diseases
Leaf ScorchThe fungus (Diplocarpon earliana) causing leaf
scorch can live through the winter in old, infected
strawberry leaves. Spores are easily spread by splashing
rain or by mechanical means, such as by machinery or people
passing through an infected planting. The first symptoms
are irregular purple spots 1/4" in diameter scattered
profusely over the upper leaf surface (Fig. 8). Spots may
appear at any time during the growing season. As the spots
age, black pimple-like fruiting bodies of the fungus are
produced in the center of each spot. Spots may rapidly
become so numerous that the entire leaf dries up and looks
"scorched" as though by fire. Spots may also occur on
petioles, stolons and fruit stalks.

Control: Plant resistant varieties. Remove and destroy
leaves after fruitbearing. The grey mold/stem end rot
fungicide program will usually control scorch. Consult
Extension Bulletin E-154 for appropriate fungicides.

Leaf Blight
Leaf blight is caused by Dendrophoma obscurans, the fungus
which also causes stem end rot. The leaf blight symptoms
first consist of roughly circular, purplish spots, 1/8 to
1/4 inch in diameter. Later, these lesions coalesce to form
large V-shaped dead areas in the leaves following the major
leaf veins (Fig. 9).

Control: See control for stem end rot.Leaf SpotThis disease
is caused by the fungus Mycosphaerella fragariae. The
fungus overwinters on old, infected, strawberry leaves. In
the spring, the pathogen attacks young leaves, leaf stalks,
calyces (caps) and stolons. Infection is favored by cool,
wet weather. Leaf spots are circular and 1/8 to 1/4 inch in
diameter (Fig. 10). The center of the lesion or spot is tan
and has a purple border. This same fungus can also cause a
fruit disorder called "black seed," but it is not usually
important in Michigan.

Control: Certain varieties are more resistant than others.
The normal fungicide sprays applied to control fruit rots
will us
ually control leaf spot. However, leaf spot disease can
build up during the first year after planting if growers do
not apply many fungicide sprays to non-bearing plants.
Consult Extension Bulletin E-154 for fungicide
recommendations.

Powdery Mildew
Powdery mildew is not a serious disease of strawberries in
Michigan. The fungus Sphaerotheca macularis, appears as a
greyish-white coating on the undersides of leaves late in
the growing season. Diseased leaves curl upward and turn
red or purple in color. Leaf function is impaired but
symptoms develop so late in the season that plant vigor is
seldom affected.

Control: Use resistant varieties.

Virus Diseases
There are several virus diseases that can infect
strawberries. Visible symptoms may not be evident in the
plants. Plant vigor is generally affected, how-ever,
causing the plants to die when weather conditions become
unfavorable. Visible symptoms can include leaf crinkling,
variegation, cupping, and stunting. Plants showing these
symptoms should be removed and destroyed. The predominant
virus-like disease of strawberries in Michigan is "June
Yellows" (Fig. 11). The main symptoms are stunting and
yellowing of leaf margins.

Control: Purchase only certified virus-free strawberry
plants. Many strawberry viruses are transmitted by aphids.
The only way to assure that the plants remain reasonably
virus-free is to carry on an aphid spray control program
throughout the growing season. However, if the strawberry
bed is renewed every 2 to 3 years, virus build-up should
not become a problem and no control measures are necessary.

Fig. 1. Verticillium wilt-diseased strawberry plant.
Fig. 2. Strawberry roots and crowns: healthy, black root
rot, Verticillium wilt and red stele. (Diagram courtesy of
Cornell University, Ithaca, New York)
Fig. 3. Discolored root core (stele) resulting from red
stele root rot.
Fig. 4. Darkened roots from black root rot. Note the white
feeder roots at the crown of the plant.
Fig. 5. Fuzzy spore masses on a Botrytis infected
strawberry.
Fig. 6. Diseased fruit cap and beginning of berry
infection from stem end rot. Note healthy berry at left
center.
Fig. 7. Discolored vascular system with radiating brown
streaks in a Phytophthora (leather rot) infected berry.
Fig. 8. Purple spotted leaves typical of leaf scorch
disease.
Fig. 9. Leaf spots and larger lesions along veins caused
by leaf blight disease.
Fig. 10. Purple bordered tan spots found on leaves with
leaf spot disease.
Fig. 11. Stunted and discolored leaves resulting from
"June yellows" virus infection.

MSU is an Affirmative-Action/Equal-Opportunity Employer.
Extension programs and materials are available to all
without regard to race, color, national origin, sex,
disability, age or religion.

Issued in furtherance of Extension work in agriculture and
home economics, acts of May 8 and June 30, 1914, in
cooperation with the U.S. Department of Agriculture. Gail
L. Imig, extension director, Michigan State University, E.
Lansing, MI 48824.

This information is for educational purposes only.
References to commercial products or trade names does not
imply endorsement by the MSU Extension or bias against
those not mentioned. This bulletin becomes public property
upon publication and may be printed verbatim with credit to
MSU. Reprinting cannot be used to endorse or advertise a
commercial product or company. Produced by Outreach
Communications and printed on recycled paper using
vegetable-based inks.

Major revision, destroy previous editions-10:94-3M-TCM-SP.
Price 75 cents, FILE 28.361

Disease resistance in strawberry varieties

Time of
initial
ripening
relative Leaf Leaf Red* Verticillium Powdery
Variety to Spot Scorch Stele Wilt Mildew
Earliglow
Allstar 2 weeks T T R I T
Annapolis same as ? ? T ? S
Blomidon 2 weeks ? ? S ? ?
Delite 2 weeks R R R R U
Earliglow - R R R R S to I
Guardian 1 week S to I R R VR S
Honeoye 1-2 days S to T T S S I
Jewel 2 weeks R R S S T
Kent 10 days S I S S S
Lateglow 3 weeks R R R VR S
Lester 5-7 days U R R S R
Midway 1 week VS S R I
Raritan 1 week S S S S to I S
Redchief 1 week S R R I R
Scott 1 week S to T R R S to I R
Surecrop 1 week I to R I R VR U
Tribute 1 week T T R T to R R(day neutral)
Tristar 1 week T T R R R(day neutral)

S=susceptible, VS=very susceptible, I=intermediate
reaction, R=resistant (the disease does not occur on that
cultivar or only to a very small degree), VR=very
resistant, T=tolerant (the disease is clearly evident but
with little or no apparent detrimental effect on plant or
yield), U=unknown.

*Cultivars are only resistant to specific races of the red
stele fungus. If races are present in the planting or are
introduced into the planting for which resistance genes are
not available, red stele can develop on "resistant"
cultivars.

Go To Top of File Michigan State University Extension Home Page Main Page for this Data Base MSU Extension Bulletin Catalog

This information is for educational purposes only. References to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned. This information becomes public property upon publication and may be printed verbatim with credit to MSU Extension. Reprinting cannot be used to endorse or advertise a commercial product or company. This file was generated from data base FR on 04/12/01. Data base FR was last revised on 11/30/99. For more information about this data base or its contents please contact cook@msue.msu.edu . Please read our disclaimer for important information about using our site.