Introduction
Disease management strategies are very similar for both
organic and conventional small fruit production systems in the Midwest.
In both systems it is important to develop and use an integrated
disease management program that integrates as many disease control
methods as possible, the more the better. Major components of the
disease management program include: use of specific cultural
practices; developing knowledge of the pathogen and disease biology,
use of disease resistant cultivars, and timely application of
organically approved fungicides or biological control agents or
products when needed. These guidelines have been written for
caneberries (raspberry and blackberry), strawberry, blueberry and
grape. Specific information is provided for each crop in its respective
chapter. Most disease control methods or strategies are identical for
both conventional and organic production systems. Perhaps the greatest
difference between organic and conventional production systems is that
organic growers are not permitted to use synthetic "conventional"
fungicides. If disease control materials are required in the organic
system, growers are limited to the use of "inorganic" fungicides such
as sulfur (elemental sulfur and lime-sulfur) or copper fungicides
(Bordeaux mixture and fixed copper products). In addition, there are
several new "alternative" disease control materials and biological
control products that are currently available and are cleared for use
in organic production.
There are several problems associated with the use of
these inorganic fungicides and "alternative" products in small fruit
disease control programs. Among the most important are 1) Phytoxicity,
which is the potential to cause damage to foliage, fruit set and fruit
finish (this is a concern primarily with copper and sulfur fungicides);
and 2) their limited spectrum of fungicide activity, which
means they may not be capable of providing simultaneous control of the
wide range of fungal pathogens that can cause economic damage to the
crop. For example, sulfur is highly effective for controlling powdery
mildew on most fruit crops, but provides little or no control of most
other diseases.
In a climate like the Midwest, environmental conditions
during the growing season are generally very conducive (warm and wet)
to the development of several important diseases, insect pests and
weeds. Limitations in relation to which pesticides may or may not be
used, present the organic grower with some unique and very demanding
challenges. Whereas the use of various cultural practices and disease
resistance will be the "back bone" of the organic disease management
program, the limited use of organically approved pesticides or
biocontrol agents will probably be required at times.
Integrated Management
of Strawberry Diseases
The objective of an integrated disease management program
is to provide a commercially acceptable level of disease control on a
consistent (year-to-year) basis. This is accomplished by developing a
program that integrates all available control methods into one program.
An effective disease management program for strawberries must emphasize
the integrated use of specific cultural practices, knowledge of the
pathogen and disease biology, disease resistant cultivars and timely
applications of organically approved fungicides or biological control
agents, when needed. In order to reduce the use of fungicides to an
absolute minimum, the use of disease resistance cultivars and various
cultural practices must be strongly emphasized. Many strawberry
varieties adapted to the Midwest have good resistance to a number of
important diseases (Table 1). This is generally not the case with other
small fruit crops. In Strawberry, the use of disease resistant
varieties is especially important for organic production.
Identifying
and Understanding the Major Strawberry Diseases
It is important for growers to be able to recognize the
major strawberry diseases. Proper disease identification is critical to
making the correct disease management decisions. In addition, growers
should develop a basic understanding of pathogen biology and disease
cycles for the major strawberry diseases. The more you know about the
disease, the better equipped you will be to make sound and effective
management decisions. The following literature contains color
photographs of disease symptoms on strawberries, as well as information
on pathogen biology and disease development:
Strawberry
Production Guide - This is a very comprehensive book covering
most phases of strawberry production. It can be purchased from:
Northeast Regional Agricultural Engineering Service, 152 Riley-Robb
Hall, Cooperative Extension, Ithaca, NY 14853. Phone: 607-255-7654.
Compendium
of Strawberry Diseases - Published by the American
Phytopathological Society, 3340 Pilot Knob Rd., St. Paul, Minnesota
55121. Phone: 612-454-7250 (1-800-328-7560). This is the most
comprehensive book on strawberry diseases available. All commercial
growers should have a copy.
Foliar Diseases
There
are three major leaf diseases of strawberries in the Midwest. They are
leaf spot, leaf scorch, and leaf blight (Figure 1). All three diseases
can occur singly or together on the same plant or even on the same
leaf. All three are caused by fungi. Under favorable environmental
conditions, these three diseases can cause serious reductions in
strawberry yields. They damage the strawberry plant by causing
premature leaf death, reduction in fruit quality, a general weakening
of the plant, and (in some situations) plant death. In order to
maximize strawberry production, these leaf diseases must be recognized
and controlled. Fortunately, several varieties have good resistance to
leaf spot and leaf scorch (Table 1).
(Figure 1) Leaf spot and leaf scorch usually appear first in early to
mid-spring. Leaf blight is more common during the summer and early fall.

Leaf Spot
Symptoms
Leaf
spot is caused by the fungus Mycosphaerella fragariae. The leaf
spot fungus can infect leaves, fruit, petioles, runners, fruit stalks,
and berry caps or calyxes. The most obvious symptoms of the disease are
small, round spots. These spots develop on the upper surface of the
leaf and at first are dark purple to reddish-purple (Figure 2). They
range in size from 1/8 to 1/4 inch in diameter. With time, the centers
of the spots become tan or gray and eventually almost white; while
their margins remain dark purple. Later in the season, tan or bluish
areas form on the underleaf surface. Symptoms on other plant parts,
except fruit, are almost identical to those on the upper leaf surface.
On fruits, superficial black spots may form during moist weather
(Figure 3). The spots form on ripe berries and around groups of seeds.
They are about 1/4 inch in diameter, and usually there are only one or
two spots per fruit. However, some fruits may be more severely
infected.
(Figure 2) Strawberry leaf spot symptoms on leaflet.

(Figure
3) Black seed disease on strawberry fruit. This disease is caused by
the same fungus that causes strawberry leaf spot.

Disease Development
This
fungus can produce two types of spores that infect newly-emerging
leaves in spring. First, older infected leaves that remain alive during
winter may give rise to conidia (spores) that are spread to new foliage
by splashing water or by handling infected plants. Another type of
spore (ascospore) is produced in speck-sized black perithecia, which
form at the edges of the leaf spots during autumn. In the spring, these
ascospores are forcibly ejected from perithecia and are carried by wind
or water to new leaf tissue.
Infection by both types of spores occurs through the underleaf surface.
Temperatures between 65 and 75 F are optimum for infection and disease
development. Infections may occur throughout the growing season, except
during hot, dry weather. Young, expanding leaves are the most
susceptible to infection.
Leaf Scorch
Symptoms
Leaf
scorch is caused by the fungus Diplocarpon earliana. The leaf
scorch fungus can infect leaves, petioles, runners, fruit stalks and
caps of strawberry plants. Leaf scorch symptoms are very similar to the
early stages of leaf spot. Round to angular or irregular dark-purple
spots up to 1/4 inch in diameter are scattered over the upper leaf
surface (Figure 4). As spots enlarge, they resemble small drops of tar.
This tar-like appearance is caused by the formation of large numbers of
minute, black, fungal fruiting bodies (acervuli). The centers of the
spots remain dark purple. This distinguishes the disease from leaf spot
where the center turns white. If many infections occur on the same
leaf, the entire leaf becomes reddish or light purple. Severely
infected leaves dry up and appear scorched. Similar, but elongated,
spots may appear on other affected plant parts. Lesions may girdle
fruit stalks causing flowers and young fruit to die. Infections on
green berries are rare, appearing as red-to-brown discolorations or a
flecking on the fruit surface. The leaf scorch fungus can infect
strawberry leaves at all stages of development.
(Figure
4) Leaf scorch on strawberry. First symptoms are individual red spots.

Disease Development
The
fungus overwinters on infected leaves that survive the winter. In the
spring, conidia are produced on both leaf surfaces in speck-sized,
black acervuli. The fungus also produces ascospores in the early spring
within disk-shaped apothecia (fungal fruiting structures) that appear
as black dots in old lesions on the lower surface of diseased leaves
that died during winter. In the presence of moisture, ascospores
germinate within 24 hours and infect the plant through the lower leaf
surface. After symptom development, conidia are produced on the leaf
spots in large numbers throughout the growing season. Therefore,
repeated infections occur whenever weather conditions are favorable.
Conidia are spread mainly by splashing water.
Leaf Blight (Phomopsis Leaf Blight)
Symptoms
Leaf
blight is caused by the fungus Phomopsis obscurans. Leaf blight
is found most commonly on plants after harvest. The disease is
distinctively different from both leaf spot and leaf scorch. The
enlarging leaf spots of this disease are round to elliptical or angular
and a quarter of an inch to an inch in diameter (Figure 5). Spots are
initially reddish-purple. Later, they develop a darker brown or
reddish-brown center surrounded by a light-brown area with a purple
border. Similar spots may sometimes develop on the fruit caps. Usually,
only one to six lesions develop on a leaflet. Often the infected area
becomes V-shaped with the widest part of the "V" at the leaf margin.
New lesions appear throughout the summer and fall if weather conditions
are favorable. Older leaves become blighted and may die in large
numbers. This disease is usually more destructive on slow growing or
weak plants. The same fungus can cause an enlarging, soft, pale-pink
rot at the stem end of the fruit. Information on resistance to leaf
blight in currently used varieties is limited. If growers encounter a
high level of disease on certain varieties, these varieties should be
avoided.
(Figure
5) Phomopsis leaf blight on strawberry.

Disease
Development
This
fungus produces spores (conidia) in speck-sized, black pycnidia (fungal
fruiting bodies) embedded in the centers of older leaf lesions. Conidia
ooze out of pycnidia during damp weather when temperatures are high.
Conidia are splashed to new leaf tissue where they germinate in the
presence of free water to initiate new infections on leaves and fruit.
The fungus overwinters on either infected leaves that survive the
winter or in dead tissue on old infected leaves.
Powdery Mildew
Powdery
mildew is caused by the fungus Sphaerotheca macularis.
Generally the disease is not a serious problem in the Midwest; however,
under the proper environmental conditions and on highly susceptible
varieties, the disease can become serious. Disease resistance is
available in several varieties. Growers are encouraged to avoid highly
susceptible varieties.
Symptoms
Foliage
symptoms usually are the most obvious. An upward curling of leaf edges
usually is the first symptom seen. Dry, purplish or brownish patches
develop on the lower surface of infected leaves and reddish
discoloration may develop on the upper surface (Figure 6). Patches of
white, powdery fungus mycelium may appear on the undersides of leaves
as the disease progresses (Figure 7).
(Figure
6 )Reddish-purple discoloration of leaf often associated with powdery
mildew infection

(
Figure 7) Patches of white fungus growth on strawberry infected with
powdery mildew.

Disease
Development
The
fungus that causes strawberry powdery mildew infects only wild and
cultivated strawberries. This pathogen can not survive in the absence
of living host tissue. It apparently overwinters in infected leaves.
Spores are carried by wind to infect new growth in the spring.
Development and spread of powdery mildew is favored by moderate to high
humidity and temperatures of about 60 to 80 F (15 to 27 C). Unlike most
other fungi that cause plant disease, powdery mildew does not require
free water for spores to germinate and infect. In dry years, when most
other diseases are not a problem, powdery mildew can be very serious.
Angular Leaf Spot (Bacterial Blight)
Angular
leaf spot or bacterial blight of strawberries is caused by the
bacterium Xanthomonas fragariae. In the Midwest, it is the only
reported strawberry disease that is caused by a bacterium. The disease
was first reported in Minnesota in 1960, and has since been found in
other regions of the United States. It appears to be spreading rapidly
to many strawberry-growing areas of the world with the importation of
planting material. Although the disease has not been a major problem in
the Midwest, it can become serious and does represent a potential
threat to production. Copper fungicides have been recommended for
control of bacterial blight with varying degrees of success when
applied in a protectant program. Once the disease is established in the
planting there is little or nothing that can be done to control it.
Hot, dry weather is the best cure for the disease. Cultivars differ in
their susceptibility to the disease. None are completely resistant, but
Cavendish, Annapolis, Allstar, Honeoye, and Kent are all highly
susceptible.
Symptoms
Typical
symptoms of angular leaf spot appear initially as minute, water-soaked
lesions on the lower leaf surface (Figure 8). These lesions enlarge to
become angular spots, usually delineated by small veins. An important
distinguishing characteristic of this disease is that lesions are
translucent when viewed with transmitted light, but dark green when
viewed with reflected light (Figure 9).
(Figure
8) Angular leaf spot (bacterial blight) symptoms on lower leaf surface.
Note the “water-soakedチEspots.

(Figure
9) Angular leaf spot (bacterial blight) symptoms on upper leaf surface.
Note the translucent, yellow spots.

Under
moist conditions, lesions often have a viscous bacterial exudate on the
lower leaf surface. When it dries, the exudate forms a whitish, scaly
film. This exudate or film is an additional characteristic that is
useful in the identification of angular leaf spot.
Lesions
may coalesce to cover large portions of the leaf. Eventually, lesions
become visible on the upper leaf surface as irregular, reddish-brown
spots, which are necrotic and opaque to transmitted light. A chlorotic
halo may surround the lesion. At this stage, symptoms may be difficult
to distinguish from those of common leaf spot and leaf scorch.
Heavily
infected leaves may die, especially if major veins are infected.
Occasionally, under natural conditions, infection follows the major
veins, resulting in veinal water-soaking that may or may not spread to
the interveinal regions.
Infection
by X. fragariae may become systemic. The pathogen can infect
all plant parts except fruits and roots and, in some cases, even the
fruits have been infected, apparently only in the tissue adjacent to an
infected calyx (fruit cap). Calyx infection can be serious. Infected
tissues turn black resulting in unattractive fruit (Figure 10).
(Figure 10) Angular leaf spot (bacterial blight) symptoms on strawberry
calyx. Note the brown discoloration and drying.

Disease
Development
Inoculum
for the primary infection of new growth in the spring comes from
infected dead leaves where the pathogen overwintered. X. fragariae
may survive for extended periods in dry leaves or in infected leaves
buried in the soil. Spread is primarily from infected leaf debris or
infected crowns.
Bacteria
that exude from lesions under high-moisture conditions may provide
secondary inoculum. Bacteria may be disseminated to uninfected plants
or leaves by splashing water, such as rain or overhead irrigation. X.
fragariae gains entrance into host tissue either passively through
wounds or actively as motile cells that swim into natural plant
openings by means of drops of dew, gutation fluid, rain, or irrigation
water.
Very
little is known about the epidemiology of angular leaf spot.
Development of the disease is favored by moderate to cool daytime
temperatures around 68 F (20 C), low nighttime temperature (near or
just below freezing) and high relative humidity. Long periods of
precipitation, sprinkler irrigation to protect plants from freezing, or
heavy dews in the spring also favor the disease. Young leaf tissue or
leaves on healthy, vigorous plants are more likely to become infected
than those on diseased or environmentally stressed plants.
Strawberry Root Diseases
Red Stele
Red
stele is caused by the soil-borne fungus Phytophthora fragariae.
Many commercial strawberry cultivars are susceptible to the red stele
fungus; however, many varieties have good resistance to several races
of the red stele fungus (Table 1). The use of disease resistant
varieties and selection of sites with good soil drainage are the key
methods of control. This root rot disease has become a serious problem
facing strawberry production in the northern two-thirds of the United
States. The disease is most destructive in heavy clay soils that are
saturated with water during cool weather. Once it becomes established
in the field, the red stele fungus can survive in soil up to 13 years.
Normally, the disease is prevalent only in the lower or poorly drained
areas of the planting; however, it may become fairly well distributed
over the entire field, especially during a cool, wet spring. The red
stele fungus may become active at a soil temperature of 40 F. However,
the optimum soil temperature for growth and disease development is
between 55-60 F. Under favorable conditions of high soil moisture and
cool temperatures, plants will show typical disease symptoms within 10
days after infection.
Symptoms
When
plants start wilting and dying in the more poorly-drained portions of
the strawberry field, the cause is very likely red stele disease
(Figure 11). Infected plants are stunted, lose their shiny green
luster, and produce few runners. Younger leaves often have a metallic
bluish-green cast. Older leaves turn prematurely yellow or red. With
the first hot, dry weather of early summer, diseased plants wilt
rapidly and die. Diseased plants have very few new roots compared to
healthy plants that have thick, bushy white roots with many secondary
feeder roots (Figure 12). Infected strawberry roots usually appear
gray, while the new roots of a healthy plant are yellowish-white.
(Figure
11) Plants dying from red stele root rot.

(Figure
12) Root system from a red stele infected strawberry plant.

The
best way to identify the disease is to carefully dig up a wilted plant
and peel off the outside portion of several roots. The inside or
central portion of the root is known as the stele. If the stele is pink
to brick red or brownish red, the plant has the red stele disease
(Figure 13). The stele of normal plants is yellowish-white. The red
color may show only near the dead tip of the root or it may extend the
length of the root. The red stele is best seen in the spring up to the
time of fruiting. No other disease of strawberry produces this symptom.
(Figure
13) Longitudinal section of a healthy (left) and red stele infected
(right) strawberry root.

Disease
Development
The
red stele fungus is introduced into new planting sites mainly through
the distribution of infected plants. The fungus can be spread within a
field or area by anything that carries or moves infested soil
(implements, shoes, water, etc.). Once in the field, spores (oospores)
of the fungus produce large numbers of smaller spores (zoospores).
Zoospores are motile and swim about when soil moisture is high.
Zoospores invade the tips of young fleshy roots. Once in the roots, the
fungus grows and destroys the water and food conducting tissues
resulting in wilting and plant death (Figure 14). As soil temperatures
rise, the fungus forms large numbers of oospores in the stele of
infected plants. These oospores survive periods of hot, dry, and
freezing weather for several years in the soil.
Figure
14: Red stele disease cycle. Disease cycle of Red Stele Root Rot
on strawberry. We wish to thank the New York State Agriculture
Experiment Station for use of this figure. Figure taken from the Small
Fruit IPM Disease Identification Sheet No.2

Verticillium Wilt
Verticillium
wilt, caused by the soil-borne fungus Verticillium albo-atrum,
can be a major factor limiting production. When a plant is severely
infected, the probability of it surviving to produce a crop is greatly
reduced. The Verticillium fungus can infect nearly 300 different host
plants, including many fruits, vegetables, trees, shrubs, and flowers
as well as numerous weeds and some field crops. Once it becomes
established in the field or garden, it may remain alive for 25 years or
longer. Several varieties have resistance to Verticillium wilt. The use
of resistant varieties and proper site selection and crop rotation to
avoid infested soil are the key methods of control.
Cool,
overcast weather interspersed with warm, bright days is most favorable
for development of Verticillium wilt. Optimal conditions for infection
and disease development occur when soil temperatures are 70 to 75 F.
Many
soils in the Midwest contain the Verticillium wilt fungus. The fungus
can be introduced into uninfested soil on seeds, tools, farm machinery,
and from the soil and roots of transplants.
Symptoms
The
first symptoms of Verticillium wilt in new strawberry plantings often
appear about the time runners begin to form. In older plantings,
symptoms usually appear just before picking time. Symptoms on
above-ground plant parts may differ with the susceptibility of the
cultivar affected. In addition, above-ground symptoms are difficult to
differentiate from those caused by other root infecting fungi.
Isolation from diseased tissue and culturing the fungus in the
laboratory are necessary for positive disease identification.
On
infected strawberry plants, the outer and older leaves drop, wilt, turn
dry, and become reddish-yellow or dark brown at the margins and between
veins (Figure 15). Few new leaves develop and those that do tend to be
stunted and may wilt and curl up along the midvein. Severely infected
plants may appear stunted and flattened with small yellowish leaves.
Brownish-to-bluish black streaks or blotches may appear on the runners
or petioles. New roots that grow from the crown are often dwarfed with
blackened tips. Brownish streaks may occur within the decaying crown
and roots. If the disease is serious, large numbers of plants may wilt
and die rapidly (Figure 16). When the disease is not so serious, an
occasional plant or several plants scattered over the entire planting
may wilt and die.
(Figure
15) Strawberry plant dying from verticillium wilt. Note the outer
leaves die first
. 
(Figure
16) Strawberry field showing symptoms of verticillium wilt.

Disease
Development
The
fungus overwinters in soil or plant debris as dormant mycelium or black
speck-sized bodies (microsclerotia). These microsclerotia can remain
viable in the soil for many years. Under favorable environmental
conditions, they germinate and produce thread-like fungal structures
(hyphae). Hyphae can penetrate root hairs directly or through breaks or
wounds in the rootlets. Once inside the root, the fungus invades and
destroys the water-conducting tissue. Destruction of water-conducting
tissue results in reduced water uptake by the plant; thus, plants wilt
and wither. As fungal colonies get older, they produce microsclerotia
in infected host tissue and the disease cycle is completed.
Black Root Rot
Black
root rot is the general name for several root disorders which produce
similar symptoms. The disorders are not clearly understood and are
generally referred to as a root rot complex. Although the exact cause
of the black root rot is not known, one or more of the following is
thought to be responsible: soil fungi (such as Rhizoctonia and
Fusarium); nematodes; winter injury; fertilizer
burn; soil compaction; herbicide damage; drought; and excess salt,
water or improper soil pH. Black root rot has been found in every
strawberry-growing area of the United States. Injured plants may be
scattered throughout the planting or localized in one or more areas
(Figure 17). A considerable incidence of black root rot has been
observed in recent years throughout the Midwest. Once the disease is
established (shows up) in the planting, little or nothing can be done
to control it.
(Figure
17) Strawberry field showing black root rot symptoms.

To
recognize black root rot symptoms, it is necessary to know what a
normal root looks like. Newly developed main roots of a normal
strawberry plant are pliable and almost white. After several months of
growth, they generally become woody and are dark brown to black on the
surface. When this dark surface is scraped away, a yellowish-white
living core can be seen. Small feeder roots that branch out from the
main roots should be white as long as they are active.
Roots
affected by black root rot have one or more of the following symptoms:
1) the root system is much smaller than normal; 2) the main roots are
spotted with dark patches or zones (Figure 18); 3) the feeder roots are
lacking or are spotted with dark patches or zones; 4) all or part
of the main root is dead (Figure 19). A cross-section of the dead root
shows it blackened throughout. Plants with black root rot are less
vigorous than normal plants and produce fewer runner plants. Severely
affected plants usually die.
(Figure
18) Strawberry root with black root rot symptoms. Note black
discoloration (lesions) on the root.

(Figure
19) Strawberry root system with advanced stages of black root rot. Note
the dead, black "Rat-tail" roots.

Strawberry Fruit Rots
Botrytis Fruit Rot (Gray Mold)
One
of the most serious and common fruit rot diseases of strawberry is gray
mold. Gray mold is caused by the fungus Botrytis cinerea. Under
favorable environmental conditions for disease development, serious
losses can occur. The gray mold fungus can infect petals, flower stalks
(pedicels), fruit caps, and fruit. During wet springs no other disease
causes a greater threat to flowers and fruit. The disease is most
severe during prolonged rainy and cloudy periods during bloom and
harvest. Abundant gray-brown, fluffy, fungal growth on infected tissue
is responsible for the disease's name "gray mold".
During
wet, cool springs, gray mold will be a major threat to organic
strawberry production. In conventional production systems, application
of fungicide during bloom generally results in good disease control.
Fungicides used in organic systems (sulfur and copper) are not very
effective for control of Botrytis. Several biological control products
are currently available for Botrytis control; however, their
effectiveness under moderate to heavy disease pressure is questionable.
Resistance is not available in most varieties; therefore, the use of
several cultural practices are the key control methods in organic
plantings.
Symptoms
Young
blossoms are very susceptible to infection. One to several blossoms in
a cluster may show blasting (browning and drying) that may spread down
the pedicel. Fruit infections usually appear as soft, light brown,
rapidly enlarging areas on the fruit (Figure 20). If it remains on the
plant, the berry usually dries up, "mummifies", and becomes covered
with a gray, dusty powder (Figure 21). Fruit infection is most severe
in well-protected, shaded areas of the plant where the humidity is
higher and air movement is reduced. Berries resting on soil or touching
another decayed berry or a dead leaf in dense foliage are most commonly
affected. The disease may develop on young (green) fruits, but symptoms
are more common as they mature. Often, the disease is not detected
until berry picking time. During harvest, the handling of infected
fruit will spread the fungus to healthy ones. After picking, mature
fruits are extremely susceptible to gray mold, especially if bruised or
wounded. Under favorable conditions for disease development, healthy
berries may become a rotted mass within 48 hours after picking.
(Figure
20) Immature strawberry fruit with symptoms of Botrytis fruit rot (gray
mold). Note the symptoms usually develop first on the calyx end of the
fruit.

(Figure
21) Botrytis fruit rot (gray mold) on a mature strawberry fruit.

Disease Development
The
fungus is capable of infecting a great number of different plants. It
overwinters as minute, black, fungal bodies (sclerotia) and/or mycelium
in plant debris, such as dead strawberry leaves in the row. In early
spring, these fungal bodies produce large numbers of microscopic spores
(conidia), which are spread by wind throughout the planting. They are
deposited on blossoms and other plant parts where they germinate in a
film of moisture. Infection occurs within a few hours (Figure 22).
(Figure
22) Disease cycle of gray mold on strawberry. We wish to thank the New
York State Agriculture Experiment Station for the use of this figure.
Figure taken from the Small Fruit IPM Disease Identification Sheet No. 1

Disease
development is favored by wet conditions accompanied by temperatures
between 41 F and 86 F. Conditions that keep flowers and fruit wet, such
as rain, dew, or sprinkler irrigation encourage Botrytis rot.
Strawberries are susceptible to Botrytis during bloom and again as
fruits ripen. During the blossom blight phase of the disease, the
fungus colonizes senescing flower parts, turning the blossoms brown.
The fungus usually enters the fruit through flower parts, where it
remains inactive (latent) within the tissues of infected green fruits.
As the fruit matures, the fungus becomes active and rots the fruit.
Thus, while infection actually occurs during bloom, symptoms are
usually not observed until harvest. This is important to remember when
one considers control. Temperatures between 70 and 80 F and moisture on
the foliage from rain, dew, fog, or irrigation are ideal conditions for
disease development.
Leather Rot
Leather
rot is caused by the soil-borne fungus Phytophthora cactorum.
Leather rot has been reported in many regions throughout the United
States. In many areas, it is considered a minor disease of little
economic importance. However, excessive rainfall during May, June, and
July can lead to severe fruit losses and quality reduction. In 1981,
many commercial growers in the Midwest lost up to 50 percent of their
crop to leather rot. The leather rot fungus primarily attacks the fruit
but may also infect blossoms. Organic fungicides (sulfur and copper)
are not effective for control. The key control methods in organic as
well as conventional systems are maintaining a good layer of straw
mulch between fruit and the soil, and site selection or improvement for
good water drainage (avoid saturated soil).
Symptoms
The
leather rot pathogen can infect berries at any stage of development.
When the disease is serious, infection of green fruit is common. On
green berries, diseased areas may be dark brown or natural green
outlined by a brown margin (Figure 23). As the rot spreads, the entire
berry becomes brown, maintains a rough texture, and is leathery in
appearance. The disease is more difficult to detect on ripe fruit. On
fully mature berries, symptoms may range from little color change to
discoloration that is brown to dark purple (Figure 24). Generally,
infected mature fruit is dull in color and is not shiny or glossy.
Infected ripe fruit are usually softer to the touch than healthy fruit.
When diseased berries are cut across, a marked darkening of the
water-conducting system to each seed can be observed. In later stages
of decay, mature fruits also become tough and leathery. Occasionally, a
white moldy growth can be observed on the surface of infected fruit. In
time, infected fruit dry up to form stiff, shriveled mummies.
(Figure
23) Leather rot symptoms on an immature strawberry fruit.

(Figure
24) Leather rot symptoms on a mature strawberry fruit. Note the
purplish discoloration.

Berries
that are affected by leather rot have a distinctive and very unpleasant
odor and taste. Even healthy tissue on a slightly rotted berry is
bitter. This presents a special problem to growers in pick-your-own
operations. An infected mature berry with little color change may
appear normal and be picked and processed with healthy berries.
Consumers have complained of bitter tasting jam or jelly made with
berries from fields where leather rot was a problem. Leather rot is
most commonly observed in poorly-drained areas where there is or has
been free-standing water or on berries in direct contact with the soil.
Disease
Development
The
fungus survives the winter as thick-walled resting spores, called
oospores that form within infected fruit as they mummify (Figure 25).
These oospores can remain viable in soil for long periods of time. In
the spring, oospores germinate in the presence of free water and
produce a second type of spore called a sporangium. A third type of
spore called a zoospore is produced inside the sporangium. Up to 50
zoospores may be produced inside one sporangium. The zoospores have
"tails" and can swim in a film of water. In the presence of free water
on the fruit surface, the zoospores germinate and infect the fruit. In
later stages of disease development, sporangia are produced on the
surface of infected fruit under moist conditions.
(Figure
25) Disease cycle of leather rot on strawberry. We wish to thank the
New York State Agriculture Experiment Station for use of this figure.
Figure taken from the Small Fruit Crop IPM Disease Identification Sheet
No. 4

The disease is spread by splashing or wind-blown water from rain or
overhead irrigation. Sporangia and/or zoospores are carried in water
from the surface of the infected fruit to healthy fruit where new
infections occur. Under the proper environmental conditions, the
disease can spread very quickly. A wet period (free water on fruit
surface) of two hours is sufficient for infection. The optimum
temperatures for infection are between 62 and 77 F. As the length of
the wet period increases, the temperature range at which infection can
occur becomes much broader. As infected fruit dry up and mummify, they
fall to the ground and lie at or slightly below the soil surface.
Oospores formed within the mummified fruit enables the fungus to
survive the winter and cause new infections the following year, thus,
completing the disease cycle.
Strawberry Anthracnose
Anthracnose is a disease that can affect foliage, runners, crowns and
fruit. Various forms of anthracnose can be caused by several fungi. In
the Midwest, the most common form of the disease is fruit rot, caused
by the fungus Colletotrichum acutatum. Although the disease is
not very common, if it becomes established in the planting, serious
losses can occur. Organic fungicides (sulfur and copper) are not
effective for control. Midwest varieties with resistance are not
available. Once the disease develops on fruit in the planting there is
little that can be done to control it. Managing the movement of pickers
into and out of infested areas and adjusting irrigation practices can
be beneficial in preventing disease spread.
Symptoms
Affected
stems are sometimes girdled by lesions, causing individual leaves or
entire daughter plants to wilt. Under warm, humid conditions,
salmon-colored masses of spores may form on anthracnose lesions.
When
crown tissue is infected and becomes decayed, the entire plant may wilt
and die. When infected crowns are sliced open, internal tissue is firm
and reddish brown. Crown tissue may be uniformly discolored or streaked
with brown.
On
fruit, symptoms first appear as whitish, water soaked lesions up to 1/8
inch in diameter which turn brown and enlarge within 2 to 3 days to
involve most of the fruit (Figure 26). Lesions are covered with
salmon-colored spore masses. Infected fruit eventually dry down to form
hard, black, shriveled mummies. Fruit can be infected at any stage of
development.
(Figure
26) Anthracnose lesion on strawberry fruit.

Disease
Development
The
disease is probably introduced into new plantings on infected plants.
Spore production, spore germination, and infection of strawberry fruits
are favored by warm, humid weather and by rain. Spores require free
water on the plant surface in order to germinate and infect.
Anthracnose fruit rot is considered to be a warm-weather disease with
an optimum temperature for disease development near 80 F. Thus, the
disease is generally a problem in the Midwest when abnormally high
temperatures and rainfall occur during fruit set and harvest. Spores
are dispersed primarily by water splash. Once the disease is
established in the field, the fungus can overwinter on infected plant
debris, primarily old-infected, mummified fruit.
Plant Parasitic Nematodes
Plant
parasitic nematodes are microscopic round worms and are common in soils
throughout the Midwest. Lesion and root-knot nematodes are probably the
most destructive kinds in midwestern plantings. These organisms
restrict root growth by feeding directly on roots. This makes plants
less efficient at taking up water and minerals from the soil. Nematodes
can also cause strawberry roots to be more susceptible to root-rotting
fungi. Strawberry plantings in nematode infested soils are not
long-lived. Production will decline rapidly after one or two seasons.
Nematode damage is most common and most severe in replant situations,
because preceding crops increase nematode numbers and high populations
of these parasites may be present when the young plants are set. Under
these conditions, strawberries never develop strong root systems.
Symptoms
Strawberry
plants infested with nematodes are stunted and show symptoms of mineral
deficiencies and water stress, particularly as the berries form.
Because nematodes are unevenly distributed in the field, damaged plants
tend to occur in patches. Heavily infested plantings decline rapidly.
Root-knot
nematodes cause the formation of knots or galls on fine roots. Heavy
galling may cause abundant adventitious root formation and lead to a
"whiskery root" condition. Other types do not form such distinct root
symptoms. Infested roots are not well developed. Lateral roots may be
few. Roots attacked by lesion nematodes are dark in color.
Causal
Organisms
The
lesion nematode (Pratylenchus penetrans) and the northern
root-knot nematode (Meloidogyne hapla) are common in the
Midwest. The dagger nematode (Xiphinema americanum) is
frequently found. The dagger nematode is the vector of tomato ring-spot
virus, which it can acquire from common weed hosts, such as dandelion.
Ring nematodes (Criconemella spp.) and lance nematodes (Hoplolaimus
spp.) are also found in soils in the Midwest. Their effect on
strawberries is not known.
Use of
Disease Resistant Varieties
In the integrated
disease management program, the use of cultivars with disease
resistance must be emphasized. Many commercial cultivars have
resistance and/or tolerance to Leaf Spot, Leaf Scorch, Red
Stele, Verticillium Wilt and Powdery Mildew. The more disease
resistance within the program, the better. Table 1 lists ratings for
disease resistance in several of the more commonly grown cultivars.
This type of information is available from a number of sources. Most
nurseries should be able to provide information on disease resistance
for the cultivars they sell.
Table 1. Disease
resistance of Several Strawberry Cultivars Commonly Grown in the Midwest
June
Bearing
|
Disease
Resistancea
|
Cultivar
|
Season
|
Verticillium
Wilt
|
Red
Stele
|
Leaf
Diseasesb
|
Powdery
Mildew
|
Comments
|
| Earliglow
|
Early |
R |
R |
R |
PR |
Standard for early cultivars; berry size
medium. Excellent flavor, but only moderately productive.
|
| Veestar
|
Early |
T |
S |
T |
PR |
Early, productive. Has performed well in
Southeast PA, with medium bright berries. Fruit shows some Botrytis
resistance. |
| Annapolis
|
Early |
I |
R |
S |
S |
Fruit medium-large, firm, and glossy with
good flavor. Plants runner freely. Fairly susceptible to Botrytis.
|
| Northeaster
|
Early |
R |
R |
I |
S |
Very large, early, and firm fruit with
aromatic flavor and aroma. King berries slightly rough. Well adapted to
heavy soils. In PA, for trial only. |
| Mohawk |
Early |
R |
R |
PR |
T |
Medium-sized fruit, comparable to Earliglow.
Good flavor. Tolerant of Botrytis. Has been very variable, as
two lines of plant material exist. Plant only small quantities.
|
Avalon
|
Early |
R
|
R
|
T
|
R
|
Large berry with good color and
flavor,
average productivity and vigor. Has performed well in Southeastern PA.
For trial only. |
|
Sable |
Early |
U
|
R
|
PR
|
S
|
Veestar x Cavendish cross. Productive, well
suited to U-pick operations. Available in small quantities. For trial
only. Produces dense beds; Botrytis control may require more
effort than usual. |
Evangeline
|
Early |
U
|
S
|
U
|
R
|
Medium yields of conical, firm berries.
Flavor good if fully ripe. Berries produced on stiff, upright stalks.
May not runner well. For trial only. Limited quantities available. |
| June
Bearing |
Disease
Resistancea |
| Cultivar |
Season |
Verticillium
Wilt |
Red
Stele |
Leaf
Diseasesb |
Powdery
Mildew |
Comments |
| Honeoye
|
Early-mid |
S |
S |
PR |
T |
Large fruit, productive; has performed well
in PA, but lack of red stele resistance a concern. Tends to become soft
in hot weather. Flavor distinctive, "Perfumy". |
| Cavendish
|
Early-mid |
I |
R |
PR |
S |
Very large firm fruit with good flavor. Very
productive (yields 85% of Kent) and moderately vigorous. Tends to ripen
unevenly in certain years. |
| Brunswick |
Early-mid |
U |
R |
U |
U |
Good size and flavor. May perform better in
cooler locations. Susceptible to Phytophthora crown rot. For trial.
|
| June
Bearing |
Disease
Resistancea |
| Cultivar |
Season |
Verticillium
Wilt |
Red
Stele |
Leaf
Diseasesb |
Powdery
Mildew |
Comments |
Raritan
|
Mid |
S
|
S
|
S
|
S
|
Widely planted cultivar in spite of disease
susceptibility; first fruits are large, but size decreases more rapidly
than most cultivars. Very flavorful. |
| Guardian
|
Mid |
R |
R |
R |
S |
Very productive, firm, large fruit,
sometimes rough (uneven) looking. Botrytis is generally more
prevalent in Guardian. Tends to get a "Long neck" which breaks down and
allows easy entry for slugs and sap beetles. Susceptible to Sinbar
injury. |
| Redchief
|
Mid |
PR |
R |
R |
R |
Productive, with good color and size. Flavor
average. Excellent disease resistance. |
| Lester |
Mid |
S |
R |
R |
U |
Productive, good-sized berry. Flavor is
good, though size tends to "run down" quickly. Fruit if fairly
susceptible to Botrytis. |
| Kent
|
Mid |
S |
S |
S |
T |
Extremely productive berry with large firm
fruit. Tends to yield fruit in middle of rows, resulting in high rot,
so keep rows narrow. Flavor average. Susceptible to Sinbar injury.
|
| Settler
|
Mid |
T |
U |
T |
S |
Large attractive moderately firm fruit. Very
susceptible to Sinbar injury. In PA, for trial only.
|
| DelMarvel
|
Mid |
R |
R |
R |
U |
Very vigorous plants, with high production,
large, firm, aromatic fruit. In PA, for trial only.
|
| Primetime
|
Mid |
R |
R |
R |
| |