of Strawberry 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 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
(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.
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 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.
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
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
(Figure 5) Phomopsis leaf blight on strawberry.
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
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
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.
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
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
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-soakedEspots.
(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
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.
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
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.