of Strawberry Diseases
Strawberry Fruit Rots
Fruit Rot (Gray
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.
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
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
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).
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
(Figure 23) Leather rot symptoms on an immature
(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.
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.
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.
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
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.
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.