Mold of Strawberry
is one of the most common and serious diseases wherever strawberries are
grown. It is caused by the fungus, Botrytis cinerea. In wet seasons
on unsprayed plants, 80-90% losses of flowers and fruit can occur. The
disease thrives during prolonged rainy and cloudy periods just before
or during harvest, and on dense, lush, foliar growth. Unless suitable
controls are used, frequent irrigation for frost control can lead to serious
losses; greatest losses occur from blossom infections. Fruit infections
often start on injuries to the flower stalks (pedicels) and caps (sepals),
on green fruit damaged by frost, or where dead petals adhere to the developing
Young blossoms are very susceptible to infection. One or several
blossoms in a cluster may show blasting (browning and dying) that
usually extends down the pedicel. Light gray masses of dusty spores
soon appear and are easily dislodged and carried by air currents
to other blossoms. Such infections are most common in well protected
areas of the plant, where the humidity is high and air movement
on image for larger version
Figure 1. Strawberry with Botrytis Fruit Rot. Infected berries
turn light brown but remain firm. This fungus produces a velvety
gray growth with powdery dry spores.
on soil and touching another decayed berry or a dead leaf in dense foliage
are commonly infected. Fruit infections appear as soft, light brown, rapidly
enlarging spots (see figure). The berry soon dries out, turns a darker
brown, "mummifies," and is covered with a gray, dusty powder
the spores of the Botrytis fungus. Immature berries may develop
infection, but they become more susceptible as they ripen. The disease
is often not detected until berry picking time, when many soft, brown,
rotted fruits are found. Pickers handling infected fruits will spread
infection to healthy fruit, causing good berries to become a rotted mass
within 48 hours after being picked.
The gray mold fungus overwinters as many minute, irregular, black, fungal
bodies (sclerotia) and as dormant mycelia on many kinds of plant debris,
such as dead strawberry leaves, stems, and fruit, and even on annual weeds
in the strawberry patch and adjoining fence rows. As spring approaches,
these sclerotia produce large numbers of microscopic spores (conidia).
Wind, splashing water, and human activity spread the conidia throughout
the strawberry patch, depositing them on blossoms, stems, young fruit,
and leaves. Parts of the strawberry plant may become infected within three
hours. Temperatures between 70° to 80°F (21° to 27°C)
and free moisture on the foliage from rain, dew, fog, or irrigation are
ideal conditions for spore germination and infection. Infections may occur
at lower temperatures when plants are wet for longer time periods. The
fungus usually attacks through dying, dead, or injured petals, stamens,
flower stalks, berry caps, or other plant tissue. Fruit infections commonly
originate at the stem end. The Botrytis fungus can penetrate the
unbroken skin of the strawberry fruit. One affected berry may contaminate
many others in the field or even after fruit has been harvested.
Several strawberry cultivars, which include Canoga, Guardian, and Honeoye,
appear to be partially resistant to gray mold. Under certain conditions,
however, these cultivars may also become infected. Cultivars that produce
the most exposed fruit suffer the least damage.
Select a sunny planting site with good air and soil drainage that
is not subject to frost injury.
Proper spacing of plants and timing of fertilizer applications are
important preventive measures. Avoid wide, matted rows of densely
spaced plants. Narrow the row to 8 to 12 inches at renovation. Apply
a suggested fertilizer in the summer on the basis of a soil test.
Heavy applications of nitrogen fertilizer in the spring produce excessive
amounts of dense foliage. Thick foliage shades berries and prevents
rapid drying of the fruit after wet periods, thus creating ideal conditions
for development of gray mold rot.
Mulching strawberry plants and row middles with clean straw or other
dry organic matter or with black polyethylene sheeting to keep fruit
from direct contact with the soil, reduces disease incidence.
Whether plants are mulched or not, cultivate as little as possible
from early bloom until after harvest. Control weeds at renovation
time by cultural and chemical methods. Limiting wounding of plants
slows disease spread.
Whenever possible, pick fruit frequently and early in the day as
soon as plants are dry. Cull out all diseased berries but do not leave
them in the field.
Always handle berries with care to avoid bruising. The fungus enters
Refrigerate picked fruit promptly to control gray mold growth.
Follow a fungicide spray schedule as needed to control gray mold.
Protect blossoms, fruit, pedicels, and leaves from infection by uniformly
spraying all above-ground plant parts. Preventing blossom infection
may double the yield of top-quality fruit. Applications should be
repeated at 7- to 10-day intervals through the fruit harvesting period.
Insecticides are usually added to fungicide sprays to control strawberry
weevils, "cat-facing" plant bugs, spittlebugs, leafrollers,
thrips, aphids, leafhoppers, other insects, and mites. Do not use
these insecticides during bloom, when pollinating insects might be
Spray at weekly intervals if the weather is rainy, foggy, or overcast,
or if dews are heavy; if sunny and dry, stretch the spray interval
to 10 days. If irrigations are used for frost control, more frequent
fungicide applications may be needed. Where feasible, spray a day
or two before rain is predicted.
Home fruit growers (lobbyists) should follow the spray program outlined
in Illinois Extension Circular 1145, "Home Fruit Pest Control".
Commercial growers should obtain a copy of the Illinois Commercial
Small Fruit and Grape Spray Guide. These two publications are
updated frequently to provide current recommendations on new products
and labelled rates.1
For more details concerning suggested cultural management practices, including
fertilization, those who grow strawberries at home should consult Illinois
Extension Circular 935, "Growing Small Fruits in the Home Garden."
Commercial growers should check the most recent Proceedings of the
Illinois Small Fruit and Strawberry Schools. Copies of the annual
proceedings can be obtained by writing to Jeff Kindhart, Dixon Springs
Agricultural Center, Simpson, Illinois 61985. Copies of Circular 935
may also be available at your nearest Extension office or from the Office
of Agricultural Publications, 54 Mumford Hall, 1301 West Gregory Drive,
Urbana, Illinois 61801.
Stephen M. Ries (email@example.com)