|
back
| home
University
of Rhode Island GreenShare Factsheets
Botrytis
Fruit Rot/Gray Mold on Fruits Botrytis
cinerea
Many
fungi are capable of rotting mature or near-mature fruits of strawberry,
grape, raspberry, blueberry and blackberry. Serious losses can occur
under favorable environmental conditions for disease development.
One of the most serious and common fruit rot diseases is gray mold.
The gray mold fungus can affect petals, flower stalks (pedicels),
fruit caps and fruit. In wet, warm seasons, there is probably no
other disease capable of causing a greater loss of flowers and fruit.
The disease is most severe during years with prolonged rainy and
cloudy periods during bloom or during harvest. Although berries
of all grape varieties are susceptible to bunch rot, losses generally
are greater on tight-clustered varieties of Vitis vinifera
and French hybrids. Young
blossoms are usually very susceptible to infection. One or several
blossoms in a cluster may show blasting (browning and drying)
that
may extend down the pedicel. Fruit infections usually appear as
soft, light brown areas on the fruit which enlarge rapidly.
Infected
fruits remaining on the plant usually dry up, "mummify," and become
covered with gray, dusty spores, which give the disease its name
"gray mold." Fruit infection is most severe in well-protected areas
of the plant, where the humidity is high and air movement is poor.
On strawberry, 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 fruits become
more susceptible as they mature. The disease is not usually detected
until fruits are mature at harvest time. After picking, mature
fruits
are extremely susceptible to gray mold, especially if bruised.
The handling of infected fruit while picking will spread the fungus
to healthy ones. Under favorable conditions for disease development,
healthy berries may become a rotted mass within 48 hours of picking. The
gray mold fungus is capable of infecting a great number of different
plants. The disease cycle is very similar for strawberries, grapes,
blueberries and brambles. The fungus overwinters as minute, black
fungus bodies (sclerotia) or as mycelium in plant debris, such as
dead strawberry or raspberry leaves. Recent research has shown that
nearly all of the overwintering inoculum in strawberry plantings
comes from mycelium in dead strawberry leaves within the row or
planting. In early spring, the mycelium becomes active and produces
large numbers of microscopic spores (conidia) on the surface of
old plant (leaf) debris in the row. Spores are spread by wind throughout
the planting where they are deposited on blossoms and fruits. They
germinate when a film of moisture is present and infection can occur
within a few hours. Temperatures
between 70 and 80 degrees F and free moisture on the foliage from
rain, dew, fog or irrigation water are ideal conditions for
disease
development. The disease can develop at lower temperatures if foliage
remains wet for long periods. Grapes and berries are susceptible
to Botrytis during bloom and again as fruits ripen. Recent
research
indicates that most fruit infection actually occurs during bloom;
however, symptoms usually do not develop until close to harvest.
During bloom, the fungus colonizes healthy or senescing flower
parts,
often turning the blossoms brown. These blossom infections establish
the fungus within the receptacle of the young fruit as a "latent"
or "quiescent" infection. The fungus generally remains latent in
developing (green) fruit until the fruit starts to mature (the
fruit
sugar content increases and the acid content decreases to a level
that supports fungus growth), at which time the fungus becomes
active
and symptoms (rot) appear. Thus, the most critical period for applying
fungicides to control gray mold is during bloom. This is an important
point to remember when considering fungicide applications for controlling
this disease. • Select a planting
site with good soil drainage and air circulation. Plants should
be exposed to direct sunlight. Plant rows with the
direction of the prevailing wind to promote faster drying of foliage
and fruit. • A good layer
of straw mulch (or other material) between the rows or around
the plants aids greatly in controlling fruit rots. The
mulch acts as a barrier that reduces fruit contact with the soil. • Proper spacing
of plants and timing of fertilizer applications are also important.
Excessive applications of nitrogen fertilizer, especially
in the spring before harvest, can produce excessive amounts of
dense foliage. Shading of fruit by thick foliage prevents rapid
drying
of the fruit during wet periods and creates ideal conditions for
disease development. • Good weed control
is very important. Weeds prevent air movement in the plant
canopy. This slows drying time of flowers and fruits
and increases the chances for infection. Pick fruit frequently
and early in the day as soon as plants are dry. Cull out all
diseased
fruit but do not leave them in the field. Handle fruit with care
to avoid bruising. Refrigerate promptly at 32 to 50 degrees
F to
inhibit the gray mold fungus. • Fungicides
are an important disease management tool in commercial plantings,
but are generally not effective unless they are timed
properly and used in conjunction with the above cultural practices.
Homeowners are encouraged to emphasize the use of cultural
practices
in order to avoid the use of fungicides. Adapted
from Michael A. Ellis, Ohio State University Extension, 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
|