GRAY MOLD OF STRAWBERRY
John R. Hartman and Donald E. Hershman
Several fungi can cause developing
and ripe strawberries to rot. In Kentucky the most common and economically
important of these diseases is gray mold, caused by the fungus Botrytis
cinerea. Also called Botrytis rot or ash mold, it is an ever-present
threat to strawberries both in the field and after harvest. It can be particularly
devastating because the disease strikes after investments in the crop have
been made, but before returns have been fully realized.
Causal Organism and Disease Development
B. cinerea is both very common
and well adapted for survival. It occurs on a wide range of hosts and overwinters
in dead leaves and decaying plant tissue. As the temperatures warm in the
spring, infective spores are produced and disseminated to susceptible strawberry
tissue by air currents, splashing rain or insects. When these spores contact
water, they germinate and infect plants within hours. B. cinerea
has an advantage over many pathogens in that it has the ability to colonize
either living or dead tissue. Many times, the fungus first becomes established
in dead or dying tissue and then moves into healthy tissue.
Fruit infection is often most severe
in shaded and protected areas under the canopy of plants where air movement
is poor and humidity high. Also, fruit resting on the soil or touching
other diseased berries are commonly affected. The fungus often first colonizes
deteriorating floral parts. From there, infections may quickly destroy
the developing berry, or less often remain latent until fruit are well
ripened or even harvested. Fruit infections usually appear as soft, light
brown areas on the fruit surface. These areas spread rapidly throughout
the berry until it is completely destroyed. Rotted berries retain their
general shape and become tough and dry. Little or no leak is associated
with the disease. As the berries dry out, they become covered by a distinctive
gray, dusty-appearing growth of the fungus, from which the disease gets
Though B. cinerea is active
over a wide range of temperatures and humidities, optimum conditions for
infection and disease development occur between 60 and 70°F, and at
relative humidities above 90%. Under these conditions, gray mold can explode
in an unprotected berry patch and produce a severe epidemic in as little
as 28 hours. As the disease becomes more and more established in a particular
field, it becomes proportionally harder to control.
Certain cultural practices help control
gray mold by promoting faster drying of foliage and fruit while other practices
reduce exposure to fungal inoculum.
•Select a planting site with good soil
drainage and air circulation.
•Expose planting to full sun.
•Orient plant rows toward the prevailing
•Apply appropriate nitrogen levels
to prevent excessive foliage from developing
•Mulch plants with straw to reduce
fruit contact with soil.
•Pick fruit frequently.
•Cull out and remove diseased berries
from the planting.
•Handle berries with care to avoid
bruising and refrigerate harvested fruit promptly at 32-50°F (0-10°c).
Gray mold control can be aided by applying
protective fungicides beginning at or before bloom and continuing until
harvest. Where gray mold has been a significant problem before, applications
should begin at the white bud stage of flower development. Also, where
frost has damaged a planting and a marketable crop remains, great care
should be taken to maintain a strict fungicide spray program.
To reduce the possibility of fungicide-resistant
strains of Botrytis developing, apply the systemic fungicides as
a tank mix or alternating with nonsystemic fungicides not likely to produce
resistance problems. For specific information regarding chemicals available
and use rates, consult ID-94, Kentucky Commercial Small Fruit &
Grape Spray Guide, available at your County Extension office.