Botrytis Fruit Rot/Gray Mold
(Botrytis cinerea) 

CAUSAL AGENT AND SYMPTOMS.  Botrytis fruit rot (gray mold) is caused by the fungus Botrytis cinerea and is the most important disease of strawberry worldwide.  In Florida, this disease causes severe preharvest losses primarily due to infections of fruit and flowers, especially under humid conditions when daytime temperatures are moderate to warm (60 to 75F).  Botrytis fruit rot is also an important postharvest disease, since the fungus grows at refrigeration temperatures.  This pathogen infects a wide range of plants including many fruit, vegetable, and weed species.  On strawberry, infection begins at the flower stage but symptoms are typically observed on green or ripening fruit (Figure: Botrytis 1).  Fruit lesions are typically found on the stem end of the fruit and are generally associated with infected stamens, with dead petals adhering to the fruit (Figure: Botrytis 2) or trapped under the calyx.  Lesions begin as small, firm, light brown spots (Figure: Botrytis 3) that quickly enlarge and become covered with white fungal mycelia and gray to brown spores (Figure: Botrytis 4).  Botrytis can eventually consume and mummify the whole fruit (Figure: Botrytis 5).  When diseased fruit are disturbed, large numbers of spores are often released and are visible as gray puffs.

DISEASE DEVELOPMENT AND SPREAD.  Botrytis fruit rot epidemics are typically started by spores produced on dead strawberry foliage within the field (Figure: Botrytis 6).  The fungus colonizes young expanding strawberry leaves without producing any symptoms.  As the leaf senesces, the pathogen spreads quickly through the dying tissue and sporulates.  Spores (conidia) are dispersed by air, water or harvesting and ultimately infect different floral parts including stamens and petals.  After infecting the flower, the fungus eventually invades maturing fruit and causes rot.  Direct infection of fruit by spores is not considered important.  The fungus can also spread to adjacent fruit by direct contact.  As the epidemic progresses, the pathogen sporulates on diseased flowers and fruit, and these become important sources of inoculum.  The fruit rot phase of the disease can be particularly severe in annual production where plants produce flowers and fruit over several months.

CONTROL.  Control of Botrytis fruit rot and blossom blight requires a combination of chemical and cultural control methods.  Although no strawberry cultivars are highly resistant to Botrytis fruit rot, cultivars with large clasping calyxes can be more susceptible, probably because moisture collects between the calyx and the receptacle and promotes the spread of the pathogen from stamens and petals to the developing fruit.

Control of Botrytis fruit rot involves protecting flowers and leaves from infection, or to control sporulation of the fungus.  Effective disease management in annual winter production systems involves regular applications of a general protective fungicide combined with timed applications of specific fungicides during peak bloom periods.  Regular applications of protectant fungicides should begin immediately after overhead irrigation for plant establishment is done, and continue throughout the season.  Additional applications of protectant fungicides may be made during periods of moderate temperatures and rainy or humid weather.  Timed applications of fungicides specifically labeled for Botrytis should be made during peak flowering periods.  The first application should be made at 10% bloom and again 7 days later.  In Florida, it may be best to apply 3 to 4 bloom applications 7 days apart during the second peak bloom since Botrytis fruit rot is most severe during that period. 

 Control of Botrytis fruit rot requires a combination of chemical and cultural control methods.  There are differences in the susceptibility of commercial cultivars to Botrytis, and some cultivars are relatively resistant.   Growing strawberry in protected culture (large tunnels) can dramatically reduce the severity of Botrytis fruit rot epidemics (C. L. Xiao, et al, 2001. Comparison of Epidemics of Botrytis Fruit Rot and Powdery Mildew of Strawberry in Large Plastic Tunnel and Field Production Systems. Plant Disease 85:901-909) but the economics of these production systems in Florida are not currently practical.  Sanitation, the physical removal or destruction of infected plant parts and crop debris, may reduce the incidence of Botrytis fruit rot.  After plant establishment, necrotic and senescent leaves are frequently trimmed and left in the alleys between beds in Florida.  Leaf sanitation may reduce Botrytis fruit rot incidence (Mertely, et al. 2000.  Comparison of sanitation and fungicides for management of Botrytis fruit rot of strawberry.  Plant Disease 84:1197-1202).  But does not increase marketable yield and is not economical due to the high labor costs involved in trimming foliage (>$275 per acre).  The removal of diseased and unmarketable fruit, and the inoculum they produce, from within the plant canopy is critical to the management of Botrytis fruit rot. 

For the open field production system currently used in Florida, effective management of Botrytis depends on regular applications of fungicide and timed applications of specific fungicides during peak bloom periods.  Although strawberries flower throughout the winter in the main Florida production region near Tampa, there are typically two peak flowering periods each season, one in November/December and the other in January/February.  Regular applications of fungicides are needed to protect these flowers from infection and application should begin immediately after overhead irrigation for plant establishment is done, and continue throughout the season.  Timed applications of fungicides specifically labeled for Botrytis should be made during peak flowering periods.  

Our current fungicide control recommendations are to apply protectant fungicides such as thiram or captan on weekly intervals at the lower end of the labeled rate during the early season (i.e. November through mid-late, January).  During the late season when the second peak flowering period occurs, specific fungicides like Elevate and Switch (when it is labeled) should be applied at top labeled rate on a weekly schedule.  As the season moves into late February and early March, growers should resume applications of captan or thiram at higher rates.  Growers should also consider applying higher rates of captan or thiram in the early season during rainy periods. 

For more information - Chemical and cultural control of Botrytis fruit rot of strawberry in annual winter production systems. 

Control of Postharvest Botrytis Fruit Rot with Preharvest Fungicide Applications in Annual Strawberry

Management of Botrytis Fruit Rot in Annual Winter Strawberry using Captan, Thiram and Iprodione

Comparison of Epidemics of Botrytis Fruit Rot and Powdery Mildew of Strawberry in Large Plastic Tunnel and Field Production Systems

Comparison of sanitation and fungicides for management of Botrytis fruit rot of strawberry.

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Figure 1
Botrytis fruit rot infection of young green fruit.

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Figure 2
Botrytis infections often start from infected petals adhering to the fruit.

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Figure 3
Young brown lesion of Botrytis rot on a green fruit.

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Figure 4
Typical Botrytis lesion with gray/white mass of spores (conidia) on the cap end of a ripe fruit.

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Figure 5
Diseased fruit can become consumed by Botrytis and spread disease to adjacent fruit.