Practice Persistence When Battling Botrytis In Blueberries

Botrytis cinerea is a ubiquitous fungus capable of infecting more than 200 crops worldwide. Small fruits are particularly susceptible and damages can be substantial if proper management is not implemented timely. In blueberry fields, B. cinerea causes blossom blight and can severely damage flowers, especially when favorable weather conditions (extended wetness and mild temperatures) occur during the peak bloom period.

Symptoms of gray mold can be seen on this blueberry bush. Photo by Caleb Slemmons

Symptoms of gray mold can be seen on this blueberry bush. Photo by Caleb Slemmons

Additionally, the fungus attacks tender green twigs and leaves in early spring. Because of similarities between blossom blight and freeze injuries, a portion of losses due to freeze injuries may be actually caused by B. cinerea. Infections may carry over after harvest and shipment and result in subsequent losses due to gray mold development on fruit.

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Figuring Fungicides

Blueberry producers in Florida who also grow strawberries are aware of the difficulties of managing gray mold. The mild weather conditions occurring in Florida between February and May can put at risk even the highbush blueberry varieties, predominantly cultivated in Florida, and known to be less susceptible to B. cinerea than rabbiteye and lowbush blueberries.

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Fungicide applications, including multisite and single-site fungicides, are necessary to protect flowers, twigs, and leaves during the season and fruit prior to harvest. Beside its ability to infect multiple crops, B. cinerea produces a high number of spores that are spread easily by wind, and can mutate quickly which warranted its classification at the ‘highest risk’ level for developing resistance to fungicides. Although resistance to multiple fungicides (MFR) in the gray mold fungus has been extensively documented in strawberry fields, very little is known about the situation in blueberry. With almost half the number of fungicides registered for B. cinerea control in blueberry compared to strawberry, blueberry growers in Florida may be in jeopardy if resistance to common fungicides was to occur.

Read The Signs

In 2013, 130 isolates of B. cinera were collected between April and June from flowers and fruits originating from 10 blueberry fields in Hillsborough, Citrus, Polk, and Lake counties. These isolates were tested for their sensitivity to six fungicides: Pristine (boscalid and pyraclostrobin, BASF); Fontelis, (penthiopyrad, DuPont Crop Protection); Switch, (fludioxonil and cyprodinil, Syngenta Crop Protection); and Elevate (fenhexamid, Arysta LifeScience). Experiments included sensitivity tests on agar media containing a sub-lethal dose of each fungicide and fruit trials to confirm results from these lab studies.

Cabrio (pyraclostrobin, BASF) is registered as single product to control other diseases in blueberry in addition to being premixed with boscalid to control B. cinerea. It is likely that spays not targeting B. cinerea caused the high frequency of resistance (more than 90%) observed to Cabrio (see chart below). Resistance frequencies as high as 50% were registered to three fungicides: Elevate, Pristine, and cyprodynil (one of the active ingredients of Switch). Low resistance frequencies were observed for the newly registered fungicide Fontelis and particularly to fludioxonil, which is holding the efficacy of Switch. Although differences in resistance frequencies were observed between the surveyed blueberry fields, it remains unclear why such high resistance frequencies are observed. One would have expected lower frequencies knowing that blueberries are usually sprayed less with fungicides compared with strawberries. The presence of other host of B. cinerea, such as strawberry or tomato next to blueberry fields, may serve a source of resistant inoculum.

Another risk with B. cinerea is its ability to develop resistance to multiple fungicides simultaneously. Only a small portion of the isolates (7%) were sensitive to all fungicides, whereas 14%, 31%, 25%, and 12% of isolates were resistant to three (MFR3), four (MFR4,) five (MFR5), and six (MFR6) fungicides, simultaneously (see pie graph). Multiple fungicide resistance limits the options of spray programs and reduces disease control efficacy.

Future Trends

In order to manage blossom blight and gray mold effectively, one needs to answer two questions: When and what fungicide should I spray?
Because Botrytis infections are favored by mild temperatures (64°F to 77°F) and periods of wetness exceeding 14 hours, it is vital to monitor weather conditions in order to predict possible disease events and spray accordingly. Currently, strawberry growers in Florida and other states have the possibility of using the Strawberry Advisory System (SAS), which estimates a risk for Botrytis infections based on temperature and wetness duration. SAS includes six weather stations distributed across Central Florida and subscribed members can receive text and eMail alerts whenever a risk for disease development exists. The implementation of SAS in commercial strawberry fields allows growers to reduce the number of fungicide sprays by 50% without losing fungicide efficacy by targeting the potential infection events only.

Knowing the type of fungicide resistance in a particular field also is important in order to make better decisions about spraying. Tools exist nowadays to help conduct field monitoring of fungicide sensitivity for particular locations.

The importance of such issues for blueberry growers warrants future research aiming to conduct a larger fungicide resistance monitoring program including blueberry fields from North, Central, and South Florida, to investigate the role of different potential sources of B. cinerea inoculum on resistance development in blueberry. In addition, implement the weather advisory system for blueberry growers to help them make targeted sprays, reduce number of fungicide applications, and lower risks of fungicide resistance development.

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