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Humidity increases mold pressure in raspberries

September 12, 2014  By Annemiek Schilder Michigan State University Extension

September 8, 2014, East Lansing, MI – Continued humid conditions with moderate to warm temperatures are promoting Botrytis gray mold infection in fall raspberries.

Frequent rains increase disease pressure as well as lead to wash-off of fungicides, which decreases fungicide efficacy when it is needed most. As a reminder, Botrytis gray mold is caused by the fungus Botrytis cinerea and is recognized by a fuzzy gray mold on ripening berries. The infection may be spreading from infected berries to adjacent healthy berries in a cluster. Moldy berries yield millions of infectious spores, which become airborne and may also be spread by pickers touching infected berries.

To the extent possible, practice sanitation by removing overripe and rotting berries from the field to limit inoculum availability. Michigan State University Extension does not advise throwing them on the ground as they will continue to sporulate for some time. To minimize spores from being spread by air while removing rotting berries, you can throw berries into a wide-mouthed bottle, like an orange juice bottle, or into a container with soapy water. Avoid touching healthy berries in the process.

Botrytis cinerea is favoured by prolonged moisture and high humidity at temperatures of approximately 60 to 75 F. The infection starts with a spore landing on the fruit. The spore germinates when the berry remains wet for more than eight to 12 hours from rain or dew. Fall weather promotes infection due to more frequent dew and rain events. However, high relative humidity greater than 93 percent over a period of several days may also be sufficient for infection to occur.

The presence of old flower petals or other source of nutrients, such as juice on the fruit surface, is known to stimulate spore germination and infection. Environment modification to decrease relative humidity and increase airflow, such as open canopies and wide rows, can help reduce disease pressure.

There are a number of excellent fungicide choices for gray mold control in raspberries. However, under high disease pressure and frequent precipitation, even the best fungicides may not provide full control. Under those conditions, it may help to include protectant or contact fungicides in the tank-mix to kill fungal spores on contact before they have a chance to germinate as well as killing spores that are being produced on rotting fruit.

It is also important to alternate fungicides with different modes of action, as indicated by different Fungicide Resistance Action Committee (FRAC) codes, to reduce the risk of fungicide resistance development.

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