Humid weather increases Botrytis gray mold pressure in fall raspberries

Annemiek Schilder, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences - September 03, 2014

Botrytis gray mold is common in fall raspberries. Alternate fungicides with different modes of action and add protectants to the tank-mix.

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 them 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 favored by prolonged moisture and high humidity at temperatures of approximately 60-75 degrees Fahrenheit. 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. Fungicide resistance is more likely with systemic fungicides, such as the strobilurins (FRAC code 11) and succinate dehydrogenase inhibitors (SDHI’s) (FRAC code 7). Fungicide resistance is unlikely to develop to protectant fungicides like captan.

Fungicides for Botrytis control in raspberries

Product	Active ingredient	FRAC code*	Behavior	Risk of resistance	Efficacy rating	PHI (days)
Captan	captan	M4	Protectant	Low	Moderate	3
Captevate	captan + fenhexamid	M4 + 17	Protectant + locally systemic	Low	Good	3
Elevate	fenhexamid	17	Locally systemic	Low-Med	Mod-Good	0
Pristine	pyraclostro-bin + boscalid	11 + 7	Systemic and locally systemic	Med-High	Good to Excellent	0
Rovral**	iprodione	2	Contact/ locally systemic	Med-High	Good	0
Switch	cyprodinil + fludioxonil	9 + 12	Systemic, locally systemic	Low-Med	Excellent	0
Environmentally friendly/OMRI
Fungastop	citric acid, etc.	NC	Contact	Low	Fair	0
Oxidate	hydrogen peroxide	NC*	Contact	Low	Fair	0
Regalia	giant knotweed extract	P5*	Induced resistance	Low	Fair	0
Serenade	Bacillus subtilis	F6	Contact	Low	Fair	0
Sil-Matrix	potassium silicate	NC	Contact	Low	Mod	0

*M4 = Multisite contact activity; NC= Not classified; P5 = Plant defense inducer; F6 = microbial disruptors of pathogen cell membranes.
**Add surfactant to Rovral to improve efficacy.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).