Recommendations for Botrytis fungicides for 2020

Mary Hausbeck<hausbec1@msu.edu> and Blair Harlan, Michigan State University Extension, Department of Plant, Soil & Microbial Sciences - April 03, 2020

Research results for greenhouse ornamentals.

Botrytis blight on geranium and aerial stem blight on vinca — Photo 1. Botrytis blight on geranium (A) and aerial stem blight and sporulation on vinca (B). Photo by Mary Hausbeck, MSU.

The greenhouse environment provides ideal growing conditions for both plants and plant pathogens. The high relative humidity and lack of air circulation often found beneath the plant canopy are especially ideal for the pathogen Botrytis cinerea, commonly called gray mold. This pathogen infects many greenhouse ornamentals and is considered the second most important plant pathogen in the world.

Common disease symptoms include leaf spots, blight and stem cankers with the fuzzy gray mold being a key way to identify the Botrytis culprit (Photo 1). Flowers are especially prone to infection by the gray mold and when the infected petals fall to the foliage, leaf infections result. Botrytis spores move by air and are carried to healthy plants where new infections begin. Infection requires water, which is needed for spore germination. Penetration of the plant by Botrytis can be direct or indirect through natural openings or wounds. Small leaf spot symptoms may quickly combine into large blighted areas under high relative humidity and wet conditions. Botrytis usually becomes established and produces spores on older leaves near the moist soil surface and under the plant canopy and flowers. Dead tissue in the plant pot or on the greenhouse bench or floor can support gray mold and spore production.

There are times in the spring when Michigan growers face a perfect storm for a Botrytis outbreak. Elements of the storm include overcast, chilly, wet weather, and maturing floriculture crops with flowers.

Preventing and controlling gray mold relies on a multi-pronged approach including cultural and chemical controls. Sanitation reduces the spore load by removing dead and dying plants, leaves and flowers from greenhouse production areas. Improving air flow to reduce relative humidity is also important. Relative humidity can also be reduced by slightly increasing the temperature via heating. Limit watering whenever possible and water at a time of day when the foliage can dry quickly. While these strategies are helpful, they are often not enough to eliminate gray mold during Michigan’s spring damp weather complete with overcast skies and fungicides may be needed.

Choose effective fungicides, use their full rate, and apply often. It is very likely that fungicides will need to play a central role as growers look to hold their crops due to Michigan’s “Stay Home” order. Botrytis is best managed preventively and that includes using fungicides prior to a damaging outbreak. Use only those fungicides that have been proven effective in research trials and apply them at the full labelled rate at seven-day intervals. It is important to alternate between fungicide modes of action in order to limit the risk of developing fungicide-resistant Botrytis strains. This pathogen has a history of adapting to fungicide use and there are strains of Botrtis resistant to many fungicide classes. The multi-site fungicide, Daconil, is effective with little risk of pathogen resistance but must be used preventively, before disease develops, for best results. The Fungicide Resistance Action Committee (FRAC) has assigned alphanumeric codes to fungicides based upon the modes of action of the active ingredients. There is also a role for biorational controls in limiting Botrytis such as Affirm WDG (FRAC code 19), which has demonstrated that it works against gray mold.

We have identified effective fungicides for controlling Botrytis in greenhouse ornamental crops. Each year, we include many commercially available products in our studies. The “A” and “B” Team table shows the results of many years of testing fungicide products and lists the product name, FRAC code and active ingredient. The products listed in the A Team are especially effective against Botrytis. The B Team fungicide (Chipco 26019) can also limit disease, but likely won’t be as effective as the A Team products. Always alternate among fungicide products with active ingredients that have different modes of action (FRAC codes) to delay the development of fungicide resistance in the Botrytis pathogen.

*Botrytis* A Team
Product	FRAC^**	Active ingredient
Affirm WG	19	polyoxin D zinc salt
Astun SC	7	isofetamid
Broadform SC	7/11	fluopyram/trifloxystrobin
Daconil	M05	chlorothalonil
Decree	17	fenhexamid
Emblem/Medallion	12	fludioxonil
Mural	11/7	azoxystrobin/benzovindiflupyr
Orkestra	7/11	fluxapyroxad/pyraclostrobin
Pageant	11/7	pyraclostrobin/boscalid
Palladium	9/12	cyprodinil/fludioxonil

^*Not recommended when disease pressure is high.
^**The FRAC code is an alphanumeric code assigned by the Fungicide Resistance Action Committee and is based on the mode of action of the active ingredient.

*Botrytis* B Team*
Product	FRAC**	Active ingredient
Chipco 26019	2	iprodione

Acknowledgement. This research was supported by funding from the Floriculture Nursery and Research Initiative of the USDA Agricultural Research Service under agreement #58-8062-8-014.

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