Does botrytis have you feeling too warm and fuzzy?

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The disease

Stem, leaf and flower blights caused by the fungus, Botrytis cinerea, can limit all phases of ornamental production. Botrytis is well known for its ability to produce large masses of gray conidia (also called spores, see below) that may be picked up and carried on air currents and transported to healthy plants where blight can become established. Monitoring the occurrence and build-up of this inoculum in the greenhouse can signal the need for implementing control measures. On bedding and stock plants, Botrytis typically becomes established and produces spores on aging lower leaves that are near the moist soil surface and under the plant canopy. In addition, Botrytis readily infects the broken or cut stem surface of stock plants and progresses downward, causing a dieback of the entire stem. This diseased tissue offers another source of nutrients necessary to produce spores.

Choosing fungicides

Fungicides are often important in managing Botrytis and should be chosen carefully. Each year, Michigan State University tests products for control of Botrytis blight. Geraniums are good test plants because they seem to be a “magnet” for this disease. All fungicides are applied and allowed to dry prior to introducing Botrytis spores. Each time a test is conducted, fungicides that are considered especially effective are included for a comparison. Over the years of testing fungicides for Botrytis control, I consider Decree, Chipco 26 GT and fungicides containing chlorothalonil (Daconil and Echo) standards because they consistently provide effective control. We were interested in comparing the fungicides Compass, Terraguard 50W and Fungo 50WSB against these standards. Compass is a fairly new product and is often recommended for control of powdery mildew. Terraguard is also a strong powdery mildew fungicide when applied as a spray. It can also be used as a drench for some root rots. Fungo 50WSB has thiophanate-methyl as its active ingredient and is effective against a number of foliar blights and root rots.

In our trial, Botrytis progressed and produced spores on nearly 40 percent of the foliage that was left untreated. Overall, the standards, including Echo 720, 26 GT, Daconil Weatherstik and Decree 50WDG, provided good protection and kept disease to less than 10 percent. Compass + Latron B-1956 offered Botrytis control, but was not always as effective as the standard fungicides. It is helpful to know that Compass is not only a strong powdery mildew fungicide, but can also suppress Botrytis. Terraguard 50W, even at a high rate, was not especially helpful in this trial in managing Botrytis blight. The inability of Fungo 50WSB to limit Botrytis in our trial may be due to resistance of our particular Botrytis strain to this fungicide. Due to the frequency of Botrytis isolates that are resistant to benzimidazoles (examples are Cleary’s 3336, Fungo), these fungicides are no longer recommended as the primary tool for controlling Botrytis. Although resistance to the systemic dicarboximide fungicides (Chipco 26 GT) has also been documented, resistance does not appear to be widespread, and if used wisely, they should continue to be effective. Rotating dicarboximide fungicides with protectant fungicides (Daconil, Decree, Phyton-27, Exotherm Termil, Echo, Dithane are examples) is a traditional way of delaying the buildup of resistant isolates. Protectant fungicides do not appear to be at great risk of developing resistance. However, these protectant fungicides may not be as effective as the dicarboximide fungicides. Moorman and Lease (1992, Plant Disease 76:374-376) at The Pennsylvania State University determined that tank mixtures of systemic and protectant fungicides provided good control of Botrytis and lasted longer than applications of single fungicides.

Many growers have asked about ZeroTol for Botrytis control and two different treatment programs were included. In our trial, Botrytis blighted over 30 percent of the foliage that was left untreated. The Daconil Weather Stik 6SC standard provided good protection and kept disease to less than 1 percent. The unregistered fungicides Fluazinam 500F, Boscalid 70WG, and Captan 80WDG all limited disease to 4 percent or less and performed in a way that was comparable to Daconil Weather Stik 6SC. Repeated applications of Fluazinam appeared to cause some yellowing of the geraniums’ leaves and requires additional testing. Using ZeroTol to control Botrytis was not especially helpful in this particular trial.

Identifying new fungicides that could be registered for use on ornamentals has been a major thrust of the research sponsored by the Special Floriculture Initiative of the USDA Agricultural Research Service and the American Floral Endowment. With data from screening trials, University researchers can partner with the product manufacturer, interested growers and the IR-4 Project (www.ir4.rutgers.edu) to initiate and speed registration. Input from growers regarding their need for pest management tools is encouraged and appreciated and can be made via the IR-4 website.

Sanitation and the environment are important, too

Fungicides cannot do the job by themselves; environmental manipulation and sanitation are necessary partners in a successful program. Since Botrytis requires specific environmental conditions in order to cause disease and produce conidia, control can be achieved by manipulating the environment, making it unfavorable for Botrytis establishment.

Grow plants in a well-ventilated, low relative humidity (<85 percent) environment.

  • Increase plant spacing. A less dense plant canopy will allow air circulation and thorough fungicide coverage.
  • Minimize the hours that the foliage is wet by watering in the morning so the foliage dries by the evening.
  • Reduce the relative humidity for a minimum of 24 hours immediately following the harvesting of cuttings to help “dry” the wounded stems and thereby limit stem blight.

Sanitation also plays a role in disease management. Botrytis readily colonizes and produces conidia in senescent or dead plant debris. Therefore, remove plant debris on and underneath plant benches that could serve as a reservoir for sporulating Botrytis.

Acknowledgement:
This research was funded by the USDA/ARS through a Specific Cooperative Agreement as part of the Floriculture and Nursery Research Initiative.

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