Here are some management tips especially important for periods of cloudy, rainy weather:
Clean Plants: Keep plants in retail areas clean. Remove dead and injured plants and spent flowers a couple times a day even during the busy season. Botrytis and high ethylene concentrations from decaying plant tissue will cause premature loss of foliage and flowers.
Water sparingly: Let plants run on the dry side and spot water areas as needed to prevent over-watering. Avoid watering late in the day or when water will sit on leaf surfaces for long periods of time.
Lower humidity levels in greenhouses to minimize Botrytis: Heat and vent greenhouses to lower humidity levels. See: Reducing Humidity in Greenhouses.
Switch fertilizers temporarily: To prevent ammonium toxicity, use fertilizer with 40 percent or less ammoniacal nitrogen when growing conditions are cool and wet. A common strategy used by growers is to alternate to a 15-0-15 Dark Weather Formula. This eliminates the extra ammonium input temporarily, but then allows growers to return to their normal formula when growing conditions improve.
Cool temperatures (less than 60°F average daily temperature), water-saturated (low oxygen) growing media, and low medium pH can cause ammonium to build up to toxic levels in the growing medium. Symptoms of ammonium toxicity include upward or downward curling of lower leaves depending on plant species; and yellowing between the veins of older leaves which can progress to necrosis.
Reference: Nitrogen: Not All Forms are Equal, by Cornell University.
Be aware of pesticide phytotoxicity and activity: Some pesticides such as horticultural oil and neem oil will cause phytoxicity if it remains wet on the foliage. Read pesticide label precautions.
Fungicides (for root disease) will work more slowly in cool media. It may take longer to see results of an application or an application may be less effective.
Apply beneficial nematodes: A cloudy or rainy day is an ideal time to apply beneficial nematodes (Steinernema feltiae) for fungus gnats and thrips as nematodes are very sensitive to UV light and desiccation. Consider adding a compatible biofungicide such as Cease to the nematode application to prevent Botrytis. Note that Decree (fenhexamid) is not listed on compatibility charts. Nematode pesticide compatibility charts:
Late season fungicides for Botrytis: Decree (fenhexamid) with capsil is still many grower's first choice in terms of plant safety based on past grower experiences. Note that Decree resistance has been reported. To prevent resistance, the label recommends not making make more than two consecutive applications and after the second application, using an alternative fungicide effective in controlling Botrytis for two applications before reapplying the active ingredient in Decree. The label also states not to use lower rates than is recommended.
Decree will perform better with a surfactant such as CapSil added (8 oz per 100 gallons or ½ teaspoon per gallon) to increase coverage and reduce visible residue. Considering plant safety and residue as of primary concern, other possible alternatives that growers are using are CEASE (Bacillus subtilis QST 713 strain) and Affirm (previously Veranda O) (polyoxin D zinc salt).
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension