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Greenhouse Updates: May 17, 2019

Cyclamen Mites and Botrytis
May 17, 2019

Cyclamen mites were observed on New Guinea impatiens at the UMass Extension Plant Diagnostic Lab this week. The mite feeding damage also served as an ingress for Botrytis, which became established and compounded the injury to the plants.

Populations of cyclamen mites are increased by high relative humidity and cool temperatures; infestations may occur in spring but are most common in the fall and winter. Cyclamen mites are spread by air currents, plant-to-plant contact, and workers. Chemical control is challenging because the mites prefer to feed in the buds, where they are at least partially protected from contact miticides. Miticide resistance develops rapidly and two or more applications may be necessary. When making multiple applications, be sure to alternate translaminar miticides with contacts.

Where feasible, immersion of infested plants into hot water (110°F/43°C) for 30 minutes can give satisfactory control. Test each plant species first to see if it can tolerate hot water treatment. Predatory mites (Phytoseiulus, Neoseiulus) are available for biological control. It is best to have predators in place before the population of cyclamen mites reaches damaging levels.

Botrytis readily invades wounds and all plant tissue, especially senescent or injured plant parts. Like cyclamen mites, Botrytis requires high relative humidity and cool temperatures. The pathogen is ubiquitous in the environment.

Botrytis is best controlled by a combination of management of environmental conditions, sound cultural practices, and fungicide applications. Control weeds and remove plant debris from the greenhouse. Space plants to allow good air circulation, reduce humidity within the canopy, and minimize leaf wetness. Improve horizontal air flow with fans. Reduce humidity in the greenhouse by a combination of heating and venting in the evening, particularly when warm days are followed by cool nights. Water in the morning and avoid overhead watering if possible.

Several conventional and organic fungicides are available for management of Botrytis; however, thiophanate-methyl and iprodione are no longer recommended due to widespread resistance. Always alternate fungicide applications among materials with different modes of action to prevent resistance development.

Damage to New Guinea impatiens caused by cyclamen mites and botrytis (A. Madeiras)

Report by Angela Madeiras