Insensitive strains are exchanged among greenhouses along with the exchange of plant material. After a greenhouse B. cinerea population acquires benzimidazole or dicarboximide resistance, it appears that the trait remains within the population after the fungicide is no longer being applied. Benzimidazole and dicarboximide fungicides are useful only on nonresistant strains of B. cinerea . Other fungicides registered in the United States for control of Botrytis in greenhouses, including chlorothalonil, copper hydroxide, copper sulfate pentahydrate, and mancozeb, are generally more effective against B. cinerea . The use of tank mixtures that include a single-site and a multisite fungicide at reduced dosages has been proposed to avoid control failure caused by fungicide resistance.
Product labels provide information about approved uses and tank mixtures on flowering potted plants.
Fungicide phytotoxicity may present a particular problem for growers attempting Botrytis control on crops during the finishing stages. Chlorothalonil smoke dust has been reported to injure poinsettia bracts and the flowers of several potted plants. Injury to poinsettias is often cultivar specific. High temperature and moisture on plant surfaces contribute to chlorothalonil smoke dust injury. Of several fungicides tested on exacum, only chlorothalonil gave effective control of B. cinerea with no phytotoxic effects.