Botrytis blight (caused by Botrytis cinerea) is a well-known problem among greenhouse ornamental crop producers. This ubiquitous fungus has a wide host range, and no host resistance is available. Thus, an integrated pest management approach is required to manage this disease successfully:

• Avoiding high humidity during cool temperatures
• Maintaining air flow through plant spacing and the use of fans
• Sanitation (removing dead plant material promptly)
• Preventative fungicide program

The preventative fungicide program is key, but increasingly challenging due to fungicide resistance in the pathogen as highlighted in a recent study from Michigan (MI). The authors, Lukasko and Hausbeck, tested almost 400 isolates of Botrytis from Michigan greenhouses for sensitivity to 8 fungicide active ingredients. They found that:

• most isolates (64%) were resistant to at least 4 FRAC groups
• many isolates (>25%) were resistant to 6 FRAC groups
• 94% of strains were resistant to thiophanate-methyl (FRAC 1)
• 80% of strains were resistant to pyraclostrobin (FRAC 11)
• 67% were resistant to boscalid (FRAC 7)
• 65% were resistant to iprodione (FRAC 2)
• resistance was less frequent for:
  • cyprodinil (FRAC 9),
  • fenhexamid (FRAC 17),
  • fludioxinil (FRAC 12),
  • fluopyram (FRAC 7)

Growers are encouraged to: 1) use cultural practices listed above to minimize Botrytis sporulation and spread: 2) consider avoiding fungicides to which Botrytis strains are most resistant; 3) consider rotating fungicides to which Botrytis strains are LEAST resistant; and 4) tank-mix with a broad-spectrum fungicide.

Although the study was conducted in MI, there is no reason to doubt that a similar trend would be seen in North Carolina greenhouses given that our crop production is similar and plants are bought and sold (and their pathogens) nationally and internationally.

Citation for full paper: Lukasko, N. T. and Hausbeck, M. K. 2024. Resistance to seven site-specific fungicides in Botrytis cinerea from greenhouse-grown ornamentals. Plant Disease 108:278-285. https://doi.org/10.1094/PDIS-06-23-1213-SR