DISEASE FOCUS: Botrytis time
by Jim Downer
While it is pretty clear that El Nino did not bring torrential rains to Southern California this year, we did have a fair number of “cutoff low storms” that brought isolated showers followed by cool weather (60°F or less). These are ideal conditions for (cool and moist) gray mold caused by Botrytis cinerea to develop in ornamental nurseries. While diseases caused by Botrytis are not aggressive on mature tissues or woody plants, the fungus rapidly attacks fleshy or juvenile tissues such as flower petals, new shoots or tender growth of bedding plants or other annual crops. This April, roses all over Ventura County suffered from rapid onset of Botrytis blight (fig. 1) because we had a cool, wet event during flowering of most roses.
Gray mold is an ascomycete fungus that produces abundant gray-colored mycelium and conidia or asexual spores. The perfect stage of the pathogen is rarely seen but the conidial or Botrytis stage is common in all nurseries and landscapes. Botrytis cinerea is in the Ascomycete family Sclerotiniaceae. Like its cousin Sclerotinia sclerotiorum (white mold or rot of bedding plants), it forms sclerotia (hardened, asexual resting structures) in decaying plant matter that has rotted as a consequence of infection and fungal colonization. Dead flowers and plant parts fall to the ground where sclerotia later form to insure survival of the pathogen. Sclerotia later germinate as hyphae to grow and form more conidia.
Botrytis is a necrotrophic pathogen and relies on enzymes to rot plant parts. Infected tissues soon turn brown due to enzymatic degradation of the middle lamella, cell walls and cell contents. Evidence also suggests that B. cinerea causes a hypersensitive reaction in plants and programmed cell death by its host plant in response to infection (Williamson et al. 2007).
While Botrytis rot is largely regulated by weather conditions, growers can take steps to limit damage. When cool, wet storms are predicted during sensitive growing periods (times of new growth or flowering), there are many fungicides that will offer some protection to these tissues. Fungicides in the strobulurin group and many newer (such as fludioxonil) and older (such as triadimefon) fungicide active ingredients can provide control of gray mold; however fungicides in every FRAC (Fungicide Resistance Action Category) number listing are rated as moderate to high resistance risk materials, so it is wise to alternate active ingredients in any spray program or use combination products that employ active ingredients from more than one FRAC group.
Cultural methods of control are also helpful. Increasing plant spacing to allow for more air movement, less spore splashing and less plant-to-plant contact will slow the progress of Botrytis during prolonged cool periods. Cleaning up rotted plant debris caused by Botrytis blight or rots is essential in limiting the disease and preventing future outbreaks. While spores are mostly ubiquitous, they can concentrate in decayed plant matter, so sanitation by deadheading diseased flowers is helpful in controlling outbreaks.
Eventually, as weather warms, Botrytis blight fades from importance, but survives in litter or as sclerotia in soil until cool, wet weather returns.
Jim Downer is Environmental Horticulture Farm Advisor, UC Cooperative Extension, Ventura County.
Cornell University. 2015. Botrytis blight. Plant disease Diagnostic Clinic Cornel University. http://plantclinic.cornell.edu/factsheets/botrytisblight.pdf
Karlik J, Golino DA. 2014. Roses in the Garden and Landscape — Diseases and Abiotic Disorders. UC ANR Publication 7463. UC Statewide IPM program, Univ. Calif. Davis.
Williamson B, Tudzynski B, Tudzynski P, Van Kan JAL. 2007. Botrytis cinerea: the cause of grey mould disease. Mol. Plant Path. 8:561-580.