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Controlling Botrytis by Paul Pilon

March 15, 2011

Botrytis or Botrytis blight (Botrytis cinerea) is one of the most problematic diseases that attack crops in the late winter and throughout the spring months. This disease is particularly problematic for growers in the spring because the environmental conditions are conducive for its development. Botrytis commonly occurs in both production areas and overwintering facilities.

The primary condition necessary for an infection to occur is free moisture (water) remaining on plant surfaces for 4 to 8 hours. This often occurs from watering late in the day, allowing plant leaves to remain wet into the night or when irrigation is applied during cool, cloudy conditions. Although temperatures between 60 and 70º F are most favorable for this disease, Botrytis can rapidly establish at nearly any production temperature (55 to 84º F) when water remains on the leaves for long durations and the humidity is high.

The damaged tissue first appears as tan to brown water-soaked areas that become gray as they dry out.  Infected flowers usually show small pin-prick type lesions or water-soaked areas, which enlarge rapidly and turn to a brown or black coloration. Botrytis is most commonly identified by the fuzzy gray or brown spore masses that develop on infected plant parts (this disease is also commonly referred to as gray mold).

This pathogen requires dead or damaged plant parts and optimum conditions in order for it to prosper. Once Botrytis begins to grow on wounded or senescent tissues, it can rapidly spread to healthy leaves and stems.  Severe infestations can consume entire flowers and cause leaf and stem blights.

The most important aspect of Botrytis management is to avoid extended periods when free water remains on plant surfaces. Reduce any water that may rest on plant leaves, such as dripping from roof condensation or dripping sprinklers. Avoid overhead irrigation during cloudy weather or late in the day where the leaves are likely to remain wet for long periods of time. The occurrence of Botrytis can also be greatly reduced by maintaining adequate air circulation throughout the growing area. Keep the relative humidity below 80% by heating or venting the moist air.

The incidence of Botrytis can be greatly reduced by removing dead plant materials, such as senescent leaves and flowers, from the crops. To prevent the spread of existing Botrytis outbreaks, carefully remove (rogue) any infected plants from the production site. The spores are spread by air movement and splashing water, which means many normal greenhouse activities, such as watering, spraying, moving, harvesting cuttings, and spacing, can disseminate gray mold spores throughout the production site.

Where optimum environmental conditions for Botrytis exist, growers often implement preventative spray programs using protectant fungicides containing chlorothalonil, fenhexamid, or fludioxonil. Systemic fungicides including azoxystrobin, boscalid + pyraclostrobin, iprodione, thiophanate-methyl, and trifloxystrobin are effective on both a preventative and curative basis. For ongoing programs, it is recommended to rotate systemic fungicides with protectant fungicides and between differing chemical classes to prevent resistant strains of Botrytis from developing.

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

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