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Managing Botrytis Inside Greenhouses During the Winter

February 04, 2013

Many growers experience infections caused by the fungal pathogen Botrytis cinerea during the late winter and throughout the spring months.  Botrytis or Botrytis blight is particularly problematic for growers during this time of year since the environmental conditions are frequently conducive for disease development. Botrytis commonly occurs on crops such as perennials which are being over-wintered or on actively growing plants inside greenhouses.

The damaged tissue first appears as tan to brown water-soaked areas that become gray as they dry out.  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).  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 requires dead or damaged plant parts and optimum conditions in order for it to prosper.  This can be dead petioles and leaves, senescing plant parts, or injury caused from plant maintenance activities.  Once gray mold begins to grow on wounded or senescent tissues, it can rapidly spread to healthy leaves and stems.  Severe infestations can cause leaf and stem blights and may even infect the crown of the plant which could lead to plant mortality.

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.

The primary condition necessary for an infection to occur is free moisture (water) remaining on plant surfaces for several 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 (34 to 84º F) when water remains on the leaves for long durations and the relative humidity is high.

In many instances, the free moisture is caused by high humidity levels (>85%).  This is especially true within the plant canopy where there is little air movement and moisture from the soil increases the relative humidity in this micro-environment.  In many cases, water develops on the leaves as dew or falls on the plants from condensation on the covering materials.  In both instances, high relative humidity and changes in the temperature are the causes.  Unless looking closely, it is often difficult to see this thin film of water on plant surfaces.

The most important aspect of Botrytis management is to avoid extended periods when free water remains on plant surfaces.  When possible, avoid overhead irrigation during cloudy weather or late in the day. Reduce any water that may rest on plant leaves, such as dripping from roof condensation or sprinklers.

However, even these strategies will not prevent the leaves from becoming wet if the relative humidity is over 85%.  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 (purging).  Maintaining the relative humidity at this level is the most effective strategy for preventing Botrytis infections on crops during this time of year.

Besides the cultural methods mentioned above, many growers apply fungicides preventatively or after Botrytis infections are first detected.

The most effective fungicides for controlling gray mold are boscalid + pyraclostrobin, chlorothalonil, fenhexamid, iprodione, mancozeb and triflumizole.  When applying fungicides preventively, apply them using the middle of the labeled rate.  When active Botrytis infections are present, use the highest labeled rate to maximize the efficacy of the fungicide.  Be sure to rotate fungicides with differing modes of action with each application to prevent the disease from developing tolerance (resistance) to them.

Paul Pilon
Perennial Solutions Consulting

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