Botrytis blight was observed this week on rosemary and basil in the greenhouse. The disease has a wide host range and is characterized by the fuzzy gray sporulation of the fungus. Botrytis produces abundant spores which travel easily in a greenhouse environment. Spore germination and infection occur at temperatures of 55-65°F when plant surfaces are wet for 8-12 hours and relative humidity is >93%. The fungus uses small wounds as an ingress to the plant tissue, but once established it will colonize and kill healthy tissue.
The most important element of a Botrytis prevention program is the reduction of leaf wetness and humidity in the greenhouse. This can be accomplished by heating and ventilating, increasing horizontal airflow, and avoiding overhead irrigation when possible. Water early in the day to allow foliage to dry. Do not hang baskets over plants on benches, as fallen flowers and leaves can spread Botrytis to plants below. Remove severely infected plants from the greenhouse - place these in plastic bags to prevent spores from escaping in transit.
A list of products labeled for Botrytis prevention on edibles in the greenhouse can be found in the New England Vegetable Management Guide: http://nevegetable.org/table-19-fungicides-and-bactericides-labeled-vegetable-transplants-and-bedding-plants
Angela Madeiras, Extension Educator and Diagnostician, UMass Extension Plant Diagnostic Lab, with contributions from Jim Mussoni