High relative humidity and low temperatures in the greenhouse open the way for Botrytis to develop on plants. A mix of cultural and fungicide control options will help you manage this common disease effectively.
Controlling the greenhouse environment to encourage optimal plant growth while maintaining proper humidity challenges even the most experienced grower. Many greenhouse diseases are directly related to relative humidity. When the sun goes down and the greenhouse temperature falls, condensation can develop and provide the perfect conditions for disease spores to develop. Botrytis is one of the most common greenhouse diseases, and its symptoms can vary in appearance depending upon where the disease penetrates into leaf, flower, or stem tissue.
Identify Botrytis Correctly
The humidity-chamber technique is a useful tool to help determine if the symptoms displayed are, in fact, the pathogen Botrytis. The objective is to create the ideal conditions to promote sporulation within 24 hours. Place a moistened paper towel in a plastic container or baggie. Then position the plant material displaying symptoms in the container, but not in direct contact with the wet paper towel. A bottle cap or small plastic lid can be used for this purpose. Place the container in a warm environment out of direct sunlight. If Botrytis is present, the characteristic gray, fuzzy sporulation will develop.
The ideal environment for Botrytis development is 75F to 82F and 80% humidity, but this can vary slightly. Four to eight hours of leaf wetness allows the spores to germinate directly on healthy plant tissues. Botrytis can also penetrate through wounds or natural openings.
Botrytis can lead to the formation of sclerotia, which are black, hard, irregular-shaped structures within the plant tissue. They can remain viable in plant debris at temperatures ranging from 39F to 131F. Sclerotia are the primary means of survival for this pathogen, and when conditions become favorable, they provide a source of inoculum for infection to reoccur. This is why the prompt removal of infected plant material from the greenhouse helps to reduce disease pressure.
Reduce Relative Humidity with Good Air Circulation
Many efficient portable instruments indicate relative humidity, air temperature, and dew point/leaf wetness in your production
area. Cultural controls begin by supplying adequate air circulation to reduce the relative humidity. Allow as much space between plants as is feasible for better air flow. Horizontal air flow (HAF) fans have proven to be quite valuable in mixing the air and reducing the relative humidity in the plant canopy. Don’t be tempted to open vents on rainy days. If it’s rainy or foggy outside, keep the greenhouse closed up and supply some heat. The timing of irrigation is also important. If drip irrigation is not available, limit overhead watering to the morning to allow the leaves time to dry.
Rotate Fungicides to Reduce the Risk of Disease Resistance
Even with the best cultural controls, fungicides are often still needed. Researchers continue to identify strains of Botrytis that are resistant to one or more of the commonly used fungicides. Include three or more fungicides in rotation to slow the development of resistance.
Botrytis rotations include the following fungicides: Affirm, Chipco 26019, Daconil, Medallion, Mozart (TR aerosol), Mural, Palladium, Pageant, and Spectro. OMRI (Organic Materials Review Institute)-listed products for organic growers include Actinovate SP, Cease/Milstop tank mix, Triathlon BA, and ZeroTol 2.0. It is always recommended to trial pesticides on small quantities of plants to evaluate them for phytotoxicity before making large-scale applications. Products other than those mentioned may also be safe and effective. Always read and follow all label instructions. Please note, some products may not be registered for use in all states.
Photo 1 One symptom of Botrytis blight is gray, fuzzy sporulation on foliage and flowers, similar to that shown on the flower of this hibiscus.
Photo 2 Botrytis can develop on flowers and foliage that are kept constantly wet, such as on the flowers of this catharanthus.