Greenhouse Disease Management
Botrytis Blight
Botrytis blight caused by Botrytis cinerea is one of the most common fungal disease of greenhouse crops. The disease is often referred to as gray-mold because it produces a crop of gray fuzzy-appearing spores on the surface of infected tissues. Plants may be attacked at any stage, but new tender growth, freshly injured tissues, and dead tissues are most susceptible. Botrytis readily attacks healthy or senescing, soft, nutrient-rich flower or bract tissues of most flowers including cyclamen, geranium, rose, and poinsettia. Botrytis cinerea occurs wherever there is high humidity and soft plant material. This disease is often a problem in overwintering polyhouses where stagnant moist air and day/night temperature fluctuations result in condensation on the plant. The disease is primarily a problem in container stock. However, dense seedling beds may also be affected.
Disease Cycle
The grey mold fungus is ubiquitous in the greenhouse. Microscopic spores can routinely be detected from plant material and air currents. Spores may germinate on plant surfaces in the presence of high relative humidity or standing water on plant surfaces and can be produced within a wide range of temperatures in the greenhouse. The fungus can penetrate the plant directly or enter through natural plant openings and wounds that may be created by taking cuttings or stripping leaves from a plant.
Symptoms
The most obvious initial symptom of grey mold is the rapid development of a gray “fuzzy” growth on flowers and other infected plant parts (See Figure 28.2). The grayish growth is actually large quantities of spores produced by the fungus. Sepals and petals are often the first portions of the flower to show symptoms.
Cultural Management Strategies
Once Botrytis develops, it cannot be effectively controlled with fungicides alone. The key to suppressing Botrytis is to keep the plant canopy dry especially from dusk until dawn. Using drip irrigation or watering plants at the base instead of sprinkling or watering overhead will help to prevent Botrytis and many other leaf spot diseases.
Sanitation. Botrytis is often considered a “disease of bad management.” Sanitation practices before, during and after each cropping cycle are the first important steps to achieve good control. Keeping the greenhouse as clean as possible is a good strategy of the greenhouse manager. Walkways, benches, and other structural aspects of the greenhouse should be disinfected several times a year with an approved disinfectant. Also, all recycled containers should be disinfected. A good time to disinfect greenhouse benches, sidewalls, and walks is between crops when the greenhouse is empty.
Heating and Venting Greenhouses to Lower Humidity. Greenhouse environment control computers play a significant role in maintaining the precise environmental growing conditions that are conducive to vigorous, healthy plant growth. Maintaining an environment within the greenhouse that will not permit the fungus to grow and sporulate is essential for control. To this extent, excellent control can be achieved by keeping the relative humidity below 85 percent and by reducing the relative humidity at night by maintaining warmer night temperatures and proper ventilation practices.
Chemical Management Strategies
A variety of fungicides are available to control gray mold in the greenhouse. However, some Botrytis populations have developed resistance to certain chemicals such as chlorothalonil, iprodione, and fenhexamid. Therefore it is important to rotate between fungicides of different Fungicide Resistance Action Committee (FRAC) group numbers. Fungicides with different modes of action can be mixed simultaneously or used in rotation so that the fungus does not develop resistance to one particular chemical. Reportedly, growers have had success alternating between Pageant Intrinsic or Orkestra (combination of FRAC groups 7 and 11) and Palladium (9 and 12). Appendix c, Fungicides and Bactericides Labeled for Greenhouse Use, lists fungicides and bactericides, labeled for control of plant diseases in greenhouses.
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