W. Garrett Owen, PhD Student and Floriculture Technician, Purdue University
Roberto G. Lopez, Ph.D., Associate Professor & Floriculture Extension Specialist, Purdue University
Botrytis blight (Botrytis cinerea) or gray mold is one of the most common pathogenic fungi found on floriculture crops especially those of great economic importance such as geraniums, poinsettias, New Guinea impatiens, and many annual bedding plants. Under the right environmental conditions, botrytis blight is fast growing and can infect healthy or senescing plants or plant parts causing rot. Botrytis blight is found widely throughout greenhouses on stock plants, in propagation and production areas, and is a concern for many growers.
In general, botrytis blight has a very broad host range. Infection on high value potted crops such as geraniums (vegetative; Fig. 1), poinsettias, potted rose, gerbera daisies, and cyclamens is of concern. .
Biology of Botrytis Blight
Greenhouse temperatures between 59 to 74 °F and a relative humidity greater than 90% from irrigation is the optimal environment for botrytis blight to occur. In association with rapid changes in humidity, conidia (spores) are often dispersed in the greenhouse environment by air circulation, splash from irrigation water, and/or from improper sanitation practices. Once dispersed in the greenhouse, infection can occur directly on plant stems, leaves, bracts, flower petals, and on fruits; through natural openings, or through wounds caused by cutting harvest, mechanical damage by automation, and/or by handling. Conidia germination typically occurs in less than three hours in a film of water on plant surfaces caused by increased humidity or by irrigation events. Conidia will infect plant tissue by means of conidial germ tubes or by hyphal (long, branching structure of a fungus) growth from previously colonized dead plant parts or plant debris that contacts healthy plant tissue. Infection can be stimulated by nutrient depletion of leaves. Also, shattering of flowers, fallen petals, and pollen deposition from overhead hanging baskets can provide an energy source for the fungus and facilitate infection of adjacent healthy crops. Infected cells will collapse and start to disintegrate as the fungus grows. As fungal growth occurs, plant tissue will begin to soften and rot. During this time, conidiophores (specialized stalk-like structure) will form and bear the conidia, thus sporulation and infection can reoccur within less than eight hours of initial infection.
For long-term survival in the greenhouse, sclerotium (harden mass of branching hyphae or mycelium) form in colonized plant tissues or in soilless mixes. Under appropriate greenhouse conditions, sclerotia will germinate and produce conidial inoculum. Once this occurs, previously infected crops can be re-infected or new crops can become infected and the disease cycle starts again.
Diagnosing Symptoms and Signs
Symptoms of botrytis blight on propagation material include leaf loss and cutting rot (Fig. 2). Symptoms of botrytis blight can range from discolored spots or “ghost spots” (Fig. 3) or blights on plant leaves (Fig. 4), bracts, or flower petals (Fig. 5). Premature flower bud or flower loss (Fig. 6) can also occur. Stem cankers (Fig. 7) and crown, bulb, or corm rot may occur resulting in wilting and collapse of the plant or damping-off at the substrate surface (Fig. 8).
Growers should familiarize themselves with the signs of botrytis blight. Most commonly, greenhouse growers will encounter the mass of fuzz-like, white to grayish-brown fungal colonies on plant material (Figs. 9). However, with a hand lens or with an on-site microscope, growers can detect mycelia growth or the hard, blackened, irregular shaped sclerotium structure.
Botrytis Blight Management
Control of botrytis blight is challenging for greenhouse growers because the plant pathogen has the abilities rapidly invade host tissues and quickly produce abundant conidia that are easily distributed by air currents. An integrated strategy combing cultural, environmental, and chemical or biological practices and management techniques will effectively reduce or eliminate the ever-so-present threat in the greenhouse.
Cultural control of botrytis blight in the greenhouse first starts with prevention. All plant material including plugs, liners, stock plants, and plant material for finishing should be thoroughly inspected for plant diseases and insects prior to placement in the greenhouse. An on-site quarantine area should be established, separate from the production facility for any plant material not produced by the greenhouse to eliminate the potential introduction of pathogens or pests in the production facility. If plants are damaged, diseased, or dying upon arrival, plant material should be rejected. This will eliminate future disease problems that may arise during crop culture.
Greenhouse floors or benches should be properly sanitized between crops. Production areas should be free of any plant debris from previous crops and weeds. Tools used for propagation should be thoroughly sanitized.
Monitoring and managing crops by providing adequate plant spacing, limit leaf wetness to three to four hours, and remove excessive foliage or flowers whenever possible, will reduce the potential for infection. If a plant in production is infected, it is recommended to remove the plant by placing it in a sealed plastic bag to prevent spore dispersal and discard immediately. Do not place infected plants with other organic wastes as Botrytis has the ability to survive as a saprophyte.
Manipulating and controlling the greenhouse environment by reducing the relative humidity below 85% will prevent the optimal growing conditions for Botrytis. Increased air circulation by a horizontal airflow system will prevent stagnate air from occurring and promotes drying of wet plant surfaces. Also, providing ventilation and heat at sunset to drive moisture-laden air out of production areas will elevate the opportunities for infection to occur during the night.
Chemical control of botrytis blight has become complicated for growers because of repeated applications of fungicides with the same mode of action (FRAC Group) can result in resistant populations of Botrytis. Resistance and insensitive strains of B. cinerea have occurred when growers continuously applied benzimidazole (FRAC Group 1) and dicarboximide (FRAC Group 2) fungicides. However, benzimidazole or dicarboximide fungicides are only useful in controlling nonresistant strains of B. cinerea.
Other registered fungicides effective in controlling botrytis blight in the greenhouse include, chlorothalonil, copper hydroxide, copper sulfate pentahydrate, and mancozeb. Additionally, tank mixtures that include a single-site and a multisite fungicide at reduced dosages can assist in controlling botrytis blight and decrease the possibility of resistance.
It is recommended to follow all fungicide labels as directed from the manufacture. Trialing fungicides or tank mixtures on an individual plant of each species to determine phytotoxicity and the duration of fungicide residue on plant material are recommended.
In recent years, the use of biological control agents or bioanatagonistic fungi to control Botrytis has shown to be promising. The use of biological control fungicides such as Cease (Bacillus subtilis), Actinovate (Spreptomyces lydicus), Mycostop (Spreptomyces griseoviridis), and PlantShield (Trichoderma harzianum) may be utilized as part of a resistance management plan. However, it is recommended to utilize biological fungicides only preventatively for botrytis blight management and do not rely upon them solely for control of Botrytis. Therefore, conducting small trials to determine efficacy and effectiveness is recommended.