Introduction

Connecticut has a robust greenhouse industry that produces a wide range of ornamental crops, worth over $193 million in sales annually. Bedding plants are an important category and bring in significant revenue for greenhouse operations each Spring. 

Bedding plants include any herbaceous plant started under controlled conditions, and purchased and further grown by the customer. Most bedding plants are seed propagated, and are grown from plugs. The bedding plant crop has a rapid turnaround time in the greenhouse. 

There are many factors involved in the production of high quality bedding plants, including cultivar selection, fertilizer, light, temperature, and control of pests and diseases. Growers accurately time plantings to meet contract obligations, and customer schedules. 

Disease Impact

Diseases can cause significant losses in bedding plant production. Losses can occur from stunting and plant death during production, or reduction in aesthetic quality due to leaf spots and blights. 

Pathogens are often present in the greenhouse and infect when conditions are favorable. Alternaria leaf spot and damping off, Botrytis blight, Pythium root rot, bacterial leaf spot, and Thielaviopsis can all reduce seedling quality and stand count, leading to uneven seedling trays. 

During production, Botrytis blight, Cercospora leaf spot, Myrothecium, powdery mildew, rust, Fusarium root rot, and Thielaviopsis can make production difficult, reduce the quality of planting packs, and increase disease management costs. During shipping, and at retail, gray mold is commonly observed.

Pathogens

The pathogens that infect during greenhouse production of bedding plants are numerous, and usually require specific conditions for infection. Pythium root rot causes stunting and yellowing. Many bedding plants are affected by Pythium while still in plug trays, or when moisture levels are high for extended periods of cool cloudy weather. 

Sanitation and fungicides are necessary to protect plants against this pathogen. Leaf spot diseases and powdery mildew are generally less of an issue during bedding plant production than in the landscape. Fungicides and biocontrols can be applied that limit many of the common leaf spot pathogens. Bacterial blights spread rapidly in seedling trays, as they can be seedborne, and spread when conditions are warm and humid during propagation. Vegetatively growing geranium, zinnia, and begonia can also be affected by bacterial blight during production.

Certain pathogens are very difficult to control in bedding plant production. Botrytis blight caused by Botrytis cinerea affects almost all bedding plants grown, and can infect at any stage of production. Botrytis blight is widespread and frequently observed on a wide range of crops. The pathogen is able to grow on debris in the greenhouse, and produces abundant spores that travel to neighboring plants.

Using forced heated air beneath benches is an effective method to reduce the humid environmental conditions needed for successful infection. A fungicide program that includes a rotation of highly effective systemic and protective fungicides is recommended to control this pathogen. 

Management of plant pathogens in bedding plant production starts with cultural controls such as sanitation and improving the environmental conditions for crop growth. Cleaning the benches and greenhouse walls followed by application of a chemical sanitizer will help to reduce inoculum sources in the greenhouse. If pots and trays are reused, they should be cleaned and sanitized to kill any pathogen spores that may be present.

Fungicides

There are many fungicides available. Each fungicide contains one or more active ingredients that are the active chemistry that inhibits the fungal pathogen and inactive ingredients that may include adjuvants or carriers. These inactive ingredients are proprietary and often not listed on the label. 

Fungicide formulations are made for 1) convenience, 2) stability, 3) compatibility, and 4) biology. Fungicides have a mode of action, that is represented by the Fungicide Resistance Action Committee (FRAC) code. Fungicides have many possible mechanisms of action against pathogens, for example inhibiting melanin synthesis, or disrupting mitosis. 

The FRAC codes were designed to aid in making fungicide resistance management decisions. Rotating among FRAC codes can delay the development of fungicide resistance. Some fungicides have a very high risk for resistance development, and should be used with caution to ensure maximum use and efficacy. 

A broad spectrum fungicide has a low resistance risk. In general, applying the same FRAC code multiple times in succession is not recommended. Overuse of fungicides can also have a negative impact on the crop, although this is uncommon in bedding plant production. 

Triazole fungicides may have growth regulating effects leading to bunchy or thickened leaves in sensitive species. Drenches of certain fungicides can cause phytotoxicity to the roots of plants, leading to stunting. Seedlings may be sensitive to fungicide application, but usually sensitivity is related more to the crop species. Check to make sure that the crop being drenched is listed on the label to reduce the chances of phytotoxicity. 

Biological fungicides, containing Bacillus, Streptomyces, and plant extracts, can help to reduce pathogen competition and establishment. The biological fungicides are often labeled for organic production, or can be rotated with synthetic fungicides in a program. Some potting media comes premixed with beneficial fungi. Applying a fungicide drench may negatively affect these beneficial organisms. 

Surfactants are used for improving leaf wetting and fungicide coverage. The surfactants typically fall into four categories, anionic, non-ionic, cationic, and ampholytic (positive and negative). The cationic surfactants are cost prohibitive and may cause phytotoxicity. Most commercial surfactants are non-ionic compounds, that are safe on the majority of plants, and mix well with fungicides. Oils and plant extracts can also improve uptake and leaf wetting at low rates. 

Surfactants may cause phytotoxicity to certain plant species, and can physically interact with a fungicide by extending the period of leaf wetness or increasing uptake, potentially enhancing phytotoxicity from the fungicide. 

Equipment

Using appropriate spray equipment for making applications can improve fungicide performance. There are a number of different sprayers that are used in greenhouse production. The backpack sprayer is simple and easy to use. Track sprayers and hydraulic sprayers are designed for larger greenhouses. Overhead irrigation of fungicides is generally not recommended and can lead to wasted fungicide and runoff. Cold fogging is a popular method to apply fungicides during the evening when the greenhouse is empty. Fungicides with systemic or translaminar activity should be used for most fogging applications. Spray equipment should be calibrated and cleaned regularly to ensure optimal performance. Fungicides active against specific pathogens may be applied preventively to provide optimal control.

Disclaimer: The data in Table 1 and 2 are based on information from the University of Connecticut, regional universities, Chase Agricultural Consulting, and chemical companies. A blank response does not indicate that the fungicide is ineffective against the pathogen listed. The ratings represent typical pathogen response from many trials summarized for each pathogen. Many of the fungicides listed are available in other formulations.  

Consult the label for specific use requirements and crop safety recommendations. Pesticides must be used in a safe and effective manner in accordance with the label. It is important to have the label with you prior to application. Product names are given for educational purposes and not as an endorsement.  

Resources

This work is supported by the Crop Protection and Pest Management Program [grant no. 2021-70006-35582] from the USDA National Institute of Food and Agriculture.