NC State Extension Publications

Introduction

Sanitation of previously used float trays prior to seeding each year is an important step in managing Pythium spp. and Rhizoctonia solani in the greenhouse. In the past, tobacco growers primarily utilized methyl bromide to eliminate pathogens from float trays, but it is no longer available and growers need an alternative tray sanitation method. Existing options to completely eliminate pathogens are to purchase new trays each season, which is cost prohibitive, or steam sanitation. While steaming trays has been available for many years, the simplicity and cost effectiveness of chemical fumigation has limited the widespread adoption of this method at the farm.

Steam Sanitation

Previous research has shown that trays should be steamed at 176°F (80°C) for 30 minutes (Table 1). However, work conducted in conjunction with the NC State Department of Entomology and Plant Pathology indicated that steaming trays at 140°F (60°C) for 2 hours can also be utilized and maintain disease control equivalent to MeBr fumigation. Although reduced temperature and extended time was shown to work in a single study, the higher temperature recommendation has proven to be effective over multiple years with actual on-farm results. It should also be emphasized that steam must be used, not dry heat. Simply placing the trays in a bulk barn, or other structure, and advancing the heat to the desired temperature at the corresponding time required will not be effective. Steam is more effective at penetrating cracks in the tray walls to ensure contact with all potential pathogens surviving in trays. Care should be taken to prevent overheating to trays because Styrofoam trays will shrink if exposed to elevated temperatures (> 200°F), and the amount of damage will increase withe exposure time. It should be noted that steam also will not eliminate black root rot (Thielaviopsis basicola) or tobacco mosaic virus organisms. Proper protocol should be followed when handling the trays and placing them in storage to avoid re-contamination of the trays.

Tray Summary

Table 1. Control of Rhizoctonia solani achieved using various alternatives to methyl bromide.

System Specifications

If trays are going to be steam sanitized prior to seeding during winter months, then the tray storage structure should be insulated due to the lower ambient temperatures. At least 1 inch of foam insulation is recommended to minimize heat loss and to decrease the time required to complete the process. Any structure should also incorporate a manifold to distribute the steam around the trays. Ideally, the manifold would be positioned beneath the trays in a lower air plenum that is approximately 2 to 4 inches high. This would be the distance measured from the floor to the bottom of the trays. A general guideline for the steam distribution system is to include ¼-inch or larger diameter holes spaced 12 inches to 18 inches apart on the manifold lines for the steam to exit, to minimize flow restrictions, and to improve distribution around all the trays. The steam unit outlet hose should have an inner diameter of 1 inch or larger to minimize flow restrictions and have a temperature and pressure rating greater than the output of the steam unit. Any structure purchased or constructed for steam trays should have at least one thermometer incorporated on the structure that can be easily seen to accurately monitor the process temperature. Additionally, a thermometer and pressure gauge should be incorporated in the steam supply line to monitor the steam parameters entering the structure.

Modified shipping container

Modified shipping container used as a storage structure for steaming trays.

Steam generator modifications

Modifications used to convert an add-on heating unit to a steam generator.

Steam generator

Steam generator made from a modified pressure washer add-on heating unit.

Commercial insulated storage structure

Commercially available insulated structure to store trays for steam sanitation.

Manifold

View of a manifold used to evenly distribute steam around the trays.

Safety Precautions

Steam can cause serious injuries, and correct safety precautions should be used with any equipment that produces steam. The main danger of working with steam is burns or scalding to the skin. The tray sanitization application will result in exposure to steam temperatures equal to and exceeding 212°F. Wear appropriate personal protective equipment (PPE) such as gloves, eye protection, long pants, and boots. Remember the steam distribution system and related components will remain at elevated temperatures after the process has stopped during loading and unloading. Steam will reduce visibility, which could result in other accidents. Condensation of steam will cause floors to become slippery, which will increase the risk of slipping or falling. Steam can also get into electronic devices and outlets resulting in an electric shock. The system should be operated away from outlets, and be sure to cover any electrical equipment in close proximity during the process and ensure it is dry before using. Also, do not bypass or disconnect factory installed safety features incorporated on any commercial steam or hot-water equipment purchased or modified for this application.

Useful Resources

Tobacco Growers Information Portal

Flue-Cured Tobacco Guide

For more information regarding equipment specifications and costs associated with building your own steam sanitation system Contact your local N.C. Cooperative Extension agent.

References

Gutierrez, W. A., H.D. Shew, and T.A. Melton. 1997. Sources of inoculum and management for Rhizoctonia solani damping-off on tobacco transplants under greenhouse conditions. Plant
Dis. 81:604-606.

Thiessen, L. D. 2019. “Managing Diseases”. 2019 Flue-Cured Tobacco Guide. NC State Extension, pp. 118-143.

Ellington, G. H. 2019 “Curing and Mechanization”. 2019 Flue-Cured Tobacco Guide. NC State Extension, pp. 164-188.

Authors

Extension Assistant
Biological & Agricultural Engineering
Extension Assistant Professor
Biological & Agricultural Engineering
Assistant Professor & Extension Specialist
Entomology & Plant Pathology
Research Associate/Project Coordinator
Biological & Agricultural Engineering

Publication date: April 10, 2019

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