Recommendations for Starting Disease-Free Vegetable Transplants

Home gardeners and commercial growers likely have placed their seed orders or have last year’s seed saved. Over the next few weeks, many will plant those seeds in order to establish transplants for spring and summer gardens. In some cases, those seeds may germinate, wither, and die (Figure 1). In other cases, plants may establish but become diseased shortly after transplanting. This damping-off can be caused by a number of soilborne fungi or fungus-like water molds, often the result of infected seed or from contaminated soil or containers.  More information on damping-off diseases can be found in the publication Damping-off of Vegetables & Herbaceous Ornamentals (PPFS-GEN-03). Numerous steps can be taken to prevent the occurrence of seedling diseases.

(Photo: Michelle Grabowski, University of Minnesota CES)

Purchase Pathogen-Free Seed

When purchasing seeds from suppliers, select seeds that are certified free of disease-causing pathogens. Different sellers may have different seed-testing practices. Information about whether or not seeds are certified disease-free can be found online, in seed catalogs, or on seed packets.

Heat-Treat Seed

Many home gardeners choose to save seeds from year to year, particularly in the case of heirloom varieties. However, pathogens may be present on the exterior and/or interior of seeds, even if not visible. This may also be true of purchased seeds that are not certified disease-free. Hot water seed treatment may be used for certain types of vegetables to kill pathogens. In this process, seeds are pretreated in a water bath at 100°F for five minutes. Seeds are then transferred to a second water bath set at a specified temperature, typically between 118 and 125°F, for a specified period of time. The temperature and treatment time varies depending on the type of seed being treated. Reference the publication Disease Management Practices for Saved Vegetable Seeds. There are certain types of vegetable seeds that cannot be heat treated, such as peas, beans, and most cucurbits; seed pretreated with fungicides also should not be hot water-treated.

Surface Sterilize Transplant Trays

Reused transplant trays can harbor disease-causing pathogens. If trays are reused, all soil and plant debris should be removed. Trays can then be sterilized using a solution of one part household bleach to nine parts water (10% bleach) or a disinfectant. Pots and metal stakes should be sterilized in a similar way. For additional information on cleaning and disinfection of trays can be found in the Cleaning & Disinfecting Home Garden Tools & Equipment and Cleaning & Sanitizing Commercial Greenhouse Surfaces publications.

Pasteurize Planting Media

Planting media can also contain pathogen propagules that may infect seeds or seedlings. Always use new planting media for starting seeds, as most purchased media is typically pathogen-free. If media is suspected of being contaminated, pasteurization (heating up soil) can eliminate pathogens. To pasteurize, put well-moistened soil in a metal container (such as a disposable cake pan) and heat at 200°F for 46 to 60 minutes, or microwave in a glass pan for 30 seconds, mix, and repeat until soil is evenly heated (approximately 3 minutes total time).

Additional Resources

• Disease Management Practices for Saved Vegetable Seeds (PPFS-VG-09)
• Bean & Pea IPM Guide for Small Acreage & Backyard Production (PPFS-VG-22)
• Cole Crop IPM Guide for Small Acreage & Backyard Production (PPFS-VG-23)
• Cucurbit Crop IPM Guide for Small Acreage & Backyard Production (PPFS-VG-19)
• Tomato & Pepper IPM Guide for Small Acreage & Backyard Production (PPFS-VG-21)
• Cultural Calendars for the Production of Vegetable Crops (Series)
• Vegetable Production Guide for Commercial Growers (ID-36)
• Cleaning & Disinfecting Home Garden Tools & Equipment (PPFS-GEN-17)
• Cleaning & Sanitizing Commercial Greenhouse Surfaces (PPFS-GH-07)
• Cornell University – Managing Pathogens Inside Seed with Hot Water
• Damping-off of Vegetables & Herbaceous Ornamentals (PPFS-GEN-03)

Revised by Nicole Gauthier, Plant Pathology Vegetable Extension Specialist

[Original article by Kim Leonberger, Plant Pathology Extension Associate, and Emily Pfeufer, Former University of Kentucky Plant Pathology Extension Specialist]