Recommendations for Starting Disease-Free Vegetable Transplants

Many home gardeners and commercial growers have placed their seed orders or have the seeds saved from last year safely tucked away. In the coming days or weeks, it will be time to start those seeds in transplant trays. However, in some cases, seeds come up, wither, and die (Figure 1). Worse is when plants establish but become diseased shortly after transplanting. Several steps can be taken to prevent disease before it occurs.

Figure 1: Vegetable growers may experience frustration when seeds come up only to wither and die. (Photo: Michelle Grabowski, University of Minnesota Cooperative Extension Service)

Purchase Pathogen-free Seed

When purchasing seeds from suppliers, it is important to note if seeds are certified free of disease-causing pathogens. Different sources may have different seed-testing practices. Information about whether or not seeds are certified disease-free can be found on seed packets or by asking your seed supplier.

Treat Seed

Many home gardeners choose to save seeds from special varieties from year to year. However, pathogens may be present on the exterior and/or interior of seeds, even if not visually obvious. 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 5 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 Cornell University’s Vegetable MD Online article entitled “Managing Pathogens Inside Seed with Hot Water” and Appendix I in UK ID-36. 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

Re-used transplant trays can harbor pathogen propagules, which can cause seedling diseases. If trays are re-used, all soil and plant debris should be removed. After organic material is removed, trays can be sterilized using a solution of one part household bleach to nine parts water (10% bleach). Pots and metal stakes should be sterilized in a similar way. Ensure that trays have been rinsed with fresh water and are completely dry before planting.

Pasteurize Planting Media

Planting media can also contain pathogen propagules that may infect seeds or seedlings. It is recommended to always use new planting media for starting seeds, and most purchased media is typically pathogen-free. If media is suspected of being infested with pathogens, pasteurization (heating up soil) can eliminate them. To pasteurize, put thoroughly 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 15 to 20 seconds, mix, and repeat several more times.

Additional Resources


By Kim Leonberger, Plant Pathology Extension Associate and Emily Pfeufer, Extension Plant Pathologist


Posted in Vegetables