Hot Water Seed Treatment

Some plant pathogens, including specific fungal, oomycete, bacterial, and viral pathogens, can be carried on seed; some can only infest the seed surface and others are able to penetrate the seed coat and survive within the seed. In both cases, when the seed is planted, the pathogen can grow along with the seed, resulting in an infected plant. Therefore, starting with disease-free seed is an important step towards growing disease-free crops. Seeds can be treated with chlorine or pesticides to eliminate pathogens that are associated with the surface of seeds. However, these treatments cannot penetrate the seed coat, and therefore leave internal pathogens untouched. Hot water can penetrate the seed coat and kill pathogens without the use of pesticides, making it a useful tool for managing seed-borne diseases. With the right tools, growers can hot water treat seeds effectively at home. Additionally, we offer a hot water seed treatment service through the UMass Extension Vegetable Program. This article includes instructions on treating your own seed as well as information on how to submit seeds to be treated.

Treating your seeds with hot water can help prevent the establishment of seed-borne diseases on your farm, or prevent their reintroduction year after year. However, while hot water seed treatment (HWST) will kill pathogens on and within your seeds, it does not prevent reinfection and does not guarantee disease-free crops. Plant diseases can spread via wind, water, and insects throughout the growing season. Crop rotation and field sanitation are key for preventing the spread of diseases, regardless of whether you hot water treat your seed. 

HWST also has the beneficial effect of priming seeds, resulting in faster germination than untreated seed. However, the treatment can decrease germination rates, especially of older seed (more than 1 year old) or seeds that were grown under stressful environmental conditions. Treated seed does not remain viable for as long as untreated seed and should be planted during the growing season immediately following treatment.

Deciding which seeds to treat

• Determine the likelihood that seed-borne pathogens are present based on the seed (see Table 1 for the seed-borne diseases affecting different crops). If you are saving your own seed and diagnosed one of the diseases in Table 1 in your crop, you should hot water treat that seed. If you buy in seed, it can be harder to determine if HWST is necessary. Tomato, pepper, and brassicas are good candidates for hot water seed treatment because there are common bacterial and fungal diseases of these small-seeded crops that can be easily killed through treatment. Having a conversation with your seed supplier is also a good idea: ask them if the seed was produced in a way to minimize exposure to seed-borne pathogens and if the seed was tested for their presence. Find out if the seed has already been treated with hot water or if it has been primed (pre-soaked to promote earlier and more uniform germination), as treating again could adversely affect the seed. Only a few companies routinely hot-water treat seeds—many are reluctant because there is a risk that germination rate will drop if the water is too hot or if the seeds were already exposed to stressful environmental conditions.
• Don’t treat seed that has a fungicide or insecticide treatment coating, or pelleted seed, as the treatments or pelleting will wash off during treatment and the water bath will be contaminated with pesticides.
• Treat only the seeds that you will use next season. HWST reduces the shelf-life of seeds, so don’t treat seeds that you plan to seed more than a year in the future.
• Don’t treat old seed. Treat only seeds that were produced for the current growing season. HWST is more likely to decrease the germination of old seed.
• Large-seeded crops (beans, cucurbits, peas, corn etc.) are usually not effectively disinfested with hot water treatment because the temperature required to heat the whole seed would kill the outer seed tissue and the seed will not germinate. In some cases, hot water has been used to disinfect just the surface of larger seeds, for example when treating anthracnose on beans.

Treatment procedure

The general protocol for seed treatment is the same across all crops, with just the water temperature and treatment time varying depending on the crop. The temperature of water for treating seed varies from 115 to 125°F, and the treatment period varies from 10 to 60 minutes. See Table 1 for treatment times and temperatures for different crops. It is important to use the appropriate protocol for each crop to control pathogens without damaging the seed; a difference of just a few degrees can either damage your seed or fail to kill pathogens. While hot water seed treatment can be done effectively on a stovetop in a large pot with an accurate thermometer and careful temperature control, it is easier and safer to use precision water baths which provide an even, stable, and accurate temperature.

Before you treat all of your seed, you may want to conduct a seed germination test, as different varieties and seed lots may react differently to hot water treatment. Treat a 50- or 100-seed sample using the procedure below, then test the germination of both the treated seeds and an equal number of untreated seeds, either in the same growing medium that you plan to use for transplant production, or in a moist paper towel. If the test gives acceptable germination rates, treat as much seed as you expect to use in the coming season. 

1. Preheat water baths. Heat one bath to 100°F and another to your treatment temperature. The first bath will be used to preheat the seed so that the temperature of the treatment bath doesn’t drop significantly when the seeds are added. Heat enough water to allow water to move around seeds freely. We treat about 0.5 liters of seed at a time in our six-liter water bath. Use an accurate laboratory thermometer. It is important that the water be maintained at a uniform temperature throughout the bath, that the recommended temperature not be exceeded, and that the seed be treated no longer than the time interval specified. A stirring hot plate helps to provide continuous agitation and uniform water temperature, though it can be done with continuous, consistent manual agitation or an aquarium bubbler. Keep a separate container of room temperature water close by to add, if necessary, to prevent overheating. An immersion circulator (a hand-held water heater used for the sous vide cooking method) is a great tool for treating your own seed.
2. Prepare the seed. Make a packet for the seeds out of cheesecloth, screen, a coffee filter, or insect netting. Fill each packet no more than halfway with seed, to allow for water movement throughout the packet. Include a metal bolt, coin, or other weight to keep the seed submerged. Label all packets, especially if you’re treating more than one variety at once! We label our packets by including a small piece of a plastic transplant tray label tag, labeled with permanent marker inside the packet.
3. Pre-heat the seed. Submerge the seed in the pre-heat bath for 10 minutes, constantly checking the temperature to ensure that it does not rise above 100°F.
4. Treat the seed. Move the seed to the treatment bath and treat for the recommended amount of time. Again, check the temperature constantly to ensure that it does not rise above the recommended temperature. Remove the seeds promptly and run them under room temperature tap water to cool them.
5. Dry the seed. Pat dry with towels, then air dry at 70 to 75°F by spreading the seed on dry paper towels. We leave treated seeds in their mesh packets and dry them in a simple food dehydrator on fan only to dry the seeds quickly. Make sure there is no added heat! Not all dehydrators have this option—check before you buy) to dry the seeds quickly.

Equipment: There are many options for water bath equipment; cheaper options likely require you to watch and adjust the temperature constantly where more expensive options may be more precise and hands-off. Stirring hot plates start at about $400. Both analog and digital precision water baths run at about $700 minimum. Laboratory thermometers are about $15. There are many brands of sous vide immersion circulators that sell for $50-100.

UMass Hot Water Seed Treatment Service: If the procedure above sounds daunting or you’re not sure you want to invest in hot water treatment equipment, we can treat your seed for you! We are only able to treat seed that will be used by the submitter—we cannot treat seed that will be resold or distributed. Submissions are treated and returned to the submitter within 10 days of receipt.

Submit a hot water seed treatment sample