Hot Water Seed Treatment
Benefits of hot water seed treatment
Starting with disease-free seed is an important step towards growing disease-free crops. Some plant pathogens are able to penetrate and survive within the seed, out of reach of surface seed treatments. They include many bacterial pathogens as well as some fungi, oomycetes, and viruses. 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. Hot water seed treatment is a valuable tool for prevent-ing establishment of seed-borne diseases on the farm, or their reintroduction year after year. Hot water seed treatment also has the beneficial effect of priming seeds, resulting in faster germination than untreated seed.
However, it’s important to note that while hot water seed treatment will kill pathogens on your seeds, it does not protect seeds or crops from disease and does not guarantee disease-free crops. Crop rotation and field sanitation are key for preventing diseases that overwinter on crop debris, and crops need to be scouted regularly for wind-, water-, and insect-borne diseases.
Deciding which seeds to treat
To decide whether to use hot water treatment, first determine the likelihood that seed-borne pathogens could be present based on the crop (see Table 1 for refer-ence). Next, ask your seed supplier 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 uni-form germination), as treating again could adversely affect the seed. You should also not treat seed that has a fungicide or insecticide treatment coating, as it will wash off during treatment. Only a few companies routinely hot-water treat seeds - many are reluctant because there is a risk that germina-tion rate will drop if the water is too hot or if the seeds were already exposed to stressful environ-mental conditions.
Treatment procedure and seed requirements
The temperature of water for treating seed varies from 115 to 125°F, depending on the crop, and the treatment period varies from 10 to 60 minutes. 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, treating anthracnose on beans. Hot water treatment will also negate other seed treatments, and so chemically-treated, pelleted, or primed seed should not be hot-water treated. Hot water seed treatment can decrease germination rates, especially for older seed (more than 1 year old) or seeds that were grown under stressful environmental conditions. You may want to test the germination of a small sample of your seed both before and after hot water treating to evaluate the potential effects of treatment on germination. Treat no more seed than you think you will use in the course of a season, as hot water treated seed may not remain viable for as long as untreated seed. It is important to use the appropriate protocol for each crop to control pathogens without damaging the seed. 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 much better to use precision water baths which provide an even, stable, and accurate temperature.
Before you test all of your seed, you may want to conduct a seed germination test, as different varieties and 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 media 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.
Preheat water baths. Heat one bath to 100°F and another to your treatment temperature (see Table 1). The first bath will be used to preheat the seed so that the temperature of the treatment bath doesn’t drop when the seeds are added. Heat enough water to allow water to move around seeds freely. We treat about six packets 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 vessel, that the recommended temperature not be ex-ceeded, 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. It helps to have a separate container of room temperature water close by to add, if necessary, to prevent overheating.
Prepare the seed. Make a packet for the seeds out of cheese cloth, screen, 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!
Pre-heat the seed. Submerge the seed in the pre-heat bath for 10 minutes, constantly check-ing the temperature to ensure that it does not rise above 100°F.
Treat the seed. Move the seed to the treatment bath and treat for recommended time (see Table 1). 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 tempera-ture tap water to cool them.
Dry the seed. Pat dry with towels, then air dry at 70 to 75°F by spreading the seed on dry paper towels.
|Crop||Treatment Temp||Treatment time||Diseases Controlled|
|Broccoli||122°F||20 minutes||Alternaria leaf spot, Bacterial leaf spot, Black leg, Black rot|
|Brussels sprouts||122°F||25 minutes|
|Brussels sprouts||122°F||25 minutes|
|Carrot||122°F||20 minutes||Alternaria leaf blight, Bacterial leaf blight, Cercospora leaf spot, Crater rot/foliar blight|
|Celery/Celeriac||118°F||30 minutes||Bacterial leaf spot, Cercospora leaf spot, Septoria leaf spot, Phoma crown and root rot|
|Eggplant||122°F||25 minutes||Anthracnose, Early blight, Phomopsis, Verticillium wilt|
|Lettuce||118°F||30 minutes||Anthracnose, Bacterial leaf spot, Lettuce mosaic virus, Septoria leaf spot, Verticillium wilt|
|Onion||122°F||20 minutes||Purple blotch, Stemphylium leaf blight, Basal rot, Botrytis blight, Smudge, Black mold, Downy mildew|
|Pepper||125°F||30 minutes||Anthracnose, Bacterial leaf spot, Cucumber mosaic virus, Pep-per mild mosaic virus, Tobacco mosaic virus, Tomato mosaic virus|
|Parsley||122°F||30 minutes||Bacterial leaf blight, Alternaria leaf blight, Black rot, Cerco-sporoid leaf blight, Septoria blight|
|Spinach||122°F||25 minutes||Anthracnose, Cladosporium leaf spot, Cucumber mosaic virus, Downy mildew, Fusarium wilt, Stemphylium leaf spot, Verticillium wilt|
|Tomato||122°F||25 minutes||Alfalfa mosaic virus, Anthracnose, Bacterial canker, Bacterial speck, Bacterial spot, Cucumber mosaic virus, Early blight, Fusarium wilt, Leaf mold, Septoria leaf spot, Tomato mosaic virus, Verticillium wilt, Double virus streak|
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.
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! For information about this service, including shipping and pricing information, please see the hot water seed treatment submission form below.