One of the ways plant pathogens are introduced into a crop is on seeds. Bacterial pathogens are particularly notorious for this means of dissemination. In general, the earlier a pathogen encounters the crop, the greater the potential for a serious disease problem to develop. This is why it is very important to start with "clean" seed. Properly used, hot water treatment kills most plant disease-causing bacteria both on and within seeds. This treatment is suggested for seeds of eggplant, pepper, tomato, carrot, spinach, lettuce, celery, cabbage, turnip, radish, and other crucifers. Primed, old, or poor-quality seeds may be damaged by this treatment. Seed coatings, including pelleting materials, pesticides, and biologicals are partially or completely removed by hot water treatment, so only raw seed should be treated. Seeds of cucurbits (squash, gourds, pumpkins, watermelons, etc.) may be damaged by hot water. It is highly recommended that a small sample of each seed lot be treated and tested for germination (see below) prior to treating the entire lot.
Chlorine treatment is easier to perform than hot water treatment and does not require specialized equipment. However, chlorine treatment only kills pathogens on the seed surface. This type of treatment is recommended for pumpkin and other large-seeded vegetables that may be damaged by hot water treatment and do not commonly harbor pathogens beneath the seed coat. Cucumber seeds may internally harbor the bacterial pathogen that causes angular leaf spot, so hot water treatment is recommended. While pepper seeds tolerate hot water treatment, the major pathogens causing bacterial spot and Pseudomonas leaf spot are carried on the seed surface, so chlorine treatment is recommended. Chlorine treatment also kills fungal, viral, and human pathogens on the seed coat that may not be killed by hot water treatment. Chlorine treatment may be combined with hot water treatment.
Hot Water Treatment
When treating vegetable seeds with hot water, follow these instructions exactly to ensure that the seeds are not damaged by the treatment, and/or the pathogen is not completely eliminated. Maintaining a water temperature within ±1 degree Fahrenheit (± 0.5 degrees Celsius) of the recommended treatment temperature is critical. While this can be done on a stovetop with constant, careful monitoring, using water baths or precision water heaters (sous vide—see below) is recommended. We have tested a sous vide-type precision cooker and found it to be an effective, inexpensive alternative to commercial water baths.
Instructions
The following equipment and supplies are needed to treat vegetable seeds with hot water.
- Water bath*—preferably two: one for pre-warming and one for treatment, along with:
*Water baths can be purchased from sources such as Fisher Scientific Co., Thomas Scientific, or VWR Scientific.
- Sous vide precision cooker with digital controls, along with:
Hot Water Seed Treatment: Water Baths
Step 1: Add seeds to a clean container holding water that is pre-warmed to 100 ± 1 F (37 ± 0.5 C). If two water baths are not available, the pre-warming step may be done with tap water warmed to 100 ± 1 F (37 ± 0.5 C) or water heated on a stovetop to this temperature.
Step 2: Pre-warm seeds for 10 minutes in 100 ± 1 F (37 ± 0.5 C) water, stirring occasionally.
Step 3: Drain seeds through a fine mesh sieve and transfer them to another clean container with clean water heated to 122 F (50 C), or the temperature indicated for the seed type being tested (see Table 1). Place container in a water bath that will maintain the desired temperature (±1 F or ± 0.5 C) for the recommended amount of time. Stir occasionally.
Step 4: After treatment, drain seeds through a fine mesh sieve and then run cold tap water over them for five minutes to stop the heating action.
Step 5: Spread seeds in a single, uniform layer to dry on a screen. Use a fan to increase air movement.
Hot Water Seed Treatment: Sous Vide Precision Cooker
Step 1: Add seeds to plastic cups labeled with the seed variety and lot number, cover with mesh screening, and seal the screening over the top of the containers with rubber bands. Alternatively, seeds may be placed in a cheesecloth bag. Make sure that seeds are loosely packaged to ensure even heating.
Step 2: Place the plastic cups or bags containing seeds in water pre-warmed to 100 F (37 C), ensuring that the air is completely displaced by water. Warm the seeds at this temperature for 10 minutes.
- Set the desired time and temperature on the cooker and allow the water temperature to rise to the set point. The timer will engage when the selected temperature is reached. It is not necessary to remove the seeds during this time.
- If two precision cookers are available, or pre-warming is done with warm tap water or water heated on a stovetop to 100 F +/- 5 F, keep the seeds in the pre-warmed water for 10 minutes, and then transfer the seeds in their containers to the water set at the higher temperature.
Step 3: After the pre-determined time in the higher temperature water has been reached, place the seeds in clean, cool water to stop the reaction.
Step 4: Place seeds on a screen to dry.
Chlorine Seed Treatment
Step 1: Agitate seeds in a solution of 25 oz of Clorox plus 100 oz of water with one teaspoon of surfactant (e.g. liquid detergent) for one minute. Use one gallon of disinfectant solution per pound of seed (conversions provided below) and prepare a fresh solution for each batch.
Step 2: Rinse seed thoroughly in cold running tap water for five minutes.
Step 3: Spread seeds in a single, uniform layer on screen to dry. Do not dry the seeds in an area where fungicides are present.
Optional Chlorine Rinse After Hot Water Treatment
Some growers use an additional step after the hot water treatment to promote complete sanitation of the vegetable seeds from viruses and potentially harmful bacteria, such as human pathogens, that are not killed by the hot water treatment.
- If a water bath was used for the hot water treatment, at Step 4 (after draining the treated seeds in a sieve), use the sieve to dip the seeds into a 20% solution of chlorine bleach [1 part regular bleach (5.25% sodium hypochlorite): 4 parts cold water] for one minute. Immediately rinse the seeds for five minutes in running, cold, tap water.
- If a sous vide cooker was used, at Step 3 drain the seeds from each cup separately into a sieve, then use the sieve to dip them into a 20% solution of chlorine bleach (1 part bleach: 4 parts cold water) for one minute. Immediately rinse the seeds for five minutes in running, cold, tap water.
Dry the seeds in a single layer on a drying screen as shown above.
How to Test for Seed Germination After Hot Water or Chlorine Treatment
- Mix seeds in each seed lot and count out 100 seeds per seed lot.
- For the sanitizing seed treatment of your choice, treat 50 of the seeds exactly as described in this fact sheet.
- After the treated seeds have dried, plant the two groups of seeds separately in flats containing a planting mix according to standard practice. Label each group as "treated" or "untreated."
- Allow the seeds to germinate and grow until the first true leaf appears (this allows for differences in germination rates to be observed).
- Count seedlings in each group separately.
- Determine the percent of germination in each group using the following equation:
- Compare germination within each group. They should be within 5% of each other.
Conversions |
1 teaspoon = 5.7 g |
1 pound = 454 g |
8 oz = 1 cup = 237 ml |
16 oz = 1 pint = 473 ml |
32 oz = 1 quart = 946 ml |
128 oz = 1 gallon = 3.8 L |
Acknowledgments
We thank Margaret Moodispaw and Ashlina Chin for technical support, and Michelle Grabowski, University of Minnesota, for helpful suggestions on the use of the sous vide cooker.
Resources
Check out the following resources for information, equipment, and supplies used for hot water treatments of vegetable seeds:
- FisherScientific - fishersci.com/us/en/home.html
- VWR - us.vwr.com/store
- Thomas Scientific - thomassci.com