Infested seed can be the source of the pathogen for important bacterial diseases occurring in the northeastern United States. These diseases include black rot affecting crucifer crops, bacterial spot affecting tomato and pepper, and bacterial speck and bacterial canker affecting tomato. They can cause substantial damage if not controlled.
The first strategy to use for controlling any disease is to eliminate or reduce the amount of the pathogen available to initiate disease. Therefore, the use of disease-free seed and transplants are some of the most important management practices for bacterial diseases. Some seed companies have the resources to produce seed in areas where these diseases do not occur and to test seed for the pathogens. First look at the seed package to determine if your seed has been tested for these pathogens and/or has been treated. Check with the seed company if the package does not contain this information. Hot water, hydrochloric acid, calcium hypochlorite, sodium hypochlorite, and peroxyacetic acid are treatments that seed companies use for bacterial pathogens. Infested crop debris, infested planting supplies (especially wooden stakes for trellising tomatoes) and infected weeds are additional sources of bacterial pathogens and must also be managed in an effective control program.
Seed can be treated by growers with hot water or Clorox® bleach (labeled formulations only) to kill the pathogen. Hot-water treatment is more thorough than Clorox because it affects bacteria inside the seed; however, high temperatures can adversely affect germination if proper precautions are not taken and bacteria deep inside seed may survive treatment. It is best to have seed custom treated, which some seed companies will do. Realize before you start that when you treat the seed, the seed company’s liability and guarantees are null and void. Seed that is coated or fungicide-treated should not be treated with hot water or Clorox. Also do not treat old seed. Make sure seed has not already been hot-water treated as a second treatment can kill the seed.
Precise control of conditions is essential for successfully hot-water treating seed yourself. Realize that there is a small margin between the temperature and length of exposure needed to kill pathogens and the treatment conditions that will kill seeds, and that the highest temperature seed can tolerate varies among crops. Use the following temperatures and times. (See Managing Pathogens Inside Seed with Hot Water for a more complete list.)
- Tomato seed treat at 50oC (122oF) for 25 minutes or 51.5 oC (125oF) for 20 minutes.
- Pepper, cabbage and Brussels sprout seed treat at 50oC for 25 minutes.
- Cauliflower and broccoli seed treat at 50oC for 20 minutes.
- Carrot seed treat at 50oC for 20 minutes.
- Celery seed treat at 50oC for 30 minutes.
- Lettuce seed treat at 47.8oC (118 oF) for 30 minutes. Some feel lettuce is too sensitive to treat.
Hot water treatment can be damaging or impractical for seed of other crops including pea, bean, cucumber, sweet corn, and squash. Water temperature needs to be carefully controlled during treatment. The best way to control temperature while treating seed is to use precision water baths (See Managing Pathogens Inside Seed with Hot Water). Next best is a stirring hot plate and a precision laboratory thermometer. Hot plates and thermometers can be purchased from a laboratory supply company such as Fischer Scientific or VWR International. A recommended thermometer is Fischer catalogue # 15-114 (0 – 80 C range), which sells for $35.43 (2005 price). Fischer carries a 10′ x 10′ stirring hot plate that sells for $525 (Cat. No. 11-600-100SH) and a 7′ x 7′ stirring plate for $418 (Cat. No 11-600-49SH). A magnetic stir bar is also needed. A 3-in long bar sells for $13.20 (Cat. No 14-513-68). A large glass container will be needed because metal can crack the hot plate surface. The larger the container used, the easier it is to maintain water temperature and the less the impact on temperature of adding room-temperature seed. Hot-water treatment can be done successfully using a large pot on a stove top and a precision laboratory thermometer.
With any of these methods, expect to spend some time adjusting settings to achieve the desired temperature and have it maintain, especially with the stovetop. A very low hot plate or stove setting will probably provide the correct temperature. Wait to begin treatment until the water temperature in the pot is maintained. Have containers of hot and cold water nearby in case the water temperature does not stay at the desired temperature. Place seed in a tea infusion ball or in a piece of cotton cloth. Add a metal weight to keep the seed container submerged, but make sure it is not on the pot bottom. Agitate the water continuously. A wooden spoon works well when using a stovetop. Check the temperature constantly. Keep the thermometer off the hot bottom of the pot; this can be accomplished by taping it to the inside of the wooden spoon used for stirring. Upon removing, cool the seed under tap water. Spread the seed out on paper towels to air dry at 70-75°F. It is recommended to conduct a preliminary germination test with a small quantity of treated and untreated seed from each variety and lot number before treating all the seed. Some seed lots produced from stressed plants may not stand up to hot water treatment and germination may be adversely affected (though this is rare with pepper seeds).
Clorox Commercial Solutions® Clorox® Germicidal Bleach (EPA Reg. No 5813-100) and Clorox Commercial Solutions® Ultra Clorox® Germicidal Bleach (EPA Reg. No 67619-8) are labeled for pepper (bacterial spot pathogen) and tomato (bacterial canker pathogen) treatment. There is less chance of seed being damaged with bleach than hot water; however, chemical controls such as Clorox are effective for pathogens on the seed surface only; hot-water treatment can kill bacteria inside as well as on the outside of seed. These have 7.85% and 5.84% available chlorine, respectively. To Clorox treat seed, prepare a solution with 10,000 ppm available chlorine. Mix 16.7 or 22.2 fl oz of these products, respectively, with 1 gallon of water to obtain treatment solution. Use 1 gallon of this solution per pound of seed. Put up to 1 pound of seed in a cheesecloth bag, submerge in this solution and provide continuous agitation for 40 minutes, rinse seed under running tap water for 5 minutes, then dry seed thoroughly on paper towel. Put the seed in a new package, not back in the original one. Prepare a fresh batch of the dilute Clorox solution for each 1-pound batch of seed. The soak can stimulate germination, so if the seed is dried and held too long, germination will be reduced. To legally make this treatment, only these formulations can be used and the full label with this use must first be obtained from the Clorox company (800-446-4686) or by going to the New York DEC website. Enter ‘Clorox Germicidal Bleach’ in the ‘Product Name’ box on the form. Click on ‘Search’ below the search section. When the results are displayed, click on the ‘More’ button in the lower right corner of the box for either product. Note that the search results will include other products that do not have use directions for seed treatment.
Either seed treatment should be done within a few weeks of planting. Best is doing right before planting as treatment can prime seed for germination. Afterwards a fungicide can be applied to prevent damping-off and other pathogens from infecting seeds. There are several formulations of Thiram registered for application as a dust or slurry.
Please Note: The specific directions on fungicide labels must be adhered to — they supersede these recommendations, if there is a conflict. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.
See also:
More information/prepared by:
Margaret Tuttle McGrath
Associate Professor
Long Island Horticultural Research and Extension Center (LIHREC)
Plant Pathology and Plant-Microbe Biology Section
School of Integrative Plant Science
College of Agriculture and Life Sciences
Cornell University
mtm3@cornell.edu