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Managing Pathogens Inside Seed with Hot Water

Ensuring seed is not a source of pathogens causing diseases is an important first step in management. Some pathogens can be on or in seed. Fortunately, not all are capable of becoming associated with seed, but some important pathogens can. Seed-borne fungi include pathogens causing Septoria leaf spot of tomato and Alternaria leaf spot of crucifers. Diseases caused by seed-borne bacterial pathogens include black rot of crucifers, bacterial leaf spot of pepper, and bacterial canker of tomato. Contaminated seed can be an important first source of a pathogen on a farm or even a larger area (most notable example is the new downy mildew of basil in the US). Additionally, a severe disease outbreak can result when a pathogen is present at the start of plant growth. Pathogens able to get inside seed are especially difficult to manage because a surface disinfectant or fungicide treatment won’t affect them. Only heat treatment can get in to these pathogens.

What seed should be treated?

Likelihood that a particular batch of seed could benefit from hot water treatment depends on the crop, the pathogens affecting it, and the seed’s history. Most large-seeded crops (beans, cucurbits, and peas) cannot be effectively treated with hot water. Some pathogens that can be in seed occur more commonly than other seed-borne pathogens. For example, the pathogen causing black rot in crucifers is common in the northeast while the pathogen causing black leg has not been detected for years. Tomato, pepper, and crucifers are crops affected by some of the more common pathogens that can be seed-borne. Vegetable seed that can be treated are listed in Table 1 and diseases caused by pathogens that can be seed-borne are listed in Table 2. The seed’s history is another important consideration when deciding whether treatment is warranted. If you save your own seed or buy from a small producer, hot-water treatment may well be worthwhile. Some seed companies have the resources to minimize the chance of their seed becoming contaminated and also to test their seed. Producing seed where key pathogens do not occur and/or where environmental conditions are not favorable, such as in a greenhouse, are used to obtain clean seed. Determine the likelihood that pathogens could be present by asking your seed supplier if the seed was produced in a way to minimize potential for exposure to seed-borne pathogens and ask if the seed was tested for their presence. It is also important to find out if the seed has already been treated with hot water as treating again could adversely affect the seed. Pelleted seed cannot be treated, and primed or old seed should not be treated.

How should seed be treated?

Sous videWhile hot-water seed treatment can be done effectively on a stovetop, it is much better to use a precision water bath or sous vide machine. The temperature of water for treating seed varies from 118 to 125 F, depending on the crop, and the treatment period likewise varies from 15 to 30 minutes. Pre-heating seed at 100 F is recommended. Equipment for treating seed, including precision water baths, were purchased for several locations in the mid-Atlantic and northeast regions through a project funded by the Northeastern IPM Center. Additionally, extension specialists were trained so that they could assist growers who want to hot-water treat their seed. Contact Meg McGrath to find the nearest location. It is important to use the appropriate treatment protocol for a crop to achieve control of pathogens without damaging the seed. Protocols are listed in Table 1.

Sous vide (right) is a new machine that became available after the workshop training project.  It is an immersion circulator precision cooker pod for cooking food in vacuum-sealed bags.   Precise temperature control combined with water circulation makes it suitable for hot water seed treatment at a lower cost than scientific water baths.  Several units are available on the web for less than $200.  A unit used for treating seed is shown right and demonstrated in a video prepared by Amy Ivy (retired Horticulture Educator, Cornell Cooperative Extension Clinton County).  Seed needs to placed in a porous container as done with water baths (not a sealed plastic bag used for food cooking) and also kept submerged during treatment.

See also Treatments for Managing Bacterial Pathogens in Vegetable Seed for guidelines for treating seed on a stovetop.

What to do after treating seed?

Treating seed with hot water is one component of an integrated disease management program. Sometimes this procedure is not completely effective. But even when it is, achieving effective control of a disease typically necessitates implementing other practices targeting additional potential sources of the pathogen. Other practices to use include sanitation (greenhouse, planting materials, tomato stakes, etc. ) and crop rotation. Resistant varieties, fungicides, and water management will slow disease development. Specific practices to use vary with the pathogen.

Please note: while this is a well-documented procedure, any treatment done to seed after purchase voids any guarantees of the seed company.

Table 1:  Hot-Water Seed Treatment Protocols

Prepared by Margaret  Tuttle  McGrath, Cornell University, Long Island Horticultural Research
and Extension Center, 3059 Sound Avenue, Riverhead, NY. mtm3@cornell.edu

Crop                                 Temperature and time        Reference

Brussels sprouts                122°F     25 minutes              1, 3, 4

Broccoli                             122°F     20 minutes              1, 2, 3

Cabbage                            122°F     25 minutes              1, 3, 4

Carrot                                122°F     20 minutes              1, 2, 3

Cauliflower                       122°F     20 minutes              1, 3, 4

Celeriac                             118°F     30 minutes              3

Celery                                118°F     30 minutes              1, 3

Chinese cabbage               122°F     20 minutes              1, 4

Collards                             122°F     20 minutes              1, 3, 4

Coriander                          127°F     30 minutes              4

Cress                                 122°F     15 minutes              1, 3, 4

Cucumber                          122°F     20 minutes              1, 4

Eggplant                            122°F     25 minutes              1, 3, 4

Kale                                   122°F     20 minutes              1, 3

Kohlrabi                            122°F     20 minutes              1, 3, 4

Lettuce                              118°F     30 minutes              1, 3, 4

Mint                                  112°F     10 minutes              4

Mustard                             122°F     15 minutes              1, 3, 4

New Zealand Spinach       120°F     60-120 mins            4

Onion (sets)                       115°F     60 minutes              4

Parsley                               122°F     30 minutes              5

Pepper                               125°F     30 minutes              1, 3, 4

Radish                               122°F     15 minutes              1, 3

Rutabaga                           122°F     20 minutes              3, 4

Shallot                               115°F     60 minutes              4

Spinach                             122°F     25 minutes              1, 3, 4

Sweetpotato (roots)          115°F     65 minutes              4

(cuttings, sprouts)         120°F     10 minutes              4

Tomato                              122°F     25 minutes              1, 3, 4

Turnip                                122°F     20 minutes              1, 3, 4

Yam (tubers)                     112°F     30 minutes              4

125°F = 51.5 °C    122°F = 50 °C       118°F = 48 °C

125.6°F = 52 °C    131°F = 55 °C       132.8°F = 56 °C

1    Seed Treatments for Commercial Vegetables in Kentucky.  by Bill Nesmith 7-94

2  Vegetable seed treatments, Department of Primary Industries and Regional Development’s Agriculture and Food, Government of Western Australia.

3    Hot Water and Chlorine Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens.  Ohio State University Extension Fact Sheet.   By Sally Miller and Melanie Ivey.

4    Vegetable Seed Treatment.  University of Illinois Extension.  RPD No. 915.  March 1992.  By Mohammed Babadoost.

5    Hot water treatment of vegetable seed – an alternative seed treatment method to control seed borne pathogens in organic farming. Journal of Plant Diseases and Protection 110(3):pp. 220-234.  2003.  By Eva Nega et. al.

Note: Hot water treatment can be damaging or not practical for seeds of peas, beans, cucumbers, sweet corn, beets and some other crops. Some hybrid varieties of cauliflower may be damaged by the recommended treatment.   From: Vegetable seed treatments

Table 2:  Diseases of Vegetable Crops Caused by Seed-borne Pathogens

Seed companies manage and test for many of the diseases listed here.
Prepared by Margaret Tuttle McGrath, Cornell University, Long Island Horticultural Research and Extension Center,
3059 Sound Avenue, Riverhead, NY. mtm3@cornell.edu

Crucifers (Cabbage, broccoli, cauliflower, Brussels sprouts, kale)

  • Alternaria leaf spot
  • Bacterial leaf spot (peppery leaf spot)
  • Black leg
  • Black rot

Carrot

  • Alternaria leaf blight
  • Bacterial leaf blight
  • Cercospora leaf spot
  • Crater rot and foliar blight

Celery 

  • Bacterial leaf spot
  • Early blight (aka Cercospora leaf spot)
  • Late blight (aka Septoria leaf spot)
  • Phoma crown and root rot

Eggplant

  • Anthracnose
  • Alternaria early blight
  • Phomopsis
  • Verticillium wilt

Lettuce 

  • Anthracnose
  • Bacterial leaf spot
  • Lettuce mosaic virus
  • Septoria leaf spot
  • Verticillium wilt

Onion 

  • Botrytis neck rot
  • Downy mildew
  • Purple blotch
  • Smut
  • Stemphylium leaf blight

Parsnip

  • Phoma canker

Pepper

  • Anthracnose fruit rot
  • Bacterial leaf spot
  • Cucumber mosaic virus
  • Pepper mild mottle virus
  • Tobacco mosaic virus
  • Tomato mosaic virus

Spinach

  • Anthracnose
  • Cladosporium leaf spot
  • Cucumber mosaic virus
  • Downy mildew (aka Blue mold)
  • Fusarium wilt
  • Stemphylium leaf spot
  • Verticillium wilt

Tomato

  • Alfalfa mosaic virus
  • Anthracnose
  • Bacterial canker
  • Bacterial speck
  • Bacterial leaf spot
  • Cucumber mosaic virus
  • Early blight
  • Fusarium wilt
  • Late blight (requires both mating types)
  • Leaf mold
  • Septoria leaf spot
  • Tomato mosaic virus
  • Verticillium wilt
  • Double virus streak

Turnip, Rutabaga and Radish

  • Alternaria leaf spot, brown spot
  • Black rot
  • Black leg

Table 3:  Equipment for Hot-Water Treating Seed

Prepared Feb 2013 by Kris Holmstrom, Rutgers Cooperative Extension Vegetable IPM Program, Blake Hall Rm. 243, 93 Lipman Dr., New Brunswick, NJ  08901
and Margaret  Tuttle  McGrath, Cornell University, Long Island Horticultural Research and Extension Center, 3059 Sound Avenue, Riverhead, NY. mtm3@cornell.edu

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

Prepared in collaboration with:

  • Andy Wyenandt, Rutgers University, Cooperative Research and Extension
  • Kris Holmstrom, Rutgers Cooperative Extension Vegetable IPM Program
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