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Managing Pathogens Inside Seed with Hot Water

Meg McGrath
Department of Plant Pathology and Plant-Microbe Biology, Cornell UniversityLong Island Horticultural Research and Extension Center; 3059 Sound Avenue
Riverhead, NY 11901; mtm3@cornell.edu


Andy Wyenandt
Rutgers University, Cooperative Research and Extension
121 Northville Road,
Bridgeton, NJ 08302


Kris Holmstrom
Rutgers Cooperative Extension Vegetable IPM Program, Blake Hall Rm. 243
93 Lipman Dr. ,
New Brunswick, NJ  08901

Presentation given at CT Vegetable & Small Growers' Conference on 1/11/16

Hot water seed treatment service available at UMass

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. Diagrams showing how pathogens can get inside seed plus additional useful information about seed-borne pathogens are in a pdf file prepared by Lindsey du Toit.

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.

sou videHow should seed be treated?  While 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 (Fig 1, Table 3). Additionally, extension specialists were trained so that they could assist growers who want to hot-water treat their seed (Fig 2). 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.        

Additional information about how to hot-water treat seed plus other management steps to also implement in an IPM program for seed-borne diseases are in a ppt file prepared by Kris. Guidelines for treating seed on a stovetop are at http://vegetablemdonline.ppath.cornell.edu/NewsArticles/All_BactSeed.htm.

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.

checking temperature of hot water bath Presenting
Fig 1.  Kris Holmstrom, Rutgers IPM Program, checking the temperature of hot-water baths before treating seed. Fig 2. Kris presenting information about managing seed-borne diseases with hot-water seed treatment plus other management practices during a workshop.


packaging seed Seed in hot water
Fig 3. Kris and Andy Wyenandt packaging seed in fiberglass window screen and cone coffee filters for hot-water treatment. Fig 4. Seed being treated with hot water.


Table 1.  Hot-Water Seed Treatment Protocols (also available as PDF)

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    http://agspsrv34.agric.wa.gov.au/agency/pubns/farmnote/1990/F09090.htm

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: http://agspsrv34.agric.wa.gov.au/agency/pubns/farmnote/1990/F09090.htm

Vegetable seed treatments.  Farmnote 90/1990

Table 2.  Diseases of Vegetable Crops Caused by Seed-borne Pathogens
(also available as PDF)

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, Parsnip
      Brussels sprouts, kale) Phoma canker
Alternaria leaf spot
Bacterial leaf spot (peppery leaf spot) Pepper
Black leg Anthracnose fruit rot
Black rot Bacterial leaf spot
Cucumber mosaic virus
Carrot Pepper mild mottle virus
Alternaria leaf blight Tobacco mosaic virus
Bacterial leaf blight Tomato mosaic virus
Cercospora leaf spot
Crater rot and foliar blight Spinach
Celery Cladosporium leaf spot
Bacterial leaf spot Cucumber mosaic virus
Early blight (aka Cercospora leaf spot) Downy mildew (aka Blue mold)
Late blight (aka Septoria leaf spot) Fusarium wilt
Phoma crown and root rot Stemphylium leaf spot
Verticillium wilt
Anthracnose Tomato
Alternaria early blight Alfalfa mosaic virus
Phomopsis Anthracnose
Verticillium wilt Bacterial canker
Bacterial speck
Lettuce Bacterial leaf spot
Anthracnose Cucumber mosaic virus
Bacterial leaf spot Early blight
Lettuce mosaic virus Fusarium wilt
Septoria leaf spot Late blight (requires both mating types)
Verticillium wilt Leaf mold
Septoria leaf spot
Onion Tomato mosaic virus
Botrytis neck rot Verticillium wilt
Downy mildew Double virus streak
Purple blotch
Smut Turnip, Rutabaga and Radish
Stemphylium leaf blight Alternaria leaf spot, brown spot
Black rot
Black leg

Table 3.  Equipment for Hot-Water Treating Seed (Also available as a PDF)

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

Carolina Biological Analog 10L bath: $716

Fisher Thermo Scientific Precision Digital 5.5 L bath: $1088

Thermometer from Carolina Biological Supply: $12.20
Enviro-Safe Partial Immersion 12" Thermometer (-20 to 150 C) Item #745443
Need 2.

Enaly Ozone Generator (optional): $39.99
Ozone is used to disinfect water when multiple seed lots are being treated in succession

Supplies: distilled water, aquarium bubbler, roll of fiberglass window screen, coffee filters, weights (such as metal nuts or coins), plastic labeling stakes, and permanent