Chapters 7.5

7.5 - Heat treatment of vegetable seeds

Nick Moroz and Aleyeh Waberi, University of Guelph, Canada

Suggested citation for this chapter.

Author name (2022) title of chapter. In Farmpedia, The Encyclopedia for Small Scale Farmers. Editor, M.N. Raizada, University of Guelph, Canada. http://www.farmpedia.org

Introduction to Heat Treatment of Seeds

Physical seed treatment technologies aim to eliminate seed-borne pathogens without the use of pesticides or synthetic active ingredients. Heat treatments of seeds have been widely used due to their broad-spectrum of disease control and ability to give more productive yields when pesticide seed treatments are not possible (Mancini & Romanazzi, 2014). Thus, heat treatment of seeds is a practice that can be used when problems such as low germination and poor pest or pathogen control arise.

Heat treatment is based on the premise that applying controlled heat to seeds, the heat will destroying pathogens or pests. These treatments function by killing pathogens on and within seeds, denaturing their proteins and membranes, while leaving host tissues viable (Mancini & Romanazzi, 2014). There are various methods for the physical heat treatment of seeds, but the most common physical treatments employed today include hot water, hot air (steam), dry air and electron treatments (Mancini & Romanazzi, 2014). The hot water method is the most popular and was used widely to sanitize vegetable and cereal seeds before the rise of synthetic chemical treatments (Gullino & Munkvold, 2014; Mancini & Romanazzi, 2014). This method requires the immersion of plant seeds in hot water for a specific time at a specific temperature, with stirring, and this method is now being used prior to planting for numerous seed varieties (Mancini & Romanazzi, 2014). Cornell University researchers were able to create a table listing different crop types matched with the optimal temperature and time of heat treatment necessary to successfully control the diseases respectively listed with each crop. This table is presented below as Table 1.1.

Benefits of Heat Treatment of Seeds

Physical heat treatment of seeds provides numerous advantages over other seed treatments: they have a wide spectrum of activity (seen in table 1.0), they leave no toxic or pollutants in the environment, and they do not require registration or approval in most countries (Gullino & Munkvold, 2014). Because these treatments are non-toxic, treated seeds can be used for multiple purposes, including animal feeds (Gullino & Munkvold, 2014). Outlined below are some more advantages:

•Target pathogenic microorganisms, bacteria, fungus, and viruses, in large variety of plants and plant parts (Grondeau, Samson, & Sampson, 1994).

•Main means of producing virus-free explants from infected mother-plants (Grondeau, Samson, & Sampson, 1994).

•Satisfactory control of several bacterial diseases impacting seeds (Grondeau, Samson, & Sampson, 1994; UMA, 2017)

•Disinfects seeds, preventing disease transmission and potential outbreaks (Grondeau, Samson, & Sampson, 1994) and prevention of soil-borne pathogen establishment on the farm (UMA, 2017)

•Hot water treatments are still vital for the treatment of vegetative plant propagation material (Gullino & Munkvold, 2014)

•Significant interest increasing around physical treatments that reduce disease while increasing earliness and percent emergence of crop seeds (Taylor & Harman, 1990).

• Decreased time for seeds to germinate by priming seeds (UMA, 2017)

It is important to emphasize the utility of hot water treatment for treating plant propagation materials. This is used on a commercial scale for the eradication of the causal agent of ratoon stunting disease of sugarcane from seed canes, and the management of nematodes that are transmitted through banana and plantain (Gullino & Munkvold, 2014). There are multiple reviews available for description of the treatment of propagation materials by physical means (Grondeau, Samson, & Sampson, 1994; Gullino & Munkvold, 2014).

A comparison between hot water treatment and hot steam treatment is also warranted. Compared to hot water seed treatment, the key advantages of hot air steam treatment are more accurate temperature control, seeds that require less dying after treatment, and steam often results in reduced insufficiency of seed germination (Gullino & Munkvold, 2014). Dry air treatments have the advantage of being easy to apply and lack high-level equipment (Gullino & Munkvold, 2014). Furthermore, dry air treatments have been shown to inactivate viruses in seeds and vegetative propagation materials (Grondeau, Samson, & Sampson, 1994; Gullino & Munkvold, 2014). However, there are many disadvantages of hot water, hot air, and dry treatments, which are discussed next.

Disadvantages of Heat Treatment of Seeds

The main disadvantages for the physical heat treatment of seeds are as follows: the need for optimization of the treatment (time and temperature) for each new seed-lot (i.e. no uniform treatment for all seeds), possible high energy and investment costs for equipment, and no effects on or prevention of soil-borne pathogens (Gullino & Munkvold, 2014). It is also important to note that many of the non-chemical heat treatments of seeds, such as hot water treatments, are much less effective and/or reliable compared to pesticide seed treatments, which is the primary method for treating seeds globally (Gullino & Munkvold, 2014).

Hot water and hot steam, and dry air treatments has the advantages mentioned in the previous section, but they have disadvantages, which are mentioned here. Hot water treatment disadvantages are mentioned in the preceding paragraph and are not discussed further. Hot steam treatment has been found to be only marginally successful in the prevention of bacterial diseases (Gullino & Munkvold, 2014). Dry treatments can range from a few days to as long as two weeks durations, representing a long time for a fully completed seed treatment (Gullino & Munkvold, 2014). This treatment also leads to reductions in seed germination (Gullino & Munkvold, 2014). Unlike the notable ability for dry heat to inactivate viruses, there are few documented cases of seed-borne bacterial or fungal control by dry heat (Gullino & Munkvold, 2014).

Precautions are necessary when using heat treatments. Phytotoxicity symptoms can occur if the heat treatment is not followed correctly (Paula & Pausas, 2008). If the temperature is too hot, it may kill the seed. Users are advised to perform a simple germination test: treat one batch with heat and the other without heat (control) then sow the seeds and check if the heat treatment caused a decrease in seed germination. If the answer is yes, then the heat treatment was too extreme.

Like any agricultural tool, heat treatments of seeds have many advantages and disadvantage. These tools require frequently require more conclusive evidence and research to be optimized, and the results vary by the plant used, as well as by disease (Martine, 2009). Although the heat treatment of seeds is often a beneficial treatment for various seeds and plants, it should not be seen as a magic bullet to treat every seed or crop impacted by pests and pathogens. If a farmer chooses this cheap and simple treatment over more established treatments that he or she could afford (e.g. pesticide seed treatments), it may be an unnecessarily risk.

Procedures and Tools for Heat Treatment of Seeds

Since many subsistence farmers in countries around the world do not have the means to purchase the most efficient seed treatment techniques and tools, some affordable heat techniques and procedures that might aid them are listed in this section:

1) According to Cornell University researchers (McGrath, Wyenandt, & Holmstrom, 2017), a step-by-step process for the heat treatment of seeds is outlined in the following steps:

Procedure:

1. Put weight in seed package (i.e. a coffee filter). A quarter is an ideal weight.

2. Add seed to partly fill container. Do not fill to the point that water will not be able to easily move into the center of the seed mass or that there will be stress on the seems of the coffee filter.

3. Roll top of package over twice to close then staple shut with no gaps that could enable seed to escape.

4. Treat seed at 37°C for 10 min to pre-heat and then at the treatment temperature for the seed type. Then cool with cold water.

5. Promptly either plant or dry seed. Large quantities should be spread out on paper towel to dry.

6. Once dry, put in a new envelope to hold until planting time. Seed should be put in a new envelope rather than returned to the envelope it came in, as this may cause contamination. Store as usual (e.g. refrigerate if planting won’t occur for multiple weeks, as this will preserve the seed).

2) Another procedure similar to the one previously mentioned is outlined next. It should be noted that certain treatment procedures might not work for a specific varieties of fruits, vegetables, seeds or grains. Outlined below is a description of a specific for Tomato seeds, developed by a research group at Rutgers University in New Jersey (Guest, 2012). This treatment is useful for crops prone to seed-borne infections, such as pepper and tomato (Guest, 2012).

Procedure:

1. First is to make two baths, which are temperature controlled. The first bath is for the initial pre-heat cycle and the second bath is for the effective temperature cycle. For subsistence farmers, a thermometer would be ideally used for each of these steps to monitor temperature.

2. Place seeds in porous containers (multiple holes cut in), and dip in to the hot bath

3. Turn the temperature of the water up to 37°C, remove container and put in the second hot water bath (which is for pathogen killing)

4. Remove container from second bath and rinse seeds with cold water to stop the heating cycle (also known as seed priming)

One tool, which is essential for measuring the temperate levels of a hot water bath, is a thermometer. Sites such as Aliexpress.com and Alibaba.com can provide subsistence farmers with cost-effective thermometers that will provide a direct reading of the temperature while a farmer is heat-treating seed, enabling the farmer to ensure pathogens are killed without damaging the plant tissue. Tools such as a thermometer can be purchased individually or in bulk, potentially reducing prices for farmers.

Comparison of Heat Treatment of Seeds with Pesticide Seed Treatments

There are many available chemical and non-chemical seed treatments documented (Gullino & Munkvold, 2014). Physical treatments were shown to be the most effective seed treatment, after chemical treatments, in a study that compared chemical and various non-chemical seed treatments (Mancini & Romanazzi, 2014). This study compared fungicide seed treatments to physical treatments (heat treatments), plant extracts, bacterial control agents, and plant extracts plus bacterial control agents treatments (Mancini & Romanazzi, 2014). Table 2.0 shows the results of the comparison between fungicide treatment efficacy to that of physical treatment efficacy, specifically hot water treatment (Mancini & Romanazzi, 2014).

Often, no single pest control method provides adequate control of pests and pathogens, and seed treatments can be supplemented with other control measures (Paulsrud et al., 2001). As can be seen form this section, pesticides seed treatments, although highly effective, are not the only method to help control pests and pathogens. Heat treatment of seeds is not the golden standard used by major agricultural industries, but in many countries, heat treatment of seeds is seen as a cheap and easy way to combat pathogens.

Resources Moving Forward

This seller on Aliexpress currently has 200 thermometers, which could be bought at once:

https://www.aliexpress.com/item/Underground-Soil-Thermometer-Stainless-Steel-Thermometers-Thermometric-Tool-Gardening-Tools/32648981758.html?spm=2114.01010208.3.1.Qrc5za&ws_ab_test=searchweb0_0,searchweb201602_4_10065_10068_10084_10083_10080_10082_10081_10060_10061_10062_10056_10055_10054_10059_10099_10078_10079_426_10073_10102_10096_10052_10050_425_10051,searchweb201603_8&btsid=8eb0275a-6e1b-4484-b602-333893836620

Here is a link that step-by-step allows farmers/homeowners to properly monitor the temperature levels in their soil: https://www.todayshomeowner.com/how-to-measure-soil-temperature-for-planting/

Excellent research book of many aspects of seed treatments: Gullino, M. L., & Munkvold, G. (Eds.). (2014). Global Perspectives on the Health of Seeds and Plant Propagation Material (Vol. 6). Springer.

References

1. Green, M., Lima, W. A. A., Figueiredo, A. F. D., Atroch, A. L., Lopes, R., Cunha, R. N. V. D., & Teixeira, P. C. (2013). Heat-treatment and germination of oil palm seeds (Elaeis guineensis Jacq.). Journal of Seed Science, 35(3), 296-301.

2. Guest Author. (2012). Seed heat-treatment: A management strategy for controlling bacterial diseases. Rutgers University - Sustainable Farming on the Urban Fringe. Retrieved from: https://sustainable-farming.rutgers.edu/seed-heat-treatment-manage-bacterial-diseases/

3. Gullino, M. L., & Munkvold, G. (Eds.). (2014). Global Perspectives on the Health of Seeds and Plant Propagation Material (Vol. 6). Springer.

4. Grondeau, C., Samson, R., & Sands, D. C. (1994). A review of thermotherapy to free plant materials from pathogens, especially seeds from bacteria. Critical Reviews in Plant Sciences, 13(1), 57-75.

5. Lee, J.M. (2004). Seed Enhancement for Healthy Seedlings. Technical Report No 40. Seoul, South Korea. APSA – The Asia and Pacific Seed Association. Retrieved from: http://apsaseed.org/index.php/publications/publications-for-members/techical-reports/21-tr-no-40-seed-enhancement-for-healthy-seedlings/file

6. Mancini, V., & Romanazzi, G. (2014). Seed treatments to control seedborne fungal pathogens of vegetable crops. Pest management science, 70(6), 860-868.

7. McGrath M., Wyenandt A., & Holmstrom K. (2017). Managing Pathogens Inside Seeds with Hot Water. Vegetable MD Online. Cornell University, Department of Plant Pathology. Retrieved from: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/HotWaterSeedTreatment.html

8. Paula, S., & Pausas, J. G. (2008). Burning seeds: germinative response to heat treatments in relation to resprouting ability. Journal of Ecology, 96(3), 543-552.

9. Paulsrud, B. E., Martin, D., Babadoost, M., Malvick, D., Weinzierl, R., Lindholm, D. C., ... & Maynard, R. (2001). Oregon pesticide applicator training manual. Seed treatment. University of Illinois Board of Trustees, Urbana.

10. Martine, B. M., Laurent, K. K., Pierre, B. J., kouakou Eugegrave, K., Hilaire, K. T., & Justin, K. Y. (2009). Effect of storage and heat treatments on the germination of oil palm (Elaeis guineensis Jacq.) seed. African Journal of Agricultural Research, 4(10), 931-937.

11. Retrieved from: http://www.foodnutritionscience.com/articles/why-do-farmers-spray-chemicals-on-crops/

12. Taylor, A. G., & Harman, G. E. (1990). Concepts and technologies of selected seed treatments. Annual review of phytopathology, 28(1), 321-339.

13. UMA- UMassAmherst. (April 2017). Hot Water Seed Treatment. The Center for Agriculture, Food and the Environment. Retrieved from: https://ag.umass.edu/services/hot-water-seed-treatment