Hot Water Treatment of Seeds

Some pathogens can occur on or within seed and can be responsible for spreading diseases or introducing them to areas where they had not previously occurred.  Even when only a small percentage of seeds are infested, disease can spread among transplants in the greenhouse or in the field, causing significant crop loss or increasing the need for sprays.  In some cases, , whole seedlots may be infested and this can result in severe disease outbreaks, as all seedlings will be affected and young plants may not be able to overcome early, systemic infections.  Using disease-free seed is an important first step in management of many diseases, especially for small-seeded crops where seed-borne diseases are common such as tomatoes, peppers, spinach, onions, carrots, and all the brassicas. Hot water seed treatment is a cheap and effective way to penetrate the seed and kill pathogens that might be present.

Not all pathogens can penetrate and survive within the seed but bacterial pathogens are commonly seed-borne, while some fungi (e.g. Alternaria leaf spot of brasscias), oomycetes (e.g. some downy mildews), and many viruses (e.g. TMV and CMV) are also seed-borne.  Tomato, pepper and brassicas are good candidates for hot water seed treatment because there are common bacterial diseases of these small seeded crops (eg. bacterial leaf spot of pepper and tomato, black rot of brassicas, bacterial canker of tomato, etc.). Even though bacterial pathogens do not survive well in soil once infected crop residues have decayed, they can be difficult to manage once established on a farm. Preventing establishment of these diseases, or reintroduction year after year, is a critical management tool.   Large seeded crops (beans, cucurbits, peas, etc.)  cannot be effectively disinfested with hot water treatment because the temperature required to heat the whole seed inside and out would kill the outer seed tissue and the seed will not germinate.

To decide whether to use heat treatment, first determine the likelihood that seed-borne pathogens could be present based on the crop (see table). Next, ask your seed supplier if the seed was produced in a way to minimize exposure to seed-borne pathogens and if the seed was tested for their presence.  Find out if the seed has already been treated with hot water, as treating again could adversely affect the seed.

The temperature of water for treating seed varies from 115 to 125°F, depending on the crop, and the treatment period varies from 10 to 60 minutes.  It is important to use the appropriate protocol for each crop to control pathogens without damaging the seed. While hot-water seed treatment can be done effectively on a stovetop, it is much better to use a precision water bath and an accurate thermometer.  For details on treatment procedures for each crop and for a list of supplies needed please see the following factsheet published by Cornell and Rutgers Cooperative Extensions: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/HotWaterSeedTreatment.html.