Hot Water Treatment for Pest and Disease Control  
by Graham Barclay

Originally published in NCS Yearbook 1987

Treatment originated in Scotland
In the UK, hot water treatment has its origin in Scotland where it was employed in the mid 19th century on private estates as a treatment for tarsonemid mites on flower bulbs and indeed its main commercial use nowadays is for the control of stem nematode in narcissus bulbs. 

Over the years many different types of plant have been similarly treated to eliminate nematodes, insect pests and fungal diseases. This was especially so early in the present century and until after the First World War, when hot water was widely used on chrysanthemum stools, strawberry runners and mint as a control measure for aphids, nematodes, rust fungi and tarsonemid mites. However, during this period, treatment sometimes tended to be rather 'hit and miss' and as a consequence the technique tended to fall into disrepute. 

Research in 1930's
It was not until research was carried out in the 1930s, particularly for the benefit of commercial narcissus growers, that the problems were identified and successfully solved. This work quickly led to the establishment of effective, reproducible and efficient methods of handling, treating and maintaining clean plant stocks. Developments and improvements then continued throughout the '40s, '50s, '60s and into the 70s and resulted in the introduction of, for example, tanks with recirculating pumps, thermostatic temperature controls for water heaters, palletised bulk loading and automated 'drive in' tank systems. 

Nowadays, many of the commercial systems for narcissus bulbs are designed on a 'through-flow' principle which ensures that treated material is prevented from becoming recontaminated and so remains clean. As a result of such innovation it has proved possible over the years to meet a steady improvement in standards of nematode control. 

In recent years, chrysanthemum nematode (Aphelenchoides ritzemabosi) or eelworm as it used to be called has become rare on commercially grown plants because of the change to all-the-year-round growing systems and the attendant use of soilless composts and routine applications of aldicarb insecticides. Consequently, nematode incidence is now virtually restricted to the smaller commercial nursery and to households and gardens where amateur growers in particular are not permitted the use of aldicarb.

Hot water treatment environmentally safe
For such growers, hot water treatment is the only environmentally safe, effective control measure and some commercial growers of natural season chrysanthemums do still continue with the method for just these reasons. There is an argument based on environmental grounds for much wider use to be made of hot water treatment, but unfortunately all-the-year round plants, for example, produce soft-tissued, small stools which are too easily damaged by heat for this to be applied sensibly in commercial flower production. 

Effective against white rust
However, in addition to nematode control, hot water treatment can also be effective against leaf miner, stool miner and aphids. During the past five years or so, White Rust has become a disease of significant proportions and in present circumstances where the available fungicides are perhaps not quite as good as they could be, it is interesting to see hot water treatment being recommended by the Ministry as part of a package of control measures designed to eliminate this disease. Hot water treatment offers a unique opportunity at the end of each growing season to thoroughly clean up any retained stock plants and thereby ensure that next season's starting material is in the best possible health. 

Regular treatment should be carried out even when a nematode or other pest or disease attack has not been seen for several years. The benefit of this approach is largely as insurance against the possibility of slight undetected infestations causing reductions in plant performance and flower quality, but also in preventing any such sources from subsequently creating a devastating general infestation. An additional benefit which can be obtained as a result of hot water treatment is that cutting quality and numbers seem to be improved, and this is particularly useful in shy shooting cultivars. 

Some Basic Principles 
The sole object of the hot water treatment method is to kill any pest and disease without adversely affecting a plant's ability to produce cuttings. In practice this is achieved by selecting a temperature and time of exposure which on the basis of experience is known to be consistently successful. There is, of course, as one might expect, a temperature below which nematodes and fungi are virtually unharmed and also a higher one at which death is practically instantaneous. 

Between these extremes there is a range of lethal time/temperature combinations which might useful! be employed and research in the 1960s showed that nematode death occurs almost immediately at 52°C, in 10 minutes at 46°C and 60 minutes at 43.5°C (Fig. 1). 

Nevertheless, the Ministry recommended treatment is 46°C for five minutes and this works well in practice, because, while every nematode is not necessarily killed at the end of the treatment, survivors are so badly damaged that they cannot subsequently penetrate the host or reproduce and therefore die soon afterwards. 

Amateur experiments on temperature ranges
Such a treatment does not adversely affect chrysanthemum stools, but on the other hand, many cultivars appear to be considerably less tolerant of prolonged treatment at a lower temperature, e.g. 20 minutes at 43.3°C (110°F). The use of somewhat higher temperatures in the range of 51-52°C (123-125°F) was reviewed in a 1983 Garden News article by Derek Bircumshaw, who reported on the methods used for many years by Stan Jeffery. Stan's experiments were designed to test the extent to which there might be latitude around the Ministry recommended treatment so that growers not possessing thermostatically controlled heaters could still use the technique without fear of damaging the stools. 

Using a five minute exposure period, Stan obtained the following interesting results: 

TEMP LOSSES °C °F % 57 135 100 54 130 >25 52 125 2 49 120 0 46 115 0 

As can be seen, for temperatures between 46-49°C there is no evidence of an adverse effect on survival. At 52°C however, 2% of the plants failed to recover and as the temperature increased there was a dramatic increase in failures at 54°C and finally at 57°C, every plant was killed. 

This evidence suggests there would be no losses from a five minute treatment at 49°C controlled to a level of ±2°C and minimal losses when controlled within ±3°C. Accuracy of this order can easily be achieved with a watch and hand held Centigrade or comparable Fahrenheit thermometer. 

Equipment 
For the amateur grower, the choice of tank is the first consideration, although in practice this is likely to depend inevitably on what is immediately to hand. Stan Jeffery's old gas boiler is a good example of what can be done, but any sort of tank could be adapted for the desired purpose. It is important that the tank is well-lagged and if this is not already the case, then thick sheets of polystyrene should be fixed around the outside to minimise heat loss. 

The source of heat will also depend on what is available and while direct heat from a gas or solid fuel source could be used, these sources are generally difficult to control and an electric immersion heater would undoubtedly be a better proposition. During treatment it is essential to ensure there is an even distribution of heat, essentially throughout the entire volume of water, because too great a variation might well lead to a partial failure of the process. 

The problem can be overcome either by using a hand held stirrer or better still, an electrically driven stirrer or submersible pump. My own system is a bench top, laboratory hot water bath borrowed each year for the critical period from a local company. This comes complete with an electric thermostatically controlled heater and a continuously operated paddle-type of stirrer. Even so, a conventional thermometer provides a useful means of checking the temperature level, and should this drift downwards then there is also a booster with which to bring things quickly back into line, this type of water bath is relatively expensive and is likely to be beyond the scope of the individual pocket but for Societies the purchase of a communal system could be well within the bounds of probability. 

Method of Treatment 
Stools to be treated should be lifted when dormant and all leaves, shoots and debris removed. Then the roots are severely trimmed and thoroughly washed to remove all soil and the main stems shortened to about six inches. Prepared stools of individual cultivars should be handled separately and preferably labelled before packing loosely in an open mesh basket or polypropylene sack, roots facing downward and left so that surplus water will drain away. 

After this they should be plunged into hot water at 46°C for five minutes and initially vigorously agitated to ensure even wetting and heat distribution throughout the roots. In practice, it is usually necessary to overheat slightly before adding the stools, as the temperature of the water will then drop to the required level. It is important, however, to keep a close watch on proceedings and if necessary, add more heat or slightly extend the period of treatment if this is judged to be necessary. 

However, routinely used, little difficulty will be experienced in guaranteeing the required overall exposure. Nevertheless, there is a limit to the number of stools that can be properly treated in a given volume of water and for the usual heavy exhibition type of plant, a rough guide would be two stools per gallon. 

Rapid cooling after treatment
After treatment, stools should be cooled rapidly, usually by immersing in a bucket of cold water for about five minutes, and it can be beneficial to add a fungicide such as benomyl or zineb to the water to help prevent mildew and other fungal diseases. Stools are then left to lightly air-dry in a cool place, away from the treatment area, which is not usually a problem in October or November and may even be left in this way for a few days if necessary, without causing any ultimate problems with cutting production provided they do not dehydrate. 

Good post-treatment hygiene is necessary at this stage to avoid re-contaminating treated stools and for this reason infected debris and washings should be carefully disposed of, well away from the treatment area. When they are ready, stools should be boxed up in sterilised soil or compost in clean plastic or sterilised wooden boxes and kept cool, out of harm's way, in a frost-free greenhouse, cold frame or under similar modest protection. 

Cuttings tend to start growing quite quickly after the stools have been treated and cool conditions should be maintained, therefore, until about four to six weeks before the required date of striking. 

Discussion 
It is important during the growing season prior to treatment to examine flowering plants carefully for the presence of pest and disease, and then only mark up for stock purposes, healthy vigorous examples which are true to type. Should there be obvious signs of major pests such as Chrysanthemum Nematode or diseases such as Crown Gall, White Rust, Verticillium Wilt or Stunt Virus, then it is probably best to destroy the stock and start afresh with clean material brought in from a reliable outside supplier. 

However, if the stock is particularly important, heat treatment is an effective control measure for nematodes and White Rust. On the other hand, bacterial, fungal and viral diseases, as exemplified in the above list, will not be eradicated because the time/temperature exposure required to kill them is very much higher than living plant tissue can withstand. In spite of this limitation there is no doubt that hot water treatment is a very useful technique as an adjunct to routine hygiene practices and is manifestly important to the amateur grower who for legal reasons is unable to use modern, effective chemical control methods.

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