Hot water treatments of planting stock have been in practical use for nematode management since at least the early 1900's. In California, the most common usage has been for stem and bulb nematode in daffodils and garlic, root-knot nematode in grape rootstocks and foliar nematode in Easter lilies.

It is essential to realize that accurate time and temperature controls must be in place for the hot water treatment to be successful. In this photograph, Easter lily bulblets were hot water treated for 10 minutes at 125F prior to planting. Treatment at the same temperature for 20 minutes resulted in severe damage to the crop. In some cases, harvested bulbs developed multiple noses, an undesirable characteristic. Each nematode species and each crop variety or cultivar needs to be tested for susceptibility on a small scale prior to undertaking a commercial scale treatment program. Information is available in the literature for a number of nematodes and crops which will provide starting points from which to select a range to test. Some examples would be: 125F for 5 minutes for root-knot nematode on grapes; 48.9C for 20 minutes for stem and bulb nematode on garlic; and 43.9-44.4C for 180 minutes for stem and bulb nematode on daffodils. These graphs of the time for mortality to occur at various times and temperatures for Aphelenchoides fragariae (foliar nematode) and Ditylenchus dipsaci (stem and bulb nematode) illustrate the great variability in susceptibility between nematodes.

There is no one perfect setup for conducting hot water treatments. For experimental treatments or small volumes, an ice chest with an electric immersion heater may be sufficient. An aluminum stock watering tank was utilized in these hot water treatments of strawberry planting stock. Larger tanks into which stock is lowered by a hoist or forklift, treated for a period of time and then removed is one common method. Another option is to place stock onto a conveyer belt which moves through a tank of hot water at a calibrated speed. Treatment tanks have also been developed in which the entire side opens to permit a forklift to drive in to place a pallet of planting stock. After the forklift has exited, the tank is closed and preheated water from an adjacent tank is pumped in to begin the treatment. A wide variety of equipment has been used to heat treatment baths, including electric immersion coils, probes, or jackets; diesel, propane or natural gas units combined with water circulating tubing; and heating units (including wood or charcoal fires) placed under the treatment tanks.

The use of a pretreatment tank to partially raise the planting stock to the required level is utilized in some procedures particularly if planting stock is being taken from cold storage or cold soil just prior to treatment. This graph illustrates how the starting condition of strawberry crowns can affect the time required to reach a desired temperature. Consideration should also be given to how quickly planting stock must be cooled after treatment. Frequently, treatment procedures include an additional tank for cooling stock after treatment or hosing down of treated stock with cool water. These set-ups for strawberry crowns and grape rootstocks utilize a pretreatment tank, a treatment tank, and a cooling tank.

The ratio of volume of water to planting stock should be large enough to prevent a significant drop in temperature when the stock is added. This factor should be determined for each hot water treatment set up prior to initiating treatments. The shorter the treatment time, the more critical it becomes to quickly reach and maintain the required temperature. For short treatment times (e.g. 5 minutes), the temperature should return to treatment level within 1 minute. It is not always clear from the literature if proposed treatments begin at the time stock is placed into the bath or when the treatment temperature is reached. This is another reason all treatments should be tested on a small scale prior to commercial scale use.

A circulating pump or some other method should be utilized to mix the treatment water to assure uniformity within the tank. Temperature uniformity for a treatment should be monitored by the use of several temperature monitors placed within the treatment tank. Temperature monitoring equipment can be relatively simple utilizing thermometers or more complex utilizing thermocouples, thermistors, analog or digital meters, and chart recorders, or other continuous recording devices. Although contrary to current scientific thinking, it may be more accurate to monitor temperatures in the Farenheit rather than the Centigrade scale because of the greater spread within the scale. County offices for Weights and Measures (which are typically a joint function with the Agricultural Commissioners) can provide information on calibration of thermometers used to monitor treatment baths.

These experimental set-ups for evaluating temperature within daffodil bulbs and nematode mortality in water placed in a culture tube illustrate the basic components of a hot water treatment system. In the daffodil studies, it was found that although the time required to reach a higher temperature increased as bulb diameter increased , this did not appear to vary based on the starting temperature. For strawberry crowns initially at the same temperature, which were placed into constant temperature tanks, there was no difference in the time required to reach the desired temperature As originally developed, hot water treatments for daffodils and garlic included the addition of formaldehyde to the treatment bath. Because of the suspected carcinogenicity of this product, attempts have been made in recent years to find a replacement with products such as sodium hypochlorite (household bleach), gluteraldehyde, and avermectin. One problem with finding a replacement for a broad spectrum product like formaldehyde is that although its registered use was to kill nematodes, in reality it is also killing fungi and bacteria circulating in the treatment water. Another interacting factor that must be considered when evaluating hot water treatments is that they may also be reducing fungal inoculum (CFU or colony forming units) within planting stock.