Many bacteria, fungi, insects, mites, and nematodes can be controlled by briefly exposing infested plants to aerated steam, dry heat, or hot water as listed in the table Pests Controlled by Heat Treatment of Plants. Certain viruses can be eliminated from cuttings by heat treating mother plants before propagation from meristem tissue. Heat-treating seeds is commonly used to eliminate Xanthomonas bacteria and certain other pathogens. Immersing bulbs, corms, cuttings, rhizomes, roots, seeds, or other parts in hot water before planting or storage can disinfect crops such as amaryllis, daffodil, calla, gladiolus, iris, lily, palm, rose, statice, tulip, and zinnia.
The temperature and time needed to provide control depend on the crop and pests:
- Insufficient temperatures or too short an exposure may not kill the pests.
- Too high a temperature or too long an exposure can damage plants.
- Inadequate equipment or technique, such as failing to maintain the proper temperature consistently throughout the treated batch and specified time can cause variation within a treated batch, resulting in some plants heat-damaged, others inadequately disinfested of pests, only a portion of the plants properly treated, or all three outcomes together.
Conduct on-site experiments with small batches, consult an expert, or both before deciding how to perform any large-scale, heat treatments:
- Specify and record the methods for each particular situation.
- Start small and expose portions of infested crops to those specific treatments carefully controlled, then observe plants for control efficacy and potential heat damage.
- Adjust methods if warranted, then retest small batches.
- After obtaining satisfactory results, treat larger numbers of those same crops and varieties.
- If alternative control methods are available, consider applying those to a different portion of the same crop that is being heat treated and compare the results.
A general recommendation is to presoak bulbs, corms, or rhizomes for 2 to 3 hours or overnight in 75°F (24°C) water containing a wetting agent before immersing them in 111°F (44°C) water for about 1-1/2 hours. Cool plants immediately afterward with clean, cold water; dry them thoroughly in warm air or sunshine; then store them under cool, low-humidity conditions until plants are used.
Each pest species and crop cultivar should be tested for susceptibility on a small scale before treating large portions of the crop. An ice chest with an electric immersion heater may be sufficient for small volume treatments. Commercial treatment can be done in larger tanks into which stock is lowered by a forklift or hoist, treated for a specified period of time, and then removed, cooled, and dried. Or stock can be placed onto a conveyer belt that moves through uniformly hot air, aerated steam, or hot water at a calibrated speed.
Consider using a pretreatment to partially raise the planting stock to the required temperature, especially if stock is taken from cold storage or cold soil shortly before treatment. The volume of hot air or water in the primary tank should be large enough to prevent a significant drop in temperature when the stock is added. The shorter the treatment time, the more critical it is to quickly reach and maintain the required temperature. Depending on how quickly stock must be cooled after treatment, an additional cool-water tank or facilities for hosing down treated stock with clean, cool water may be needed.
Accurate time and temperature controls are critical. Use a circulating fan or pump or other method to continually mix the treatment air or water to assure uniformity within the tank. Monitor temperature uniformity for each treatment using several, properly calibrated thermometers in the treatment tank. In California, the local office of the county agricultural commissioner may be able to provide information on calibration of thermometers.
| Crop | Pest common name | Pest scientific name | Treatment suggestions |
|---|---|---|---|
| amaryllis, daffodil, gladiolus, iris, lily, tulip, and some other bulbs, corms, and rhizomes during storage | bulb flies bulb mites bulb scale mites gladiolus thrips lily bulb thrips stem and bulb nematodes tulip bulb aphid |
Eumerus and Merodon spp. Rhizoglyphus spp. Steneotarsonemus laticeps Thrips simplex Liothrips vaneeckei Ditylenchus spp. Dysaphis tulipae |
Presoak bulbs, corms, or rhizomes for 2 to 3 hr or overnight in 75°F (24°C) water containing a wetting agent. Immerse in 111°F (44°C) water for 1-1/2 hr. Cool plants immediately afterwards with clean, cold water, then dry thoroughly in warm air or sunshine. Store under cool, low-humidity conditions. |
| caladium tubers, gladiolus corms, iris rhizomes | crown rot, southern wilt | Sclerotium rolfsii | Immerse corms, rhizomes, or tubers in 122°F (50°C) water for 30 min plus the time necessary for plants to reach this temperature. Cool plants immediately afterward with clean, cold water. Dry thoroughly in warm air or sunshine. |
| calla | mosaic | Dasheen mosaic virus | Heat to 150°F (66°C) for 10 min. Then culture tissue. |
| daffodil bulbs | basal rot crown rot scorch stem and bulb nematodes |
Fusarium oxysporum f. sp. narcissi Sclerotium rolfsii Stagonospora curtisii Ditylenchus dipsaci |
Store bulbs at 60° to 64°F prior to treatment to reduce heat injury. Presoak for 2 to 3 hr or overnight in 75°F (24°C) water plus a wetting agent. Maintain water at 109° to 111°F (43° to 44°C) for 3 to 4 hr after temperature reaches 109°F. Cool and dry bulbs immediately. |
| Easter lily, heat-tolerant cuttings of other hosts | foliar nematodes | Aphelenchoides fragariae, A. ritzemabosi | Dip bulblets of Lilium longiflorum in 125°F (52°C) water for 10 minutes before planting. Dip cuttings of other heat-tolerant hosts in hot water at 122°F (50°C) for 5 minutes or at 111°F (44°C) for 30 minutes. Cool and dry tissues immediately after heat treatment. |
| gladiolus corms | blue mold, corm rot Fusarium yellows neck rot, corm disease dry rot |
Penicillium spp. Fusarium oxysporum f. sp. gladioli Botrytis cinerea, B. gladiolorum Stromatinia gladioli |
Cure corms immediately after digging by storing them in shallow trays at 95°F (35°C) and 80% relative humidity (RH). Use fans to provide continuous air circulation through trays and around corms. When old corms break off easily, usually after 6 to 8 days, break off and clean the remaining new corms. Cure new corms 4 days or longer at 95°F, 80% RH. Treat the corms with hot water: select sound, hard, fully dormant cormels grown in warm soil and harvested before cold weather. Cure corms as above. Presoak cormels for 2 days in 60° to 80°F (16° to 27°C) water. Discard any corms that float. Immerse corms 30 min in 131°F (55°C) water. Cool corms immediately afterward with clean, cold water. Dry corms thoroughly in warm air or sunshine. Apply fungicide. Store corms at 40°F (4°C), 70 to 80% RH. |
| grape cuttings | vine mealybug | Planococcus ficus | Dip dormant stock (grape cuttings) for 5 min in each of three water tanks in sequence: preheating 86 +/-5°F (30 +/- 3°C), hot-water 127 +/- 0.6°F (52.8 +/- 0.3°C), and cooling 73.4 +/-5°F (23 +/- 3°C). |
| iris bulbs | blue mold | Penicillium spp. | Within 5 days of digging, cure bulbs for several days in shallow trays at 95°F (35°C) and 80% RH. Store under cool, lower-humidity conditions. Avoid injuring bulbs and do not dig them too early or too late. |
| iris bulbs | bulb nematodes | Ditylenchus destructor | Harvest bulbs 7 to 10 days early. Immerse them in 110°F (44°C) for 3 hr, then cool and dry promptly. Store under cool, dry conditions. |
| palms, potted | ground mealybugs | Rhizoecus spp. | Submerge pots in 120°F (49°C) until the internal temperature of the rootball reaches 115°F (46°C). |
| rose | mosaic rose ring pattern |
Prunus necrotic ringspot virus unknown; probably a virus |
Hold mother plants at 100°F (38°C) for 4 weeks; virus will be inactive in 99% of the cuttings taken after this heat treatment. |
| seeds of statice, stock, zinnia and others | bacterial blight | Xanthomonas campestris | Soak seed in 122° to 131°F (50° to 55°C) water for 10 min to disinfect seed surface or apply aerated steam to seed for somewhat longer. Cool seeds rapidly after heat treatment. |
| ranunculus seed | bacterial blight | Xanthomonas campestris | Soak seed in a solution of 1 part chlorine bleach and 9 parts clean water for 30 min at room temperature or at 122°F (50°C) for 15 min. Cool seeds rapidly after heat treatment. |
| many hosts | cyclamen mite broad mite, and possibly other tarsonemids |
Phytonemus pallidus Polyphagotarsonemus latus |
Thoroughly immerse plants in 111°F (44°C) water for 30 min or hold plants at 100% humidity and 111°F for 11 hours. First test some plants before treating the entire crop to determine if they will tolerate these conditions, then carefully inspect treated plants under magnification to ensure the method is effective. |
Heat can injure plant parts, causing stunting, deformation, or flower drop. Before any large-scale treatments, consult an expert and test portions of each crop and variety for control efficacy and crop tolerance to heat.
Adapted from Integrated Pest Management for Floriculture and Nurseries. 2001. UC ANR Publ. 3402. Oakland, CA
MORE INFORMATION
- Brief Exposure to Dry Heat or Hot Water Can Disinfest Many Crops of Insects. 2013. Bethke JA. UCCE San Diego.
- Hot-Water Treatments for Control of Planococcus ficus (Homoptera: Pseudococcidae) on Dormant Grape Cuttings (PDF). 2005. Haviland DR, Bentley WJ, Daane KM. J. Econ. Entomol. 98(4):1109–1115.
- Keeping the Heat on Pests: Using a Hot Water Immersion System Effectively Can Control Certain Plant Pests During Propagation. 2007. Gill S, Schuster C, Ross D, Rosenkranz G, Shrewsbury P, Klick S. American Nurseryman 205(1):22–32.
- Root Mealybugs: Preventing the Serious Pests. Hara AH. undated. University of Hawaii-Manoa, Hilo.
- Systems Approach to Quarantine Treatments for Export Ornamentals (PDF). 2011. Hara AH. University of Hawaii-Manoa, Hilo.
- The U. C. System for Producing Healthy Container-Grown Plants. 1957. Baker KF, ed. Univ. Calif. Div. Agric. Sci. Exp. Sta. Manual 23. Oakland, CA. Reprinted 1985 by the Australian Association of Nurserymen.
Note that heat at higher temperatures can also be used to disinfect heat-tolerant containers, equipment, tools, and other materials and to pasteurize growing media. For example steam treat planting beds or otherwise pasteurize growing media to the recommended pasteurization temperature before planting or immediately after removing any pest-infested crop to kill pests before replanting beds. Steam is difficult to use in field soils but can be applied using a plowlike steam rake to raise the topsoil temperature to levels sufficient to kill most pests.