U.S.EPA
Methyl Bromide Alternative Case Study
Part of EPA 430-R-96-021, 10 Case Studies, Volume 2
December 1996

Using Nematode Resistant Cultivars As An Alternative to Methyl Bromide for Selected Crops

Nematode resistant cultivars can be used as part of an integrated approach to develop an effective alternative to methyl bromide against a wide variety of nematodes for a wide variety of crops, particularly high value vegetable and fruit commodities. Crops for which both nematode resistant cultivars have been developed and for which methyl bromide has been used include both seed crops (tomatoes, bell and hot peppers, and tobacco) and vineyard/orchard crops (grapes, peaches, plums, apricots and nectarines, walnuts, almonds, and citrus) (Slaughter 1996, Fortnum 1996, Noling 1996, McKenry and Kretsch 1995, Thies et al. 1995, Khan and Khan 1991, Lehman and Cochran 1991, Cook and Evans 1987).

Nematode resistant cultivars have a number of distinct non-commodity-specific advantages over the use of methyl bromide, including 1) complete prevention of nematode reproduction, 2) no requirements for special application techniques or equipment, and 3) comparable costs to non-resistant cultivars. Nematode resistant cultivars are particularly effective because they can be used in conjunction with other pest control practices (i.e., sanitation, soil solarization, soil amendments (compost and manure), biological control, crop rotation, and early planting scheduling to reduce or eliminate nematode infestation (Dunn 1993, Lehman and Cochran 1991, Cook and Evans 1987).

Nematodes

Nematodes (microscopic unsegmented roundworms) are defined as any member of the Phylum Nematoda, including those that are parasitic to plants and animals. Plant parasitic nematodes are extremely common in soil, where they live primarily in a film of water which surrounds soil particles. A single gram of top soil can contain over one thousand of these tiny organisms. Nematodes generally interfere with water/nutrient availability and a plant's feeding mechanisms (i.e., root function and plant growth processes) (Ashwroth 1991). Many species of nematodes exist, and they attack an enormous variety of plant species. The most common are the root knot nematodes, Meloidogyne arenaria, Meloidogyne incognita, Meloidogyne javanica, and Meloidogyne hapla. Root knot nematodes alone have a host range of over 2,000 plants (McKenry and Roberts 1985).

The response of plants to nematode infestation varies considerably according to the species of nematode/ plant and environmental conditions such as host status, soil temperature/moisture/structure, aeration, organic matter, fertility level, nutrients, nematode predators/parasites, etc. (Gaur and Seshadri 1986). Symptoms of nematode-damaged plants are generally non-specific and are characterized by poor growth, plant stunting (through the presence of root galls), crop patchiness, wilting, and chlorosis (yellowing or discoloration of leaves). As a result, preliminary examination of the crop usually does not provide unequivocal diagnosis of nematode damage. For example, plant symptoms indicative of nematode infestation can also result from other variables, such as low or excess fertilizer, low water holding capacity, or poor drainage of soil. Furthermore, plant symptoms in the field may be widespread or patchy, depending on differences in the nematode densities in the field or the placement of infested planting stocks (Lehman and Cochran 1991, McKenry and Roberts 1985). Plants stunted or diseased by nematode related activity may not die, but are likely to produce reduced yields (Hauge and Gowen 1987, McKenry 1987). Plant deaths are generally not attributed directly to nematode damage, but instead to secondary pathogens (i.e., fungi and bacteria), which invade plants weakened by nematodes (McKenry and Roberts 1985).

Outlook for Nematode Resistant Cultivars as a Replacement for Methyl Bromide

Plant breeders have developed nematode resistant cultivars in an effort to prevent the stunted growth and deformed or galled roots of plants commonly infested with nematodes. Cultivars are defined as cultivated plants which are produced by breeding programs and are distinguished by characteristics significant for agriculture, forestry, or horticulture, and which, when reproduced, retain their distinguishing characteristics (Lehman and Cochran 1991, Cook and Evans 1987). Level of resistance describes the effect of the host on nematode reproduction. A completely resistant plant allows no nematode reproduction, non-resistant or susceptible plant allows nematodes to multiply freely, and partially resistant plants support intermediate levels of reproduction (Cook and Evans 1987). Nematode resistant cultivars are plants bred specifically to inhibit nematode reproduction and resist the impact of nematodes on plant growth and production, while nematode resistant vineyard and orchard crops also can be developed by grafting high yield cultivars to resistant rootstocks (Titts 1996). Ideally nematode resistance cultivars are bred for both resistance (suppressed nematode reproduction) and tolerance (nematode feeding will have little impact on plant growth and crop yield).

Resistant cultivars are already widely used for specific crops in the United States, particularly in California and Florida. No fruit or vegetable nematode resistant cultivars are resistant to all nematodes; however, many have resistance to the most common nematodes, and often in combination with resistance to one or more other pathogens. If they are otherwise suitable, nematode resistant cultivars are typically planted when no nematicide is used, but are desirable even when other chemical treatments are used (Dunn 1993). Crops where pre-plant fumigation with methyl bromide is used and nematode resistance cultivars developed include both seed crops (tomatoes, bell and hot peppers, and tobacco) and vineyard/orchard crops (grapes, peaches, plums, apricots, nectarines, walnuts, almonds, and citrus). Crops frequently fumigated with methyl bromide for which there are no nematode resistant cultivars in significant commercial use to date include: eggplant, cucurbits, carrots, broccoli, cauliflower, melons, and strawberries (Becker 1996). However, currently there are significant research efforts underway to develop nematode resistant cultivars for many of these crops. For example, Scientists at North Carolina State University have tested five cultivars of cucumber for resistance to root knot nematodes, these cultivars (C. metuliferus and ‘Sumter') account for approximately 12% of the cucumber crop grown annually in North Carolina. Preliminary results indicate that the cultivars vary in their resistance to the four root-knot nematode species (Wehner et al. 1991). Likewise, Canadian scientists are conducting research on the resistance and tolerance of strawberry cultivars to the lesion nematode, Pratylenchus penetrans (Potter 1995).

Costs

Currently, a large percentage of crop production where nematode resistance has been commercialized utilizes nematode resistant cultivars (e.g., up to 90 percent for seed crops, 95-100 percent for orchard crops, and 70-85 percent for vineyards), often in conjunction with pre-plant methyl bromide fumigation. For these crops, gains in plant vigor and yield have been achieved through use of resistant cultivars. Applications of methyl bromide are utilized in order to protect the crop from competition from weeds, diseases from fungi, and damage caused by non-susceptible nematode species. For crops where nematode resistant technology is currently not available, development and commercialization of resistant cultivars, as part of an integrated system utilizing substitute fumigants (e.g., metam sodium) or non-chemical alternatives (e.g., solarization), may enable growers to achieve yields currently realized under production systems utilizing methyl bromide fumigation. It should be noted that other pests, such as weeds, will need to be managed on an as needed basis, which may add costs to both resistant and non-resistant crop production.

Production costs under a system that uses nematode resistant cultivars in conjunction with an alternative fumigant can be compared to a system that utilizes methyl bromide with no crop resistance (table 1). A comparison of these cost estimates is provided in Table 1. Although the cost of resistant cultivars may be slightly higher, yield increases and lower fumigant treatment costs may help to offset these increases. Furthermore, the costs of nematode resistant cultivars are expected to decrease in the future as a wider variety of cultivars become available on the commercial market. In addition, there are other financial benefits of nematode resistant cultivars including cost savings from growing crops on land most suited to their production (rather than letting the presence of nematodes in a field be the deciding factor for which crops to grow). Finally, through breeding for resistance, seed producers are also able to combine resistance with other traits including better marketability, longer shelf life, and increased yields. (Seals 1996, McKenry 1996, Slaughter 1996, Titts 1996, Emershad 1996, Cook and Evans 1987, Cotton 1996).

More research dedicated to the future development of nematode resistant cultivars is needed. The successes in the development of nematode resistant cultivars discussed earlier in this document are promising; however, more varied nematode resistant cultivars must be developed in order for this practice to develop into a broadly applicable alternative to methyl bromide.


Table 1. Comparative Costs of Resistant versus Non-Resistant Cultivars
Cost Factors($/acre) Alternative Fumigant-Resistant Cultivar Methyl Bromide-NonResistant Cultivar
Treatment Cost 750-1,000 1,200-1,500
Cultivar Costs 50-1,000 10-40
Total 1,050-1,750 1,240-1,510
Notes:
Alternative fumigant is metam sodium (Vapam®).
Assumed 1 pound of seed planted per acre.
Based on a hybrid cost range of $50-75 for cucurbits and $500-1000 for tomatoes and eggplants; and a open pollinated cost range of $10-40 for cucurbits, tomatoes, and eggplants.
Sources: Seals 1996, McKenry 1996, Slaughter 1996, Titts 1996, Emershad 1996 , Noling 1996, VanSickle 1996, Cotton 1996.

References

Ashworth, William. 1991. The Encyclopedia of Environmental Studies. New York, Fact on File Publishers.

Becker 1996 (August). Personal Communication. O. Becker. University of California at Riverside. Riverside, California.

Cook and Evans 1987. "Resistance and Tolerance." R. Cook and K. Evans. In Principles and Practice of Nematode Control in Crops. Edited by R.H. Brown and B.R. Kerry. Academic Press.

Cotton 1996 (August). Personal Communication. D. Cotton. Seedway, Inc. Elizabeth, PA.

Dunn, 1993. Managing Nematodes in the Home Garden. Robert A. Dunn. Publication of the Florida Cooperative Extension Service.

Emershad 1996 (August). Personal Communication. Rick Emershad. USDA Plant Breeding Station. Fresno, CA.

Fortnum 1996 (August). Personal Communication. Bruce Fortnum. Dee Dee Research and Education Center. Blackville, South Carolina.

Gaur and Seshadri 1986. Ecological Control in Evolving Strategies for Nematode Control in 2000 AD. H.S. Gaur and A.R. Seshadri. Proc. Indian Natn. Sci. Acad. Volume G52, Number 1, pp. 49-65.

Hague and Gowen 1987. "Chemical Control of Nematodes." In Principles and Practice of Nematode Control in Crops. N.G.M. Hague and S.R. Gowen. Edited by R.H. Brown and B.R. Kerry. Academic Press.

Khan and Khan 1991. "Response of Tomato Cultigens to Meloidogyne javanica and Races of Meloidogyne incognita." A.A. Khan and M.W. Khan. In Supplement to Journal of Nematology, Volume 23.

Lehman and Cochran 1991. How to Use Resistant Vegetable Cultivars to Control Root-Knot Nematodes in Home Gardens. P.S. Lehman and C. Cochran. Publication of the Florida Department of Agricultural and Consumer Services. Division of Plant Industry.

McKenry 1996 (August). Personal Communication. M.V. McKenry. University of California. Kearney Agricultural Center. Parlier, CA.

McKenry 1987. "Control Strategies in High Volume Crops." M.K. McKenry. In Principles and Practice of Nematode Control in Crops. Edited by R.H. Brown and B.R. Kerry. Academic Press.

McKenry and Kretsch 1995. "It is a long road from the finding of a new rootstock to the replacement of a soil fumigant." M.V. McKenry and J.O. Kretsch. In Proceedings of the 1995 International Conference on Methyl Bromide Alternatives and Emissions Reductions. San Diego, CA.

McKenry and Roberts 1985. Phytonematology Study Guide. M.V. McKenry and P.A. Roberts. Cooperative Extension University of California. Division of Agriculture and Natural Resources. Publication 4045.

Noling 1996 (August). Personal Communication. John Noling. Florida Extension Service, Citrus Research Center. Florida.

Potter 1996. Resistance and tolerance of strawberry cultivars to Pratylenchus penetrans in Ontario. John Potter. Journal of Nematodology. Volume 27, Number 4, pp. 490-528.

Seals 1996 (August). Personal Communication. Joe Seals. W. Atlee Burpee and Company. Warminster, PA.

Slaughter 1996 (August). Personal Communication. John Slaughter. Burchells Nursery. Madera, California.

Thies 1995. "Effectiveness of resistance to the southern root-knot nematode (Meloidogyne incognita) in pepper (Capsicum annuum)." J.A. Thies, R.L. Fery, and J.D. Mueller. In Proceedings of the 1995 International Conference on Methyl Bromide Alternatives and Emissions Reductions. San Diego, CA.

Titts 1996 (August). Personal Communication. Margueriette Titts. Geno's. Medera, California.

VanSickle 1996 (August). Personal Communication. John Van Sickle. University of Florida. Gainseville, Florida.

Wehner 1991. "Resistance to Root-Knot Nematodes in Cucumber and Horned Cucumber." Todd C. Wehner, et al. In Supplement to Journal of Nematology, Vol. 23, 1991.

Please note that this publication discusses specific proprietary products and pest control methods. Some of these alternatives are now commercially available, while others are in an advanced stage of development. In all cases, the information presented does not constitute a recommendation or an endorsement of these products or methods by the Environmental Protection Agency (EPA) or other involved parties. Neither should the absence of an item or pest control method necessarily be interpreted as EPA disapproval.

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