UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460



  SEQ CHAPTER \h \r 1 OFFICE OF

PREVENTION, PESTICIDES AND TOXIC SUBSTANCES

MEMORANDUM

SUBJECT:	Assessment of the Benefits of Soil Fumigation with
Chloropicrin, Methyl Bromide, and Metam-sodium on Sweet Potato
Production (DP# 337490)

FROM:	Sunil Ratnayake, Biologist

		Biological Analysis Branch

		Andrew Lee, Economist	

Economic Analysis Branch

Biological and Economic Analysis Division (7503 P)

THRU:	Arnet Jones, Chief 	

Biological Analysis Branch

Timothy Kiely, Chief 	

Economic Analysis Branch

Biological and Economic Analysis Division (7503 P)

TO:		John Leahy, Senior Policy Advisor

		Special Review and Reregistration Division (7508 P)

PRODUCT REVIEW PANEL DATE:  April 11, 2007

 Summary

This assessment describes fumigant usage on sweet potato production, as
well as the benefits to yields and grower revenues of the use of a group
of soil fumigants currently under reregistration.  Sweet potato (Ipomoea
batatas; family Convolvulaceae) is a root crop native to tropical
America and widely grown in tropical and warm temperate climates.  The
average sweet potato production in the U.S. (2004 to 2006) was 15
million cwt. and the total value of the crop was $ 261 million.  North
Carolina, Mississippi, Louisiana, and California are major sweet potato
producing states in the U.S.  Sweet potato growers use soil fumigants to
control soil pathogens, nematodes, and weeds.  Compared with California,
information on the use of fumigants in sweet potato production is
limited in other States.  In 2005, the sweet potato acreage treated with
1,3-dichloropropene in California was 53%, whereas the sweet potato
acreage treated with metam sodium, methyl bromide, and chloropicrin was
9.5, 0.1 and 0.1 %, respectively.  Depending on the type of pest
pressure in individual locations in California, applications of metam
sodium can increase sweet potato yield by up to 17%.   Based on this
information, if metam sodium were not available, the benefits of metam
sodium, in terms of net operating revenue, could be as much as $0.5
million because production on the 9.5% of  acres currently using
metam-sodium would not be viable. The higher level of 1,3-
dichloropropene usage (53%) in California indicates that the major
problem in sweet potato production in the State is the presence of soil
nematodes.  The yield loss information in sweet potato production due to
the soil pathogens, nematodes, and weeds is not available for other
States.  The Agency requests this information during the comment period
of the re-registration process.     

BACKground

As part of the Reregistration Eligibility Decision (RED) process, EPA is
assessing the risks and benefits of the use of several soil fumigants as
a group:  chloropicrin, methyl bromide, and metam (metam-potassium,
metam-sodium, and dazomet).  This document presents the assessment of
the benefits to sweet potato production provided by soil fumigants,
particularly, chloropicrin and metam sodium, fumigants currently
considered under the re-registration process.

Conceptually, the benefits of a pesticide like a soil fumigant are
comprised of the improvements in production and/or reductions in cost
resulting from the pesticide use.  The social benefits of a pesticide
are divided between the users of the pesticide, e.g., sweet potato
producers, and consumers of sweet potato products.  Consumers benefit
because higher production and/or lower costs translate into a cheaper
and more abundant supply of sweet potato products.  The impact of
fumigant regulation on this consumer benefit is not explicitly evaluated
in this document.

In evaluating the benefits of soil fumigants, this document compares the
current situation in which fumigants are available for use, subject to
existing label restrictions, to the situation that is expected to occur
when fumigants are not available.  This is somewhat different from other
BEAD assessments of the impacts of regulation, in that no specific
regulatory scheme is considered.

SWEET POTATO

Sweet potato (Ipomea batata, family; Convolvulaceae) is a starchy, root
vegetable native to the Americas (6).  It is a tropical/subtropical root
crop with varying popularity in the United States.  In the northern
States of the U.S., it is used for human consumption, whereas in the
southern states, it is used for both human food and livestock feed. 
North Carolina, Mississippi, Louisiana, and California are the major
sweet potato producing states in the United States and currently, North
Carolina produces 40% of the total U.S. annual sweet potato production
(Table 1).  The highest yields are reported in California.  The average
sweet potato farm size is about 32 acres (5). The total value of sweet
potato produced in the U.S. is approximately $ 286 million (Table 1). 

 Sweet potatoes grow best in loamy soils and the highest yields are
produced on well drained, clay-loam soils.  The sweet potato plant
produces storage roots rich in starch (2).  Sweet potatoes are
vegetatively propagated from shoots that develop from previously
harvested tubers.  Each year, farmers save a portion of their crop to
produce cuttings (slips) in the following year. Pre-sprouted sweet
potatoes are planted in seed beds (hot beds) in mid March to early April
and slips are transplanted in the field in early May.  About 400 to 500
pounds of medium size sweet potatoes are needed to obtain slips for a 1
acre field (9).  One acre of hot beds typically produces 1,000,000 slips
which are enough to plant about 60 acres.  Generally, transplanting ends
in late June and crop is harvested from late August until early November
(4).

Table 1.  SWEET POTATO PRODUCTION IN THE U.S. (2004 - 2006 AVERAGES)

State	Area harvested

1,000 acres	Yield

cwt/acre	Price1

$/cwt	Production

1,000 cwt	Value

$ million

North Carolina	39	170	14	6,617	92

Mississippi	16	170	18	2,732	49

Louisiana	15	153	17	2,339	40

California	8.52	290	25	2,465	62

Texas	3	102	14	257	4

Alabama	2	158	21	374	8

New Jersey	1	135	27	162	4

South Carolina	1	147	16	112	2

Virginia	0	123	12	45	1

USA	86	180	18	15,103	261

Source: USDA NASS. Agricultural Statistics. 2007.

12004-2005 averages.

2Computed from California Pesticide Use Report, 2005.

Use of fumiganTs 

The following section provides a summary of our understanding of the use
of fumigants in sweet potato production.  In preparing this summary, we
used data from California Pesticide Use Report, information provided in
methyl bromide critical use nomination packages (5), and data obtained
from CropLife Foundation (2), and EPA proprietary pesticide use data
(3).  

In the United States, California has the highest fumigant usage in sweet
potato production.  Therefore, data collected from California were used
in this assessment.  Pest problems found in California may not be
exactly similar to other States.  While fumigants are used in South
Carolina and Texas BEAD did not have sufficient technical and economic
information to calculate the value of fumigant usage in those states.  

Table 2.  Fumigant Usage in Sweet potato production

State	Active Ingredient	% Acres Treated	Acres Treated	Rate (lb/ac)
Pounds Applied (ai)

California1	Chloropicrin	0.1	5	149	749

	1,3 Dichloropropene	53	4,511	119	538,490

	Metam Sodium	9.5	806	305	240,559

	Methyl Bromide	0.1	5	200	1,002

South Carolina2	1,3 Dichloropropene	40	338	48	16,262

Texas2	1,3 Dichloropropene	10	219	48	10,521

	Chloropicrin	10	219	10	2,192

1California Pesticide Use Report, 2005.  Field and slip production.  

2National Pesticide Use Database; CropLife Foundation, 2002

In 2005, in California, sweet potato was fumigated with
1,3-dichloropropene, chloropicrin, methyl bromide, and metam sodium
(Table 2).  1,3-dichloropropene was most widely used with 53% of the
acres treated (4,511acres), with a total of 538,490 lbs.  California has
established township caps regulating the maximum amount of
1,3-dichloropropene that can be applied per year in a township (5).  The
usage of metam sodium is second to 1,3-dichloropropene with 9.5% acres
treated (806 acres) with a total of 240,559 pounds.   In California, use
of chloropicrin and methyl bromide (0.1% crop treated each) is
insignificant compared with other fumigants.  During sweet potato
production fumigants are applied at 2 occasions.  The first is in the
nursery beds (hot beds), where rooted cuttings were produced and the
second is in the field where cuttings (slips) are to be transplanted. 

Slips production

Preheating of seed roots is useful for more rapid slip (transplant)
production.  Preheating is accomplished by raising the temperature in
seed storage to 85o F with 85 to 95% relative humidity for two weeks. 
Before planting sweet potatoes for slip production roots are carefully
examined and diseased, mutated, and bruised roots must be discarded. 
Treating seed roots with a recommended fungicide before bedding is also
important to avoid infestations with black rot, scurf, and root rot. In
addition, most growers fumigate nursery beds (hot beds) with
1,3-dichloropropene predominantly to control soil nematodes (2).  Once
the nursery beds are prepared soil fumigants are shank injected to the
soil at recommended rates and beds are covered with polyethylene to
increase the efficacy and to reduce the emission of fumigants to the
environment.  

Application Rate 

(lb ai/acre)	Chloropicrin 	80 pounds per acre (chloropicrin alone)

	1,3 – Dichloropropene 	90  pounds per acre

	Metam Sodium	90 to  252 pounds per acre

Acres Treated per Day	1 to 10 acres per tractor

Time of Fumigation	 Nov – Jan.

Application Method	Primarily shank injected under tarp

Method of Application	Primarily strip 

Tarps or Water Caps	HDPE tarp

Target Pests	Nematodes: M. incognita, 

Fungi: Pox (Streptomces ipomea) Scurf (Monilochaetes infuscans);
Fusarium (Fusarium oxisporum) Black rot (Ceratocystis fumbriata)



TABLE 3.  FUMIGANT APPLICATION IN SWEET POTATO SLIP PRODUCTION

Source: (5)

Rates, time of fumigation, and type of fumigants applied and other
information related to use of soil fumigants in sweet potato production
is shown in Table 3.  Chloropicrin, 1,3-dichloropropene, and metam
sodium are available for sweet potato growers to control soil pathogens.
Targeted disease causing soil pathogens are Pox (causal agent;
Streptomces ipomea), Scurf (causal agent; Monilochaetes infuscans), 
Fusarium (causal agent; Fusarium oxisporum), and Black rot (causal
agent; Ceratocystis fumbriata ) and soil nematodes (M. incognita) (Table
3).  Major fumigants used and their chemical characteristics are
described in Table 4.

TABLE 4. CHARACTERISTICS OF ALTERNATIVE FUMIGANTS 

Chemical	Characteristics

1, 3-dichloropropene (Telone II)	Effective against nematodes less
effective against fungal plant pathogens. 

Metam sodium	Effective against fungal pathogens, but needs a longer
aeration period.

Chloropicrin	Effective against many soil pathogens, not effective
against nematodes when applied alone.

1,3-dichloropropene + Chloropicrin	Effective against nematodes and
fungal pathogens.

Source (5)

Field Production

Early planting is an important factor responsible for a higher
marketable sweet potato yield (7). Sweet potato is subject to a number
of diseases that can cause heavy yield losses.  Soil fumigants are
applied in sweet potato fields mainly to control soil pathogens and
nematodes.  The most common diseases affecting the field production of
sweet potato are Pox, Scurf, Fusarium and Black Rot.  Causal agents of
these diseases are Streptomces ipomea, Monilochaetes infuscans, Fusarium
oxisporum, and Ceratocystis fumbriata, respectively (Table 3).  Once
production fields are prepared, soil fumigants are shank injected to the
soil at recommended rates and beds are covered with polyethylene to
increase the efficacy and to reduce the emission of fumigants to the
environment.  

benefits of fumigation

In California, of the total acreage treated with fumigants, only 0.1
percent was treated with chloropicrin.  According to the available usage
information in California, 53% of harvested sweet potato acreage was
fumigated with 1, 3-dichloropropene.  Therefore, the presence of
nematodes seems to be the major problem in California sweet potato
production and the fumigant 1,3-dichloropropene is available for the
California growers to control the soil nematodes.  However, California
has established township caps regulating the maximum amount of
1,3-dichloropropene that can be applied in townships (5).  In 2005, 9.5%
of the sweet potato acreage was treated with metam sodium.  Based on
usage information it appears that these are fields with low to moderate
levels of soil pathogens. 

Non Chemical Alternatives

Solarization of soil: Consistent effectiveness is not achieved by this
method and effectiveness depends upon meteorological conditions. 
Generally, an adequate sunlight condition does not exist prior to
seedbed preparation. Therefore, solarization must be done in the
previous summer or fall.

Steam sterilization of soil: Not readily available for farm use and the
equipment are expensive.

Crop rotation/fallow:  Strongly recommended and usually practiced. 
However, crop rotation alone is not adequate to control the fungal
pathogens. 

Physical removal/sanitation: Not effective and completely disease free
beds are required for a successful crop production.

Resistant cultivars: Already used, but not sufficient enough to control
all the soil pathogens found in field conditions. 

The effect on Yield  

Information on yield losses in sweet potato slip production is not
available in the literature.  Field research conducted in California has
reported a 16.7 and 18.6 % yield increase with the use of metam sodium
(Vapam at 75gallons/acre rate) and 1,3-dichloropropene (at 15
gallons/acre rate), respectively (8).  However, this research was
conducted only at a single location and observed results may not
represent all the sweet potato producing fields in the State.  The
higher yield obtained with 1,3-dichloropropene may be attributed to a
high level of nematode pressure existed at this location.  Sweet potato
growers with similar field conditions can use 1,3-dichloropropene as an
alternative to metam sodium to avoid any yield losses.  However,
fumigant usage information in California (Table 2) indicates that about
9.5% of the sweet potato acreage is treated with metam sodium.  This
fumigant is more effective on soil pathogens than 1,3-dichloropene (5)
and the presence of high levels of soil pathogens could be the reason
for using metam sodium in these fields.  If metam sodium is not
available for the growers in these fields, BEAD assumes that the
estimated 17% increase in yield from metam sodium would be lost.  

Currently, metam sodium (9.5% acres treated) and 1,3- dichloropropene
(53% acres treated) are the main fumigants used on sweet potato
production in California (Table 2).  Metam sodium is used to control
soil pathogens as well as nematodes and 1,3- dichloropropene is used to
control nematodes.  Percent crop treated data suggest that, in
California, the nematode problem appears to be more widespread than the
problem in soil pathogens.  

Economic Assessment

TABLE 5.   PER ACRE GROSS REVENUE, TOTAL OPERATING COSTS, AND NET
OPERATING REVENUES, METAM SODIUM AND ALTERNATIVE,IN CALIFORNIA SWEET
POTATO PRODUCTION IN 2006.

 

 	Baseline.	Alternative.

	Both metam sodium	Metam sodium unavailable.

	& 1,3,D available	Use 1,3,D

Yield (cwt/acre)	342	285

Price, $/cwt	$30	$30

Gross Revenue  	$10,260	$8,547

Estimated Yield Loss*	0%	16.7%

Cultural Costs	3,656	3,656

Harvest Costs  	5,390	5,390

Fumigation Costs	175	175

Other Costs	477	477

Total Operating Costs  	9,698	9,698

Net Operating Revenue  	$562	($1,151)

Percentage Change in Net Operating Revenue	0%	-305%



Source: Department of Agricultural and Resource Economics, University of
California, Davis, 2006.

*Estimated yield loss, and thus percent change in net operating revenue,
applies to only 9.5% of the 

California sweet potato acreage.

Table 5 shows the per acre partial budget for sweet potato production
(transplant, hotbeds, and field production) in the San Joaquin Valley of
California for 2006 (10).  In Table 5, the crop quality, thus the price,
is assumed to remain the same under treatment by the alternative, 1,3-
dichloropropene.  Further, due to limited data, the same fumigation
costs were used for both metam sodium and 1,3- dichloropropene.  The
analysis indicates that net operating revenue is much more sensitive to
the changes in yield loss than fumigation costs.  Other operating costs
(harvesting costs, etc) are assumed the same under treatment by 1,3-
dichloropropene.  Using the information in Table 5 and a yield loss
estimate of 16.7%, a 305% decrease in net operating revenue may result
on the 9.5% of the California sweet potato acreage currently treated
with metam sodium if it becomes unavailable, and 1,3-dichloropropene is
used as an alternative.  

National Benefits

As shown in Table 1, California is one of the four largest sweet potato
producing states in the U.S., particularly in the quantity (16% of the
U.S. total) and value (24% of the U.S. total) of production.  We do not
have the fumigant usage data for other states and therefore cannot
assess the benefits of the fumigants at the national level.

CONCLUSIONS

There are over 89,000 acres of sweet potato harvested in the U.S.
annually.  The average production and value of the crop were 15 million
cwt and $261 millions, respectively (2004-2006).  North Carolina,
Mississippi, Louisiana, and California are major sweet potato producing
states in the U.S.  Sweet potato growers use soil fumigants mainly to
control soil pathogens, and nematodes.  Compared with California,
information on the use of fumigants in sweet potato production is
limited in other States.  In 2005, the sweet potato acreage treated with
1,3-dichloropropene in California was 53%, whereas the sweet potato
acreage treated with metam sodium, methyl bromide, and chloropicrin was
9.5, 0.1 and 0.1 %, respectively.  Depending on the type of pest
pressure in individual locations applications of metam sodium can
increase sweet potato yield by up to 17% in California.   Based on this
information, if metam sodium were not available, the benefits of metam
sodium, in terms of net operating revenue, could be as much as $0.5
million because production on the 9.5% of  acres currently using
metam-sodium would not be viable.

Limitations to this assessment

This document presents the assessment of the benefits provided by the
soil fumigants in the sweet potato production.  The following are
limitations of this analysis:

Compared with other sweet potato producing States, California has the
highest fumigant usage in sweet potato production.  In this assessment,
due to lack of information from other states the benefits of fumigant
use is not assessed in other sweet producing states.  Pest problems and
pest pressure found in California may not be exactly similar to other
states.  Therefore, BEAD requests the information listed in the Appendix
1 during the comment period.  

  

Social benefits of pesticide use accrue to several groups, e.g., sweet
potato producers and consumers of sweet potato.  This document considers
the benefits of fumigant use from the perspective of sweet potato
producers, and does not account for economic impacts on other parties.

The assessment is based on partial budget analysis and does not account
for price or income distribution effects resulting from changes in
supply. 

REFERENCES

1)	California Pesticide Use Report, 2005.	

2)	Crop profile for sweet potato in North Carolina.  1999.   HYPERLINK
"http://www.ipmcenters.org/cropprofiles/docs/ncsweetpotatoes.html" 
http://www.ipmcenters.org/cropprofiles/docs/ncsweetpotatoes.html 

3)	EPA Proprietary Data (2001-2005).

4)	May, D. and B. Scheuerman.  1998.  Sweet potato production in
California.  Univ. of

California Division of Agriculture and Natural Resources, Publication
7237.

  HYPERLINK "	http://anrcatalog.ucdavis.edu/pdf/7237.pdf"  
http://anrcatalog.ucdavis.edu/pdf/7237.pdf .

5)		Methyl Bromide Critical Use Exemption. 2005. (Information obtained
from Sweet Potato Council of California) United States Environment
Protection Agency.

6)		Peet, M.  2001. Crop Profiles-Sweet Potato.  Sustainable Practices
for Vegetable Production in the South.    HYPERLINK
"http://www.cals.ncsu.edu/sustainable/peet/profiles/c18swpot.html" 
http://www.cals.ncsu.edu/sustainable/peet/profiles/c18swpot.html .

7)		Publication no. F-6022.  Sweet Potato Production.  Oklahoma
Cooperative Extension Service

  HYPERLINK "http://www.osuextra.com"  http://www.osuextra.com .

8)		Sweet Potato Tips.  2003.  University of California Cooperative
Extension Service   Publication.  2145 Wardrobe Ave., Merced, CA 95340

9) 	Sweet Potato Production in California.  1998.  Publication no.7237.

  HYPERLINK "http://anrcatalog.ucdavis.edu/pdf/7237.pdf" 
http://anrcatalog.ucdavis.edu/pdf/7237.pdf .

10)	Sweet Potato Cost and Return Studies in San Joaquin Valley,
California. 2006.

Department of Agricultural and Resource Economics, University of
California, Davis, CA. 

11)	USDA NASS.  2002-2006.  Agricultural Prices, Summary.  National
Agricultural Statistics Service, U.S. Department of Agriculture, July,
at   HYPERLINK
"http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID
=1003" 
http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=
1003 .

	

12)		USDA NASS.  2002-2006. Crop Production, Summary.  National
Agricultural Statistics Service, U.S. Department of Agriculture,
January, at
http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=
1047.

Appendix 1.  Information That Would Help With Future Assessments

 

1) 	Information on sweet potato yield losses with or without fumigants
due to soil pathogens, nematodes, or weeds from sweet potato producing
States.

 

2) 	Information on severity of soil pathogens and nematodes in sweet
potato producing States.

 

3)    	Research information on the use of metam sodium and other
chemicals as potential soil fumigants from major sweet potato producing
States.

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