Agriculture Reference
In-Depth Information
persimmons. In a study of irrigation water along the Rio Grande, Materon and others
(2007) reported both E. coli O157:H7 and Salmonella in water used to irrigate
cantaloupe.
Kayed (2004) conducted a 2-year study of the occurrence of indicators and patho-
gens in two irrigation districts in Arizona. Although geometric means of E. coli varied
from 4 to 81 per 100 ml, depending on the sample location, levels above two million
per 100ml were detected after periods of rainfall. Samples collected from a water
irrigation district located in central Arizona yielded a variety of foodborne pathogens.
In 2.3% of the samples Cryptosporidium was detected, 55% yielded Campylobacte r,
and 18% were positive for norovirus. The source waters were obtained from recreation
lakes, but there was no intentional discharge of sewage into the source waters. In the
same canals Carpenter (2007) found greater concentrations of E. coli and longer sur-
vival of both E. coli O157:H7 and Salmonella in the canal sediment.
The likelihood of the edible parts of the plants becoming contaminated during
irrigation depends upon a number of factors, including growing location of the edible
portion of the produce (e.g., distance from the soil or water surface), frequency of
irrigation, surface of the edible portion (i.e., smooth, rough, or webbed), and type of
irrigation method (i.e., furrow or fl ood irrigation, sprinkler, or drip). If the edible part
of the crop grows in or near the soil surface, it is more likely to become contaminated
than fruit growing in the aerial parts of the plant. Some produce surfaces are furrowed
or have structures that retain water (e.g., a pepper vs. cantaloupe). There are three
types of irrigation systems: sprinkler, gravity-fl ow (furrow), and microirrigation
systems. Microirrigation systems include surface drip and subsurface drip irrigation.
The type of irrigation system greatly infl uences the degree of crop contamination that
occurs during irrigation. Stine and others (2005a,b) and Gerba and Choi (2006) com-
pared coliphage contamination of cantaloupe, iceberg lettuce, and bell peppers by
various methods of irrigation. Virus transfer to the lettuce was 4.2%, 0.02%, and
0.00039% for spray, furrow, and drip irrigation, respectively. Drip and fl ood irrigation
did result in some contamination of the cantaloupe, but not of the bell peppers. Alum
(2001) observed virus contamination of lettuce, tomato, and cucumber when high
levels of coliphage, poliovirus type 1, hepatitis A virus, and adenovirus type 40 were
added to irrigation water used to fl ood irrigate the crops. Flood irrigation of produce
with wastewater results in crop contamination with enteric bacteria (Heaton and Jones
2008; Ensink and others 2007; Okafo and others 2003). Extensive contamination of
lettuce was reported with spray irrigation of water seeded with E . coli O157:H7
(Solomon and others 2003 ).
Use of contaminated water for pesticide application may also serve as another
mechanism of produce contamination. Guan and colleagues (2005) found that
Salmonella and E. coli O157:H7 could grow in various pesticide solutions and con-
taminate tomato plants when they were applied as a spray.
Quantitative microbial risk assessment models for the use of reclaimed water show
that the risk varies with the crop, with lettuce posing a higher risk than cucumber, but
comparable to that of broccoli and cabbage (Hamilton and others 2006). The interval
between irrigation and harvest will affect the likelihood of pathogens surviving to
reach the consumer. However, some pathogens like hepatitis A can survive for weeks
on produce before harvest (Stine and others 2005b) and some salads are harvested
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