Agriculture Reference
In-Depth Information
well in the tomato phyllosphere of plants from seeds inoculated with the tomato plant
pathogen,
Xanthomonas campestris
pv. vesicatoria and planted in low
Salmonella
inoculum soil, indicating the potential importance of debris, plant disease, and fallow
periods in the preharvest produce production cycle (Barak and Liang 2008). Thus,
breakdown of tomato crop debris by plant pathogens may enhance the conditions for
even better survival or growth of a human pathogen (Barak and Liang 2008; Brandl
2006; Brandl and Amundson 2008). Pathogen reservoirs where tenfold or more growth
of pathogen may occur are critical risk factors relevant to food contamination. High-
shedding animals; manure; crop and/or ground cover debris; and produce plant seed-
lings, leaves, and roots are candidate sites for amplifi cation. Unidentifi ed reservoirs
of amplifi cation, such as wild animals, microorganisms, and plants, may exist also.
Source-Tracking Pathogens and Fecal Indicators of Contamination
in Watersheds
The epidemiology of major produce-associated outbreaks occurring in the last decade
has revealed that preharvest contamination occurs (Table 1.1). However, surveys of
fresh produce at different stages in the production and processing cycle indicate that
bacterial pathogens are at low incidence generally (Beuchat 1996; Harris and others
2003; Nguyen-the and Carlin 1994, 2000), even though fecal indicator bacteria
(
E. coli
) present appear to increase in prevalence during transport and distribution
(Table 1.2) to wholesale and retail markets (Valentin-Bon and others 2008). Therefore,
specifi c events following preharvest contamination are important to identify also
since they may provide clues to amplifi cation sites resulting in a high incidence or
concentration.
An important stage in preharvest contamination is movement onto fi elds, and more
importantly, onto or into seedlings or the mature plants. Water (Table 1.4; irrigation,
fl ooding), intrusion by animals either directly (Table 1.3; wildlife, domestic, humans)
or indirectly (fertilizer, compost), and dust are potential mechanisms of contamination.
Water quality is a primary factor in production of safe fresh produce, and irrigation
water comes from a variety of sources dependent upon the type of produce and
location.
The majority of leafy vegetable production in the region of the U.S. implicated in
outbreaks involves irrigation with well water of high quality relative to surface water
that may be nearby. Indeed, well water was reported to be the source of irrigation of
leafy vegetables associated with recent outbreaks (CalFERT 2007b, 2008). It is note-
worthy also that U.S. winter produce production occurs mainly in the Imperial Valley
of California and the Yuma region of Arizona, where irrigation water is sourced often
from surface water. In contrast, outbreaks associated with produce from these locations
have not occurred or have been rare (Table 1.1). Obviously, the quality of water in
lakes, ponds, reservoirs, and watersheds is critical to produce production even when
it is not used directly for irrigation. Surface water could be a major source of pathogens
affecting aquifer recharging, exposure of animals to colonization, and/or transport to
produce fi elds by irrigation, or processes as yet unidentifi ed.
Watersheds are impaired by the presence of fecal bacteria from livestock, wildlife,
and humans. Any fecal contamination increases the probability of enteric pathogen
contamination of produce either directly or indirectly. The level of impairment is
