474 Produce Degradation: Reaction Pathways and their Prevention
by workers and equipment, a distinction can be drawn based on hand-harvested (e.g.,
melon, tomato) and semimechanized (e.g., cherry, orange) and mechanized
(e.g., carrot, potato). Historically, seasonal variation in storage condition influenced
the spoilage of spring (e.g., peas, rhubarb) vs. summer (e.g., blueberry, cucumber)
vs. fall (e.g., apple, potato) crops. A distinction could be drawn between mechanized,
high-input agriculture, and low-input or organic practices. The geographical source
of the produce has a significant impact on the potential for spoilage; locally grown
produce is handled less and spends less time in storage than produce grown region-
ally, shipped from across a continent or imported from distant locales. Of the various
factors that could be considered, the issue most significant in a globalized market,
with produce shipped long distances, the relative amount of time spent in storage,
and therefore the length of the window of opportunity for a spoilage microorganism
to act, will be the initial grouping factor used herein. Relatively perishable com-
modities, those that spend relatively short periods of time in storage, will be grouped
separately from relatively storable commodities. Within these groups, arbitrary sub-
groups based on gross structure will be established.
Each of the fruits and vegetables that will be considered can fall victim to a
number of phytopathogens, and, conversely, a given pathogen may have a wide host
range. Certain common themes will become apparent in the ways in which spoilage
develops among the various commodity/pathogen combinations, yet each disease
progression has unique aspects that are instructive to consider. Note that with regard
to nomenclature many diseases are referred to by the genus of the responsible
pathogen (e.g., “Fusarium rot” is caused by Fusarium spp., while in other cases the
genus may be a synonym for another common name of the disease, e.g., “Botrytis”
is equivalent to gray mold, caused by Botrytis cinerea).
15.4.2 PERISHABLE PRODUCE
15.4.2.1 Leafy Produce
Botrytis cinerea is a fungus that attacks a wide range of fruits and vegetables and
is the cause of gray mold of leafy vegetables such as lettuce, endive, and cabbage
(Anon., 2000). Botrytis establishes itself in the inner leaves of the head, and the
mycelium can spread to encompass the entire leaf. Under conducive conditions, the
inner core of the head may be completely engulfed in mycelium with little external
sign of disease. Initial infection typically occurs when the ambient temperature in
the leaf canopy is less than 25°C; thereafter, fungal growth can continue at temper-
atures ranging from 0 to 35°C. The fungus can grow under refrigeration temperatures
(0 to 10°C), and therefore presents a particular problem in storage for produce that
was infected relatively close to harvest. When infecting leaves, Botrytis cinerea can
express cutinases, pectinases, and cellulases to digest the leaf tissue to component
oligo- and monosaccharides (Carlile et al., 2001). The degraded leaf tissue resulting
from the growth of Botrytis can lead to secondary infection by other fungi or by
bacterial pathogens, such as E. carotovora, which accelerates the extent of spoilage.
Downy mildew of lettuce, caused by the oomycte Bremia lactucae, is similar to
Botrytis in that, following inoculation in the field, subsequent spoilage during storage