362 Produce Degradation: Reaction Pathways and their Prevention
Weeds do not attack fruit and vegetable plants directly; however, their presence
may also have detrimental impacts on produce yields, quality, and safety. They
compete with the crop plants for water, nutrients, carbon dioxide, light, space, and
pollinating bees, thus potentially reducing plant productivity and product size and
quality and also decreasing plant vigor, which may ultimately lead to death. Weeds
may also serve as hosts for various pests (mainly insects, mites, and fungi) and plant
diseases and provide a habitat for rodents. In orchards, weeds increase humidity,
creating an ideal environment for the development of fungal diseases. Furthermore,
dense weed stands lower orchard temperatures, increasing the risk of frost damage
in the spring. Produce contamination with weed parts at harvest may be an additional
problem, such as the contamination of peas with berries of eastern black nightshade
(Solanum ptycanthum) or with flower buds of Canada thistle (Cirsium arvense) [72].
11.2.7.1 Classification of Herbicides, Their Application, and
Mode of ActionHerbicides can be classified into groups based on several factors: (1) main point of
entry (roots or leaves), and thus their principal place of application (soil-applied vs.
foliage-applied); (2) translocation abilities (systemic vs. contact); (3) application
time (preemergence vs. postemergence); (4) toxic or application selectivity (selective
vs. nonselective); and (5) chemical structure.
Table 11.8 and Table 11.9 present major chemical classes of soil- and foliage-
applied herbicides along with their modes of action and representative examples.
Soil-applied herbicides are absorbed mainly by roots or hypocotyls and coleoptiles
of germinating seeds and, to be effective, they must be moved into soil (usually by
rain or irrigation) and be relatively persistent in it. Soil-applied herbicides are
systemic by their nature, whereas foliage-applied herbicides can be both systemic
and contact. Contact herbicides are absorbed by leaves (and also stems or shoots),
but their translocation is very limited. Foliage-applied systemic herbicides are
absorbed mainly by leaves and to a lesser extent by roots. An example of a herbicide
that is strictly foliage-applied is the very popular herbicide glyphosate (and its salts,
such as sulfosate), which becomes inactivated on contact with soil due to strong ion
exchange interactions with soil colloids. Regardless of the entry point, most systemic
herbicides are translocated in xylem (acropetally) and some also in phloem (both
acropetally and basipetally).
Preemergence and postemergence application may refer to both crops and weeds
[73]. Table 11.10 gives examples of pre- and postemergence herbicides used in
produce production in the U.S. [3,4]. In this respect, preemergence herbicides are
applied on or in soil before weed germination and prevent weed seedlings from
becoming established. A large group of preemergence, soil-applied herbicides acts
as germination inhibitors, affecting either cell division (microtubule assembly) or
cell growth and development [1,5]. In the latter group, thiocarbamates and bensulide
interfere with lipid biosynthesis, which leads to the growth inhibition in the mer-
istematic region and reduced production of cuticular waxes. Benzonitriles (dichlo-
benil and its precursor chlorthiamid) and benzamide isoxaben inhibit biosynthesis
of cellulose and consequently of cell walls, preventing the incorporation of glucose