Role of Pesticides in Produce Production, Preservation, Quality, and Safety 355
as well as postharvest treatment of certain produce types (e.g., fumigation of potatoes
with 2-aminobutane or grapes with sulfur dioxide).
Based on their ability to penetrate the plant’s cuticle and translocate within the
plant, fungicides can be divided into two main groups: nonsystemic (surface) and
systemic. Systemic fungicides are absorbed by the plant via roots, leaves, or seeds
and are redistributed within the plant, whereas nonsystemic ones stay on the surface
and thus can be removed by rainfall or evaporation and do not protect newly
developed or unsprayed plant parts. Moreover, most pathogenic fungi penetrate the
plant’s cuticle and ramify through the plant’s tissues; thus, nonsystemic fungicides
are applied mainly as a preventive measure (a protectant) before the fungal spores
reach the plant [1]. Protectants act against spore germination to early infection
(penetration of host tissues), whereas curative fungicides combat fungi at the postin-
fection, presymptomatic stage. Eradicants are capable of stopping fungal coloniza-
tion after the symptoms develop. Most systemic fungicides combine protective,
curative, and eradicative properties.
Table 11.5 presents the classification of nonsystemic and systemic fungicides
based on their modes of action and chemical structures, including the most important
fungicide classes along with representative examples. Generally, nonsystemic fun-
gicides prevent or inhibit spore germination and penetration of host tissue and usually
have a multisite mode of action, which makes their use less susceptible to the build-
up of resistant fungi (due to mutations) compared to systemic fungicides, which
often have a specific single-site action [54].
An important group of nonsystemic fungicides, including alkylenebis(dithiocar-
bamates), phthalimides, or sulfamides, can react with various vital cellular thiols
(e.g., enzymes with a thiol group). Chlorothalonil conjugates with thiols (particularly
glutathione) from germinating fungal cells and causes their depletion, leading to
disruption of glycolysis and energy production [5]. Dimethyldithiocarbamates owe
their fungitoxicity mainly to their ability to chelate with copper ions, thus inhibiting
enzymes containing copper [1]. Chlorophenyl fungicides may interfere with miscel-
laneous biosynthesis pathways including protein or phospholipid biosynthesis, lead-
ing to inhibition of germination and growth of fungal mycelium. Cationic surfactants,
such as dodine, can alter the permeability of fungus cell walls, thus causing the loss
of vital cellular components (e.g., amino acids) [1].
Many newer fungicides have systemic properties; however, most of them are
only locally systemic, which means that, after the absorption by leaves or shoots,
they are moved only short distances within the transpiration stream (generally toward
the leaf margin) or between plant cells [55]. Demethylation inhibitors (DMIs) may
serve as a typical example. The newer strobilurin fungicides, such as azoxystrobin
or kresoxim-methyl, have a slightly different distribution, called translaminar (also
meso- or quasi-systemic) [56]. These compounds move into and through the leaf
but do not move in the transpiration stream. Some systemic fungicides, such as
metalaxyl, can be absorbed by plant roots and translocated throughout the plant,
although the translocation occurs only acropetally (upwardly), with the transpiration
stream (transport in xylem driven by the evaporation of water from the leaf surface)
[57]. Currently, fosetyl-aluminum is the only truly systemic fungicide because it is