Role of Pesticides in Produce Production, Preservation, Quality, and Safety 357
ergosterol involves many steps. DMI-type fungicides (imidazoles, pyrimidines, and
triazoles) block a steroid demethylation step by binding to the active site of the cata-
lyzing enzyme (a cytochrome P450), whereas morpholine fungicides act at later stages
of ergosterol biosynthesis by inhibition of isomerase and reductase enzymes [58].
Benzimidazoles inhibit nuclear division, particularly mitosis, by binding to beta-
tubulin, which interferes with the microtubule assembly [55]. Cyprodinil and other
anilinopyrimidines act as inhibitors of methionine biosynthesis, causing inhibition
of the secretion of fungal hydrolytic enzymes that are necessary for infection [5].
Phenylamides, such as metalaxyl and mefenoxam (metalaxyl-M), interfere with
ribosomal RNA synthesis, thus inhibiting protein synthesis, which would ultimately
kill the fungus [55]. Strobilurin fungicides inhibit mitochondrial respiration by
blocking electron transfer between cytochrome b and c 1 (at the ubiquinol oxidizing
site) [5].
The growth of pathogenic fungi can also be controlled using their fungal com-
petitors or hyperparasites (biological fungicides from the group of biopesticides).
Ampelomyces quisqualis represents an example of a naturally occurring hyperpara-
site of powdery mildews (an extremely broad group of plant pathogens). It infects
and forms pycnidia (fruiting bodies) within powdery mildew hyphae, conidiophores
(specialized spore-producing hyphae), and cleistothecia (the closed fruiting bodies
of powdery mildews). This parasitism reduces growth and may eventually kill the
mildew colony [59].
Certain microbial metabolic products, antibiotics, can also be effective fungi-
cides, but their cost is usually rather prohibitive. Moreover, their application may
result in the transfer of potential antibiotic resistance to human pathogens and thus
reduced efficacy of medical treatments for humans. As an example of a wider use
of antibiotic fungicides, blasticidin-S or kasugamycin (protein synthesis inhibitors
produced by fermentation of actinomycetes Streptomyces griseochromogenes or
S. kasugaensis, respectively) is used to control the economically important disease
rice blast (Pyricularia oryzae) in Japan [60].
11.2.5.2 Problems Caused by Fungi in Produce Production and
Their Control by FungicidesFungi can attack different parts of the plant and cause various fungal diseases that
lead to economic losses (lower yields or even plant death), decreased produce quality,
and pre- and postharvest produce spoilage. Moreover, certain fungal metabolites
may pose a risk to human health and negatively affect produce safety and quality
in general (see Chapter 18). There are numerous factors that either favor or hamper
fungal infection. These factors include mainly climatic or storage conditions (mois-
ture usually facilitates fungal growth, whereas low temperatures suppress it), plant
or produce resistance, and damage by other pests. Only a small number of fungal
pathogens are capable of direct penetration of undamaged skin of most produce.
Wounds caused by insects, nematodes, birds, rodents, slugs, or snails create excellent
entry points for fungal and other forms of infection; thus, combat of fungal diseases
and prevention of produce spoilage caused by fungi should also involve control of
these pests. Table 11.6 and Table 11.7 give examples of economically important